Modern Vespa

Managed to get a v50, should be a 1966 from Japan

It has a bunch of worked done already, 12v, polini 112cc, phbl carb, Malossi clutch plates…

Can't wait to do some decent miles on it…

Here come the questions!!

I hear about 19/20mm crank taper and 20/20, 24/25mm

But I only see one size of flywheel from different brands. Does that mean it fits no matter the crank?





Last edited by 108 on Tue, 09 May 2023 05:09:33 +0000; edited 7 times

I may have most of an answer for you, but I require more vodka and possibly cookies. and my notes, which I stupidly left at home.

expect a late night update.

but basically, it's a column A or column B situation. bearings and seals have a *ahem* bearing on said situation....

there are work arounds... see: crimaz sleeve

Vodka and cookies? That's a pretty good combo.

More than happy to give bribes out to answer questions.

Is it a question of "where" the flywheel stops on the crank? And the taper is the same? Confused…

Last edited by 108 on Fri, 13 May 2022 15:07:14 +0000; edited 1 time

Two tapers. Large taper and small taper.
19/20 uses the small taper and an M10 nut to secure the flywheel.
20/20 and 24/25 use the large taper which uses an M12 nut to secure flywheel.
Not many small taper flywheels are sold anymore. Most are large taper.
Hope this answers your query.

Saké and senbei may go better with your smallie. Nice find.




rice liquor and rice cracker

A man of taste… I like it!

That diagram is perfect, great info.

Was trying to wrap my head around it

Hope to get a few more pic soon.

The front fork and hubs seem a little confusing too… want to make sure it stops properly

Of course the 25mm crank is the strongest. It came inside both the P125ETS and the PK125XL. In order to use a 25mm crank in the older smallframe cases you have to use a "conversion" flyside bearing and seal.

It's a thinner bearing?

I assume so. I'm pretty sure the seal is thinner as well? I've only seen pictures of the conversion bearing and seal. Since they're smaller, I don't known how robust they are. But if a person it building a higher powered engine into the older cases, installing
a 25mm crank is highly recommended.

Noob crank question

https://vespasmallframeforum.proboards.com/thread/23107/engine-rebuild-which-bearings-crank

https://vespasmallframeforum.proboards.com/thread/11635/24-25mm-crank

its a needle bearing, like in a PX

More of a Roller-type like Primavera or Rally bearings. Ball version is also available but must pull the bearing into the case, in other words, PITA, but cheaper.

Didn't realise the crank is installed on the flywheel side first… or is it many ways to deal with it??

I was thinking more of the ETS crank in ETS cases.

https://www.scooter-center.com/en/needle-roller-bearing-crankshaft-malossi-nbi-253815-25x38x15mm-used-for-crankshaft-flywheel-side-vespa-px-cosa-pk125-ets-rally200-ducati-m6615445b

I went through the process of getting the conversion bearing and seal etc as per Henri's guide, only to realise that I already had the ETS style flyside on the cases i was building.

Holy crap… that's an expensive bearing

Looks bullet proof though

Your link, isn't that just a normal PX flywheel side bearing? That'll work?

Regarding the ETS conversion bearing. In my experience the small taper cranks break at the taper, not the bearing journal. Being that the conversion bearing requires some special attention to fit I'd say the large taper 20/20 crank is just as well for most builds.

quite possibly I'm confusing the issue,

The link i posted is for the bearing needed for ETS or late PK cases. Thats what I ended up needing when i built my engine with an ETS crank because i hadnt realised it was an ETS flyside. Its what i was picturing in my head when i refered to it as a needle roller bearing.

If you have vrange or earlier PK cases then you need the conversion bearing or, according to the notes i made a few years ago 47x25x12 C3 6005 and then a spacer. The spacer is the same size as the circlip used for securing the primary drive bearing.

I meant to say as well, Lovely scooter.

And I have a PK125 XL, and that model has the same crank and cases as the ETS (VMX6T). So i'm lucky that I started out with an excellent building block to work with. That also means I don't have any first hand experience with the smaller taper cranks or the conversion bearing. What I do have first hand experience with is racing my P200 and T5 for many years at racetracks with many guys who had higher powered smallframes. What I realized is, that if those tuned smallframe engines (same for the largeframe engines too) didn't have a strong clutch, crank, transmission, flywheel, ect, those same scooters would be breaking parts and not finishing the races. You can't win if you can't finish. So whenever I do a engine build (large or smallframe), I make sure to overbuild the whole bottom end, when compared to the top end.

20/20 is more than enough. I followed this route on our Special and a NU204 on the fly side is a must, instead the usual bearing…

Yeah I think I'll end up using a 20/20 because of the parts compatibility.

So good to know I can use proper needle bearings for the crank now…

Oh anyone know of any guides to take the cylinder off with the engine in the frame?

Dropping the rear shock and doing it that way seems easiest, but I don't have a lift.

I have jack stands, but that's 1 foot in the air max… I don't think the front will like it too much

According to Those Who Have Been Advising Me, after you pull the tank you can lay it on its side to get at the motor. No lifts required.

I use that technique for fork removal on the PX…

Can't say it's my favourite way of working on it… suppose it'll work

Doesn't it dent the clutch side of the frame though?

Yes it's easiest to lay it on its side. But I don't even take the tank out for that. As long as your tank cap rubber seal holds fuel in.

Put your centerstand on a 4x4 block,
Pick up the tail end and rest it on your jack stand
* be careful not to push it forward off the center stand
The front end will pivot down till the wheel touches ground - block the front wheel
* add tie downs to the headset if you want extra security
Remove rear shock bolt, your engine will drop down more than enough to get at it

Better than dropping it on the side IMHO, takes only a bit more effort

Sadly there's no way to get the cylinder completely off in pivoting configuration as the cylinder will hit the frame when you lift up. Pivoting the motor on either the rear shock bolt or the main center stand bolt can give you access to the intake manifold nuts or exhaust manifold nuts (will loosen at worst possible times). Welcome to the magical, miserable world of smallframes

If you havent already, go on Amazon and buy a motorcycle scissor lift. A wheel chock and the lift will change your life!

Don't mind the extra effort, just don't want to be disconnecting wires/cables every time…

Wow, they really don't want you to maintain small frames… lol

Extra question…

If I was running a 19/20 crank, can I install a 20/20?

Is it just swapping the oil seal?

Can I reuse the bearing? Or does everyone replace it when the cases are open?

Respectfully disagree. Unless your new paint job is still soft, laying it gently on large cushions is easier and IMHO less risky. If you must lay it engine side down, be sure to block it so it is NOT resting on kickstarter or remove kickstarter lever. There are lots of jobs where access to both bottom and top are easier.

In either case, you'll need to pull the carb first to swing the motor far enuff, muff too I recall...

I think that's true for longstroke 90/100/125 cylinders, but a shorter 50 gets by? We just did TWO fifties, but pivoting on a slightly smaller diameter long screwdriver before slipping in the real pivot bolt, following the screwdriver out. We did one on its side, laid on fat pillows and then one upside down, dropping the motor in.


Notice the stout wooden support bolted to the frame and cinched down to the table. We added clamps on the wooden support when dropping the motor in

Easy two man job, even with precious new paint. And dropping the fork in was even pleasant.

Stupid question 5:

Can I pull the crank off without taking off the clutch?

The clutch cover of course (and I think the clutch plates… what size is the clutch puller? 28x1?) but do I need to remove the basket?

Both clutch and flywheel must be removed to pull the crank. If you're going so far as pulling the crank, might as well take the whole engine out of the frame. Will be easier.

Ah ok got you

Trying to avoid resetting the gear selector cables…

But looking into swapping the 43mm crank out for a 51mm

No need to take out the clutch. You only need to remove the small cog on the crank. You just need to align the bigger "window" with the cog to get it out…

Ok yes! this is more accurate statement

Still, just take out the whole motor, reset your cables when you're done. Trying to save yourself work, will end up just being more of a hassle. Setting the gear cables is really not that hard

Thought so! Wanted to double check with you guys

Just one less thing to remove install and inspect…

If it's similar to a largeframe, just takes a while to set and best left alone imo…

agreed beat to leave the cables alone if possible. How is the wear n tear on the reat of the motor? If you're doing a crank/cyl swap, and splitting cases; worth while checking the cruciform and gear stack shimming. and might as well inspect the clutch plates and bearings while you're in there. New seals too.
Few mor things to remove and inspect, better now while you're already in there.

Everyone has there own way to work, I find it easier to work on an engine out of the frame(especially smallies); even though it means unhooking cables and wires. But like VooDoo said, he prefers to lay the bike over - both ways work!

Don't know much about the bike when it was in Japan… seems well looked after. Minimal rust

The PO had the engine resealed, but with a pk gearbox and Malossi clutch and plates and the 112c polini kit. So new seals and bearings

It hasn't been on the road much. The sip gaskets poking out of the rear hub and crankcase are squeaky clean.

But I had to rejet the phbl carb for smaller jets, the spark plug was black and oily. And reset the ignition… was in the region of 26-27 deg at 1000rpm

I'm actually not 100% certain of what crank is in there… it might be a 20/20 but I'm looking at a longer stroke now

Anyone know if a Malossi 136cc kit will fit a my small frame ?

https://www.sip-scootershop.com/en/product/racing-cylinder-malossi-mk-iv-136-cc_31176810

Getting conflicting info on it… should fit…

But apparently the wrist pin might be smaller, but a crank swap would sort it out?


This is the engine…

Fired up, but a few issues to solve