Modern Vespa

Brief background first: I know many of you like the latest/most expensive casings/kits etc., but I'm never going to be in that camp; I'd rather chop up old stuff with less outlay and less at stake. I do like a good measure, calc, design and cutting project though. My current engine is a pretty good 17Hp(ish) build with 35 year-old PX150 casings, EFL gears, and a cheap DR kit; casings and barrel opened-up enthusiastically (not particularly 'radical', as it was recently described on another thread), and four years and counting, just seized once in the first year when I got impatient with jetting and took a shortcut, and all just cleaned-up and going strong again.

I have everything I need in a big box to build another engine, and have just got my hands on the older version BGM 177 ally/Nikasil kit (BGM1770B). I bought it cheap just from photos, and I knew it had seized before, but that the barrel was OK, I also knew that the piston and head had been involved in quite a party before it all came to a grinding halt.

I've never played with Nikasil before, but know that you chaps approach with caution, and I've also seen some of your pistons after various degrees of calamity, where you've cleaned-up and carried on with new rings, as I have with my current engine, so I'm hopeful this will all be OK to play with.

So, the first thing to do is to decide whether this kit I've got is going to be worth my time and trouble – that's the question after all the preamble: Is this worth saving, and a decent basis for knocking up a mildly tuned engine?

Below are photos of the barrel after just a scrub, a squirt, and a quick rub with a bit of Scotchbrite; there are very light marks, but no deep damage, anything I can catch with a nail, no signs of any delamination, or damage to Nikasil edges round ports; I can polish the marks out a bit more unless it'll do more harm than good:

Photos below also show the head squish band and the piston crown both have quite a bit of damage . I've given both a good clean and rub-down to get a better idea of depth of damage Note that the raised ring on this head is not the sealing surface, it's the squish band in a recessed head. It seems to me that I could make both of these pristine(ish) again, if it's necessary to go further than I have, and the resulting increase in squish could be recovered by a home-done skimming of top of barrel with wet and dry/plate glass. All measurements/volumes port timings etc. yet to be measured, but removing from the top of barrel doesn't change port timings anyway.

A new set of rings would be on my shopping list (these are 1mm and some flash 'extremely break-proof' material btw), but as an idea of wear to barrel and rings before seize, the ring gap with these rings is 0.3mm. This was one thing I wanted to check before getting too keen because I have no idea about cost/viability of honing Nikasil, but doubt it's easy or cheap, so wouldn't have posted here at all if the Nikasil was already thin and I'd fallen at that hurdle. It seems new rings (and delicate hone?) would get it back within tolerances.

I have a set of head/base gaskets, which are pretty knackered, but I'd make my own anyway, to precisely suit whatever thickness is needed for my port timing, which I would intend to be similar to my current engine (about 123°TD).

I'd be opening the transfers, matching the casings, (also matching to the casings, as best I can, if possible, of the engine I'm running now, so I could swap it if needed), tweaking the inlet pad and crank for inlet timing; maybe looking again at l/s crank if it makes achieving timings easier without Dremelling the Nikasil or skimming the barrel, but not just for the hell of it, or for 10cc.

It seems to me that I could, in the future, get the new BGM 177 piston, this piston is a 'B', so maybe a hone and a 'D' piston? But I would prefer to use this piston for now, and 'B' is not in stock anyway; this used kit cost me less than the new piston.

So, that's the background and plan; question again: is this worth saving, and a decent basis for knocking up a mildly tuned engine? If not, I'll just sit on the job again; I really don't need yet another spare engine, or want to spend loads, I just thought it might be something to fiddle with over winter.













Last edited by sime66 on Sat Nov 02, 2019 1:46 pm; edited 1 time in total

Glad to hear the old tractor is still running fine.

BGM kit was a good find. Nothing wrong with any of it. New rings never a bad thing. Don't go sanding the bore as Nicasil is very thin and it won't help it run better.

Of course very tunable. A 60mm crank will make this easier. What you after this time? Exhaust? Carb? Rotary/reed? How much power you want it to have? Anywhere between 20 and 40bhp is practically possible for road use.

Are you sure that's not exposed aluminum?? It looks exactly the same as my worn out T5 cylinder...

No SaFiS, that band at the top of a BGM177 cylinder is where the head recess sits. There is no piston travel in the top ~4-5mm of the bore.

I would suggest you try the cylinder with a new piston. I've had two BGM kits. The first was a really early one, probably one of the very first. Some of the bore marks were very 'odd', even after the very first 15 minutes of running. There's a thread somewhere in this forum about it. I ended up seizing it and bought a new 'C' piston to replace the scored 'A' piston, as I always felt it was a bit noisy, as if there was a bit of piston slap. It ran really well after that.

My second one was the v2 with the black graphite coated piston. I noticed that the piston was a little longer, so there must have been known issues before. This kit seemed so much better and quicker, but it was on a different spec. engine. Absolutely reliable and strong.....cannot fault it.

You'll get your 123 TD with a 60mm crank and the stock base and copper head gaskets. My favourite setup with 57mm crank is 0.4mm base gasket and the 0.5mm copper head gasket, giving you ~1.0mm squish.

Clearly this cylinder has had a hard life but there is more in it yet. The detonation marks on the piston match the grey area nearer TDC. Under severe detonation the top of the bore always has a sand blasted appearance while the piston has crown dents in the squish band. I think it will survive just fine. Will need some Dremel work to get the power up higher but don’t they always?

Blimey, no one had posted when I went out this morning, now there's loads to look at; thanks everyone.

Hi SaFiS, thanks for posting; It isn't exposed aluminum, it's where the head recesses into the barrel; I might be skimming some of that off too. As swa45 says it's mainly the recessed head, but I did note that it is wider than the recess depth though, and also what Jack221 has since said about top of bore looking sand-blasted due to detonation. I thought a ring gap check up high in the middle of that band might indicate any significant extra wear there to satisfy the question, and the difference is barely discernible; maybe 0.35mm, but dragging.

Hi swa45, thanks for your input as well; I'd carefully read your thread a few times before starting this one, and the other thread concerning the delamination of early kits. I saw your mystery mark back when it happened, and also your thoughts about piston clearance and noise, and the newer black one being slightly longer. SC only have 'D' piston in stock, which combined with your experience of it in your thread, is why I thought a 'D' might be OK. I'll have to brush up on how to measure clearances and decide; maybe they'll re-stock 'C' before I get that far too. I'll put BGM piston sizes table below for others. I also gathered from your thread that l/s will make 123°TD simpler to achieve. I'll make my own gaskets to get it spot on to what I need; over time I've gathered together all the ally and copper plate thickness I could find to be able to do that, so it's not a problem.

Hi Jack22. The Tractor has been particularly pleasing this year; with the Kytronik variable timing, the Malossi 177 head, and a louvred Stella sidepanel I've got my revs and temps just right – for me, which isn't mad but satisfying. Just approx because I haven't been measuring; 3-4th at about 7800, 4th to about 6800, just comfortable, no need to thrash it to rev-out, and staying below 160°C at WOT all summer. All it's cost me this year is a new Sip Road 2, a new rear hub, and more recently a set of new tractor tyres to fit before winter. Early September, I'm still in summer-mode really, but this barrel came up, so bought it and thought I'd see what you all thought, although I might take a few weeks to get organised for an engine build – I want to set up a work bench in my spare room, so I can contain the mess and close the door on it when I need a break this time.

As far as this engine is concerned, nothing to get too excited about I'm afraid, cast your mind back to E#3 on the other site; it's just more old ('82 P150X this time) casings, so a stepped primary shaft for EFL gears – not particularly strong there for a silly-power build, which I don't want anyway. Mostly similar to what I have now, Si carb (I thought I'd give Patrick's vortex a go), SR2, a modified standard crank unless a l/s crank will help my timings without giving me wild ATDC inlet timings. Long wide high torque, rather than peaky power. Cool!

As I had no second opinion first thing this morning whether the kit would pass muster without major outlay, all I did over my cornflakes was measure the barrel, and had a quick play with online calculator to see what DH (and therefore what skim or base gasket) I'd be looking at for around 123°TD, and with the cleaning-up of piston and head it looks like it'd be a big skim with standard crank, so maybe l/s will be good (it's about 5-6mm with 57 – [so base packer and skim or lots of Dremel], 3.5mm with 60). But now you've given it a cautious nod, I'll dig out the box of goodies, brush off the casings and do a dry build and proper measure to see for sure. Though I have a chap now that would probably do the job, I'd rather go l/s if I can find a sensible one, than repeat the palaver of having the barrel skimmed, especially with the added factor of Nikasil, and especially if, as you say, the barrel has already had a hard life.

I'll have a dry-build measure soon, and mainly try to use what I already have as much as possible; I'll get a DH, squish and volume and see how all the numbers work, then post an update....


Port Map BGM1770B

57mm crank

60mm crank

BGM Piston sizes

Hardly suprising guys like these BGM kits. Ports are pretty big already. Exhaust port is 69% width out of the box.

Still, lots of potential for making more power. On the 57mm crank the PBT is about 4mm? Which gives 111/167/28. This will of couse go ok but well down on what it could be.

With a 60mm crank, to get the best out of it, the exhaust will need moving quite a bit. As well as a few other things.

26/26 carb, vortex and a big box exhaust will be ok. Keep the P150X gearing as is and maybe take the whole Cosa clutch from the DR.

Calling SWA45.
I trying to got hold of you mate.

Having a bad day, Del? - Hope you sort it, mate.

Hi Jack. I think this will be a 60mm crank job; dry build will confirm. I have no problem with that, but my concern before was the crank timings I will have to work with to give me sensible inlet durations without going too racy in characteristics with a long ATDC duration (high revving, short power band ain't what I want); maybe I'm wrong to think that will be a problem. It seems to me I'll want to open my inlet pad to get a good volume through the Si26 and my lovely big transfers, and the crank timings will restrict what I can do at the Inlet closing end of pad; I always seem to end up concluding that cutting a standard crank is best, but probably here the 60mm is more of a priority, and inlet timing can be the best-found compromise. Crank timing info is sparse on suppliers' websites, and I'm not sure which manufacturers are currently on the naughty step, but a bit of measuring, investigating and advice should sort it. Maybe swa45, you can say what crank you used, whether you opened your inlet pad, and what inlet timings you got.
No problem raising the exhaust port; I'd expected that job, and whether it's a tiny amount or a few mm, I'm still going to have to be extra careful, but I'm ready to tackle it.
I'm not using P150X gears; I do have a choice of EFL 125/150 gearboxes, and I'd already bought the stepped crank to be able to use them in these casings; it'll all be EFL, 21/68.......
That Sip Cosa clutch has been a beauty! I will run it with whichever engine I'm running, and whenever it needs new plates (which I will check if it comes out for this engine) I'll read-up, and copy, what you chaps have been doing with CR80 ones. I bought the 16-rivet job and doubled up on springs (without knowing really what 'grade' they were; just two sets), and the wide circlip, and it's been trouble-free for ages. Clutch, exhaust and selector box can all run with both engines. I have a brand new si26 carb I can use, might need a tweak with a drill and file first to open it up a bit – can't remember.
Everything else is in the big box, which I've dug-out and is ready to unpack in the new spare-room-workshop that I cleared-out this morning. First job tomorrow though, with a good weather forecast here, is a nice crisp-morning, early-autumn ride on scooter to properly wear my old tyres out before I swap them before we get the wet leaves, mud and ice that'll soon be with us, but at least it looks like I'll have a fun indoor job to play with.

Hello Sime.
I’m ok thank you. SWA45 has something for sale I’m interested in but he’s gone off grid. I know he’ll read this post first.

Back to your Thread. I notice in the calculator you have put 6mm from top of piston to top of barrel. The head sits inside so shouldn’t you subtract that amount?

Sorry if I got it wrong but I’m absolutely crap at Maths.

I had visions of you stranded somewhere, glad it isn't a major.

The illustration I was doing with the calculator above was just to prove to myself what swa45 and jack221 were suggesting – that 60mm crank would be easier, but I'd not worry too much about those for now; I haven't measured Deck Height yet. When I do my dry-build, I'll do a more thorough working of it, and it will become clear, but your question does go back to what I was saying in another thread – the Deck is the Deck regardless of what the head does. My Deck is the flat top of my cylinder, the fact that the head drops into it doesn't change that. Because the head drops into the cylinder the piston doesn't travel so far up, so the number is bigger; that's all. That Lambretta calculator asks for distance (which you are right, it will subtract from the port dimension to get a TDC to port dimension – the other calculator, you do it beforehand I think), and the distance is (will be for this barrel) several millimetres. The crucial dimension is TDC to top of port, how you establish it doesn't matter (we measure Deck to TDC [DH], and Deck to port). TDC to top of port translates (by calculation) into degrees per revolution that the port is covered – the higher the port, the shorter the distance, the fewer degrees per revolution it is covered. So then, with a Transfer duration in mind (in this case hopefully about 123° for a strong tractor with peak power at about 7500rpm), I'll see what I need to do to get the top of the Transfer ports there (moving them in the barrel or moving the barrel, or changing the crank), then make sure the exhaust sits OK with that (or decide what needs to be done so that it does), then make sure the squish is OK (or what needs to be done so that it does). So all I was doing in that example is seeing what sort of Deck Height I'd be looking at to have my transfers at about 123° with a 57mm and a 60mm crank, which gives an idea of how practical either will be – clearly a Deck Height of about 6mm with a 57mm crank is going to be a pain; a big skim for squish, or lots of moving of ports within the barrel, something like 3.5mm with a 60mm crank is going to be far easier to play with when I know the head sits about 3mm into the barrel – so that's another £150 before I've even got it all out the box!! (and the crank I wanted to use will stay in the box for a few more years....)
That's probably made it sound more confusing, hopefully it will become clearer when I have an actual Deck Height to fiddle about with.

Watch this space!

@sime, my setup is:

- BGM 177 cylinder with the 0.4mm base gasket/packer and the 0.5mm copper head gasket supplied with the kit. SBC - 1.0mm
- Jasil 57mm 'sports' crank
- 24/24 carb with 52/140 and 160/BE3/128 (IIRC)
- SIP Road 2 'twin tail pipe' or BigBox Touring depending on whether it's in a VBC or PX frame
- Inlet extended towards the back to give overall ID of ~180 degrees
- PX150 gearbox (same as PX200) with stock 68T primary and 23T DRT clutch cog

BGM/SCK reckon 118 TD with 57mm and 123 with 60mm out of the box

Thanks swa45, that's very interesting.
So your peak power is at around 6000rpm if Transfer duration is less than 120°, but you have high gearing with the 23/68. It rides well 2-up with that gearing and stays cool working hard at lower revs? Nice. 180° (125/55) Inlet duration is what I have in mind too; it's what I have on the engine I'm running now - but peak power at higher revs, and standard PX150 gears with 21/68.

Sounds like a great scooter. Pulling that high gearing well is a tall order Port timing doesn't quite add up though. If the squish were both set at 1.0mm then 123 on the 60 crank is good but on the 57 it would be 108 and not 118. Seems like a typo. Explains why it works, as 118 wouldn't pull that gearing so well.

Jack, those numbers are just quotes from the SCK/BGM blurb. I had to use the 0.4mm base to get the squish to the recommended 1.0mm on a 57 stroke crank (PX150 motor). It pulled really well with 68/23 and did a sterling job of getting myself + wife to Brighton on a few occasions, with lovely fast stretches of A23. The kit is supplied with 0.2, 0.4 and (I think) 0.7mm base gaskets and the 0.5mm copper head gasket. It had a BigBox Touring on at the time.

On another PX150 motor, I tried the BGM with a more racey Mazz crank (closes the inlet at ~75 degrees ATDC) and lifted the cylinder by a further 0.5mm (total 0.9mm) with no head gasket. I used a PHBH 28 and the Scorpion with the stock 68/21, but 36T 4th. It was all revs and no trousers. This kit suits torquey tourer much better.

The only time I've used the BGM kit with a 60mm crank is on my VBC motor with a hybrid crank for PX ignition. There's a thread about that build on here as well. I cannot recall which gaskets and TD I ended up with on that one, but it's in the thread. It uses a 125 PX EFL box with 68/22 and my Scorpion or the SIP Road twin pipe. I've never pushed this motor to WOT as last winter came around and then this year has been about my 2xx cc scooters.

Back from a crisp, brisk ride; no time for a dry build today, but I do already have crank case dimensions (centre of crank to face), and had a quick measure of barrel face-to face and piston pin to edge of dome. With these I can pretty accurately approximate Deck Height for both cranks; I've overmarked my old sketch to work it out. Image below shows this, and gives me, with NO base gasket, 57mm DH = 3.72mm, 60mm DH = 2.22mm.

With a 57 crank and no base gasket, I make that 110.0/165.77.
With a 57 crank and 0.4 base gasket, 0.5 head gasket (per swa), I get 112.34/167.45 and 0.77mm squish (I'm approx, so don't worry too much about slight discrepancy; I've measured the upstand on the head as 3.85mm, and I'm using this edge distance from TDC as squish without head gasket for now).

With a 60 crank and no base gasket, I make it 116.3/168.21
Then I looked at various base packers with a 60 crank and calculated squish for each with no head gasket, as follows:

1.0mm bp: 121.52/172.15/27.87, sq = 0.87mm
1.2mm bp: 122.5/172.93/25.20, sq = 1.07mm
1.3mm bp: 123.04/173.32/25.14, sq = 1.17mm

I won't attach all the Lambretta calculator sheets for all these results, they just give me a better idea of the likely options after dry-build; I'll take much greater care with my calcs and checking myself when I've got the engine together and got real numbers.

Anyway, I guess that's as far as I can go for now; maybe these approx numbers here help the discussion.

Sime, not disputing your numbers, but I'm wondering why SCK/BGM would overstate the timings by so much. This from their blog:

Timings 57mm
With an engine with a stroke of 57mm and a squish gap of 1mm, you will reach the following values:

Compression 10.3:1
Transfer timing 118°
Exhaust timing 171°

Thanks to the dimension of the cylinder you will only need a 4mm gap to centre the cylinder head in the hole. The gap will prevent the cylinder head to stick out into the combustion chamber, even if you adapt the engine displacement. With an engine displacement of 60mm and depending on the engine concept you can lift the cylinder even further to achieve significantly larger transfer port angles.

Timings 60mm
With an engine with a stroke of 60mm and a squish gap of 1mm, you will reach the following values:

Compression 10.8:1
Transfer timing 123°
Exhaust timing 173°

https://blog.scooter-center.com/en/177cc-cylinder-bgm-pro-for-vespa-px-sprint-co/

Dunno.
Seriously though, I wasn't posting to disagree with you or SC/BGM; just trying to narrow the numbers down a bit for myself until I've had a proper measure.
I'm not going to worry about anyone's published numbers for now because I'll have my own soon enough, which I will check thoroughly before posting more new timings – I think I've added to the confusion, by even the best-intentioned of estimates.

**All timings I've posted so far on this thread are approximate, best estimate until I've got the engine together**

In actual fact, don't I pretty much agree with the blog for 60mm? I'm estimating a 1.2mm base gasket with a 60mm crank will give me 1.07mm squish and:
122.54/172.93, and the blog is saying, for 1mm squish:
123.00/173.00; that's not bad for best-estimate before a dry-build.

I think jack has already suggested their figures for 57mm crank are suspect, (that the 118 was more like 110 - where I got an approximation of 112) and I approximate their 0.4 base packer, 0.5 head gasket timing as:
112.34/167.45, and the blog is saying:
118.00/171; I don't get the same here, but a discrepancy (typo) has already been suggested, and also that both their set of numbers can't be correct.

Really; I'm going to ignore BGM numbers and use my own, which I'll have soon enough, otherwise it confuses the issue trying to check them, and takes longer to get the accurate numbers.

This is what I estimated above for 60mm crank:

1.0mm bp: 121.52/172.15/27.87, sq = 0.87mm
1.2mm bp: 122.5/172.93/25.20, sq = 1.07mm
1.3mm bp: 123.04/173.32/25.14, sq = 1.17mm

This is what I had in my earlier post above in my approx port timings for 60mm crank, 1mm squish; it's close to BGM's (yellow highlight):

You and Jack could be right about the typo. I didn't measure my TD when I built my touring BGM 177. I just messed with the base gaskets until I got ~1.0mm SBC, which happened to require the 0.4mm gasket. Luckily it performed beyond what I had hoped for.

Looking forward to your results, as I want to revisit my VBC/BGM 187 build. I've never been happy with the way it runs but my attention was diverted to my 221 engine before I had time to investigate.

Evening swa. Main thing for yours is the results were beyond what you'd hoped, so maybe it isn't so important why (note below that after DH measure, I now get 115.6Td with 57crank and 0.4mm base gasket). I was assembling my engine when you posted earlier, and I'm a bit brain-dead with numbers now, but, maybe against my better judgement, I'm going to post my first set of numbers below, but I intend to check them again when I have time to concentrate better; so here goes.....

To be checked and confirmed again – but I need to record it clearly to refer to next time I've got time, and my notes have gone all scribbly; I want to sleep on it before checking myself (if I leave it here someone might check it for me).

With crank, piston and barrel bolted together now, I've tried very carefully to measure DH (with 57 Crank); it's quite difficult with the piston dome when it isn't just a small, feeler-gauge distance, but I've triple-checked with feeler gauges, digital caliper and even making a mould with Plasticine to measure as accurate as I could. I get:
57 crank DH = 4.3mm, so
60 crank DH = 2.8mm.

Using Lambretta calculator, timings with NO base gasket as follows:
57 crank: 113.37/168.20/27.41
60 crank: 119.35/170.50/25.58

(swa, trying not to get distracted by CS/BGM now, but just to note that, now I'm assembled and measured, I'm closer to them: 57 crank with 0.4mm base gasket I'd get 115.62/169.86/27.12 now).

Then, using Lambretta calculator, finding what base gasket I need for 123°Td, and what squish that gives (squish is still an approximation; subject to measuring squish with solder to check): these are those numbers:
57 crank with 1.75mm base gasket (6.05mmDH***): 122.95/175.40/26.23 *
60 crank with 0.70mm base gasket (3.50mmDH***): 122.94/173.24/25.15 **

Notes:
*: Subject to checking squish, rough calculation for barrel skim for 57 crank and 0.9mm squish = 6.05DH-3.85-0.90sq = 1.3mm barrel skim.
**: subject to checking squish, rough calculation for head gasket for 60 crank and 0.9mm squish = 3.5DH-3.85+1.25 head gasket = 0.9mm squish.
***: Encouraging (as a check) that these Deck Heights are the same as those I estimated first go.

These are the two Lambretta Port Timing Sheets for the results for 123°TD for both cranks:

The problem with the BGM 177 is it has the dreaded syndrome "jack of all trades master of none'
Its a compromise of everything.
Malossi have it sorted with a sport and MHR so they focus more on the strengths of each kit rather than compromise on the middle ground somewhere.

Now im off to the naughty corner to change my reed petals.

+1 a bit like myself
Thats why I say the exhaust port needs moving. With a 60 crank and some improved port timing it will get closer to the Malossi.

When taking these measurements its only possible to get within a few degrees of someone elses cylinder or measurements. All this does confirm that Scooter Centre make typos like everyone does.

Sime - Like the approach. Been following this since your first post and almost able to follow - think - not really water skiing so much, but more like having the rope wrapped around my ankle and being dragged fast enough behind the boat for my body to at least plane as it slaps along.

I grasp the cylinder port timings/squish trade-offs fairly well.
The inlet and crank duration at inlet targets are more of a mystery to me.
Seems you are trying to keep a broad power torquey power band as SWA achieved - and have some concerns around crank timing that will affect this.
Couldn't quite follow.

Can you elaborate a touch on what your targets are - and decipher some acronyms - specifically "DH"?

Thanks for slowing down enough to allow air.

-CM

Hi charlieman, I like the water skiing analogy - know that feeling well. I'd just composed an update to my previous post, but I'll do yours first because it is fairly simple:

I've learned to use DH (Deck Height) as the distance from the edge of piston at top dead centre (TDC) to the face of the cylinder (the face that meets the head or head gasket. I've noticed that just about everyone on here uses something like PBT (Piston Below Top), so I'm going to fall in line and do the same (but not on this post because I've already written it).

Inlet timings are a combination of the length (degrees) of the opening on the inlet pad, and the position of the cuts on the crank flange that pass over that opening. I went into it in a fair bit of detail in a previous thread, here:

my BLUE Bajaj fitting 60mm Crankshaft

Different types of engine require different degrees of opening before and after Top Dead Centre, and different total degrees of opening. With a longstroke crank, the timings on the crank (where the flange is cut) seems to me to give too much ATDC (After Top Dead Centre) duration, which is not beneficial to a wide power band, high torque, low revving engine - often called 'race-crank' is a bit of a clue it's going to be designed for a revvy engine. With a standard Piaggio crank I can cut it to give me the timings I know I'll want, and be able to open up my inlet pad to get a good volume passing through the whole system from carb, opened inlet, opened transfers, to big exhaust port. It's a point I'm about to make in the next part of this post that I'd already composed and was about to cut and paste....

Bear in mind that I'm still very much a learner myself, so some of the stuff I say/ask is with the intention of being corrected, so I can learn more as I go.

You asked about rough figures; I'm thinking 123° Transfer duration because it gives me peak power at 7500rpm, which I like (maybe a bit lower?) 123° Transfer, referring to a crib sheet I have, suggests 171° Exhaust Duration, which gives 24° Blowown. The Inlet duration wants to be about 180° total, split as 125°/55° BTDC/ATDC. - Those are the sort of figures I have in mind; 123/171/24. The two I have now are (57):123/175/26 & (60):123/173/25, so not a huge tweak to get pretty nice.

Hope that helps, back to the script:

Measuring/dimensions update:
I've measured my squish, with 57mm crank and no base gasket, and it is 0.75mm. As a check, subtracting the upstand on the head (the part that goes into the cylinder) from the measured Deck Height should give the same result; there is a discrepancy of 0.3mm. Maybe that's within tolerances, but I thought I'd try to pin it down. I'm confident in my squish measurement, so I made another attempt at measuring my Deck Height, which is now a bit easier because I've fitted the flywheel to turn the crank to measure the squish more easily – I didn't do it yesterday because I had to get the starter ring off the flywheel and cut a small lip out of the casings air scroll for it to fit. I'm revising last night's DH measurement, by 0.3mm to 4.6mm.
This does not change the final cylinder height, just where I had it before, so the head, barrel and piston remain the same (as do head gasket and skim details posted yesterday, and the Port calc sheets), but the base packers reduce by 0.3mm (to 1.45mm for 57mm crank, and to 0.4mm for 60mm crank). At the bottom of this post I'll attach a sketch of that, which is the choice as I see it.

The choice:
Positive for the 57 crank is using the crank I have (which was the purpose of the job; to use my stuff), saves me the cost of a new crank, and it gives me full control over my Inlet timing and pad cutting. Performance-wise, from my experience with the DR, I can get more than enough for me out of a 177cc and 57 crank.
Main negative for the 57 crank is having to skim the top of the cylinder 1.3mm. Maybe I could do that myself with wet and dry on glass plate, maybe I could pay someone to do it; probably both are possible, but I'd rather do it myself. Jack do I recall correctly that you said you'd do up to 1.5mm yourself?

Positive for 60 crank is simplicity; I just have to make a head and base gasket. There are probably performance benefits, but I'm really not interested in that – the earlier comment about the BGM177 kit being a compromise may be true, but I'm not trying to break any records; just use up some stuff to make an engine now that I've found a cheap kit for the job.
The main negatives for the 60 crank are the cost (and the fact that the 57 crank remains unused), and the inlet timing I'm forced to compromise with because all manufacturers seem to assume that if you want a 60mm crank you want a race crank; they seem to come with (to me) unhelpful ATDC timings.

I haven't decided which option is best, and I'm going to go searching for long-stroke crank data next. Is there much difference between the two to the amount of work to the ports within the cylinder?

Sketch of both crank options to position Transfers for 123°Td
(I haven't looked at BGM data yet, I wanted to do my own first, to make an assessment)

Thanks! Ahhh. PBT. Got it. Excellent - that was both the explanation and targets I was looking for. Cheers! a time honored approach -

From SIP:
"PINASCO long stroke crankshafts have aerodynamically adapted and polished crankwebs. In terms of the timing it is between the shafts from MAZZUCHELLI and POLINI"
Hardly a precise read - but I'm sure you will find the data for the ones that fit yours. I now am better armed to look myself for my own project.

Thanks again for the detour.

No worries; don't take anything as gospel though, I tend to post in the hope that others will correct or confirm.
I've dug-out loads of L/S crank options this morning, and am now going to see what inlet timings I can achieve with the data I've found; maybe that'll make the 57/60 decision easier.
Also, I had a bit of a nightmare overnight, and maybe someone can confirm; does a L/S crank need a 1.5mm base gasket to stop piston fouling at BDC. I'm going to take my piston out and measure the length, and see for myself if it would travel down properly. In my sketch above I drew piston crown at BDC to see where it was in relation to bottom of ports, but I forgot about the bottom of the skirt hitting the casings.

Just to keep thread up to date, here's the info I got this morning, and I'm going to do a sketch of my Inlet pad and either a 138/6 or a 123/6 crank. The Serie Pro 'Evo' can keep its 130/10; I want my engine to cost £400, not just my crank!

You could always just buy the eccentric pin for 59/60mm stroke and modify an existing crank. Depends how adventurous you want to be. I've always bought 'off the shelf' sporty cranks but one day I'll do my own. Maybe a 62mm stroke and/or a different length conrod

Looked into them before; they're about £70 and I haven't got the tools or the skill to do it myself, so it would be a 'farmed out' job as well, so all in all too much expense and faff for this job. I'll buy a L/s crank and adjust inlet pad as best I can, or cut the crank I have, adjust the inlet pad to be spot-on, and deal with the head skim. Two factors to determine:
1) How these L/S crank timings work on my casings.
2) Whether a 1.5mm skim is a sensible home job.

At last I've sat down and made a logical interpretation of how Sip state their crank timings, which actually makes sense, but it might be different to how we had it before; an example:

“138/6 Post TDC” (most of the l/s Cranks):
Crank rotating clockwise, looking from flywheel side.
@ TDC the crank pin is at 0°
@ 6° 'post' (after) TDC the Io is at 0°
@ at 138° 'post' (after) TDC the Ic is at 0°
The total opening is 138-6 = 132° (I really think 132° is correct, and NOT 138° as previously read, an over-marked image below shows why – surely someone reading this has a crank they can measure).

So, for the following working out, that's what 138/6 means, likewise 123/6 (though I doubt I'll buy that one), and my standard crank, which for uniformity is 114/5 written this way, and interpreted as above.

My inlet pad on my casings is 61°, ending 64° BTDC, so my max inlet opening is 57°, ending at 66° BTDC. The current opening is 37°, ending at 76° BTDC. (image at bottom of post). Note, the max opening with a 138/6 crank is 47°, ending at 76° because I don't want to touch Ic on inlet pad. With the 123/6 Serie Pro, an 8° cut on Ic makes it pretty damn perfik, but that's £200.

114/5: I can get 125/55 (and max inlet opening area)
123/6: I can get 125/55
138/6: I can get 125/62 (and smallest inlet opening area)

The aim would be top open the Inlet pad to the maximum to get volume through, but trying to keep ATDC down will mean that I'd probably leave the Ic on the pad as it is, and just open up the Io.

I've done the three crank calc sheets (images below), and I think they show the options well; I haven't worked out the area of the openings yet, it's on the list; check my pad and work out the areas........


138/6 crank has 132 actual opening, surely?

Hopefully this makes the Inlet pad angles easier to follow

114/5 Crank Timing Calc

123/6 crank Timing Calc

138/6 Crank timing Calc

Great point what is a safe dimension?

I checked mine; my piston is 63.5mm long, and with the 57 crank and no base gasket misses the casings by about 5.5mm; the first obstruction is 43.3mm from the mating surface (see photo and drawing). So for a 60 crank with 0.4mm base gasket, I'll miss the casings by (-1.5+0.4=) 4.4mm, on my drawing it's 4.7mm (still chasing that 0.3mm discrepancy); I think 4mm is OK.

Hibbert, as you're reading this, I'm going to check your Bajaj thread later today, but after I raised the question there, of my doubt over the actual opening time on a l/s crank (whether is was 138-6), did you actually measure your crank before you cut it? I'd really like your measurements if you did.


BDC Piston clearance with 57 crank and no base gasket

Piston skirt and clearances at BDC added to sketch

when you say obstruction are you describing casing? Is it a trim job to the bottom of the piston to correct? 5mm is safe?

Looks like my June 18 post is with the 60mm 112/10 Mazzucchelli w/ stock rotary pad. With more precise measuring 157° total duration. This is before any crank/pad alteration.

Yes, the first part of the casings the piston would hit if it went lower; I think my clearances are OK – though I do not have a golden number for the limit.
Yours is running isn't already it? If you think you may have overlooked something, you'd probably know already, and it's best to raise it on your Bajaj thread for others already familiar with your build; I'm not about to start telling you whether you need to or whether it's OK to trim your piston!
Did you not actually measure the degrees on the crank itself before cutting it? Don't worry; I'll check for myself this evening – lots of other work to do today.

Shortening the ally barrel is not a great idea. The 60 crank will work out the best. If the piston does touch the crank at BDC (unlikely) just Dremel the piston to give 1mm clearance.
Can always sell the 57 crank. Someone will want it.

The stock blowdown on the BGM is 28 degrees. I would try to maintain this or even a little more

I agree about the l/s crank, the numbers work better (as long as Ic @ 66°ATDC is OK, which I believe it is) and the power and torque will be better, so I can't ignore it; no problem, I'll still get rid of most of my box of old rubbish, and I would have had to start a shopping list sometime anyway – doesn't mean I'm doing loads of other upgrades though; 'now I've got the l/s I might as well get....'.
I'll still be able to get enough open area in the pad with l/s (around 560mm²), though my ATDC is higher; I'll do my best to get better crank info, or a confirmation of my interpretation that 138/6 is actually 132° open; I've made a bit of progress with that, actually;

Scootercentre state, on their standard Mazz 60-105 crank page (1620007), that a 33mm (unmodified pad opening) gives 172°.
My unmodified pad opening is 33mm, which I know is 39°.
It follows that SC are saying the timing on the crank = 172-39=133°.
This reinforces my interpretation of Sip's 138/6 being 132° open, which makes me happier.

I'm going to update all my numbers now I've pinned-down some more facts and dimensions a bit anyway, (it'll all be spot-on before I get the Dremel out), and I'll see how 28° blowdown looks; at first glance, it looks like raising the exhaust about 1.5mm; is that what you meant? 123/179/28 (& 125/66); I did a quick sheet for for that showing the exhaust mod (bottom of post). I will get round to reading the BGM stuff; lots to do, and working through it, but no hurry 'till I start spending or cutting.
What's the theory for Blowdown up at 28°? - Also, I see 123/173/25 on BGM data sheet for 60 crank, here:

https://www.scooter-center.com/media/pdf/3c/86/82/datenblatt_data_sheet_bgm1770n_.pdf

I'm not concerned about the BDC piston skirt on mine; I'd already satisfied myself with a quick measure.
Various updates here below to help me keep track of the latest tweaks as it develops:


New timings, and exhaust mod for 28° Blowdown

Pad cutting angles and areas, and revised Inlet timing mods to suit 138/6 decision

Inlet calcs look like they are all under control. 66 ATDC is still ok. I think 'too much ATDC' has moved out to about 90 degrees these days.

Using your measurements I did the calculations again. On that datasheet SC only mention 0.2 base gasket and I didn't see anything about PBT or head gaskets. If you ignore all that, it is just about possible to get near 118/171/26.5 and 123/173/25 with 1.5mm extra on the head gasket. To get to 118 on a 57 crank PBT has to be over 5mm, seems like a lot, is that possible? using the 60 crank PBT for a 57 ends up with 108/165/28. Will all be obvious once you do the dry build and measure it.

So if at 118 original blowdown is 26.5 (similar to a stock P200). Wouldn't go less than that but 28 is still a better number with only one exhaust port. And with the inlet getting more in, it will need to get out the other end somehow.

I deliberately did did my calcs before reference to BGM timings, so as not to be tempted to fudge them to agree, but they do pretty much agree anyway (I got 'near': 123/176/26, and keep in mind my PBT was not measured with a 60 crank). My port measurements are accurate, my PBT will get a tweak, so will my gaskets.
My PBT for my 60 crank in my calcs (3.1mm) was taken from measured PBT for 57, which is 4.6mm (4.6–1.5=3.1mm). When I have a 60 crank I'll measure PBT and re-establish what base gasket and head gasket I need; the numbers are achievable, but still subject to a little tweak.
I didn't do a 57 calc sheet for 118 but can if you wish, but my 57 sheet for 123 shows a PBT of 6.05mm, hence the barrel skim, which is now rejected.

To keep it simpler, all my port timing numbers from now onwards will be for a new 60mm crank, and they'll all get a tweak when I have it here to do another dry build with it.

But, can we confirm that you're suggesting 123/179/28; I can do the 28° Bd by raising the Exhaust port – is that what you're suggesting? I think your post explains why you're suggesting it (I'll play with your numbers later to try to understand the point you're making), but it doesn't confirm my query about whether you meant the exhaust mod for 179°, to get Bd 28°.

I would take the porting a little higher 125/181/28 will go better while still being more of a touring set up. As this only has a single exhaust port the timing has to be a little more aggressive to get the same as the DR. With the 60 crank this will be a 187cc. I don't think this will rev as high as the DR pre de tune.

Righto, thanks Jack.
Raising the cylinder another 0.4mm (total now 3.1+0.8 = 3.9 PBT) gives me Td 124.95 ( 125/175/25 ).
Cutting the exhaust 1.6mm higher (from 39.63 to 38.03) then gives me Ed 180.94 ( 125/181/28 ).
I'll finalise the base gasket when I have a crank.

Is this cutting of the exhaust port within the cylinder what you are suggesting to achieve 28°? It must be, but I'll be happier once I know you've seen that it is necessary.

Yes, exhaust port needs to move. Some careful grinding needed. I think 1.6mm will be enough but more is possible, although depends on the general port shape. As max power is no great concern and you're running a box exhaust, a rounder shape could be used with port raised 2.0mm in the centre. This will stretch the powerband a little .