GS Piston melt down
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Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
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Location: So Cal
Tue Sep 10, 2019 11:48 am quote
Scene: One hour into a long ride, hot day, 95įF + outside, just pulled a long incline WOT, came down backside of said long incline, CHT topped around 310į, stopped for short cool off, took off again on up a slight incline with CHT down to around 240į, half a mile later ... fzzzzzzzzz .... no compression. Pulled top end and found near new GS piston piston fully cooked all the way around. See pic.

It looks like the thing literally expanded like a balloon inside the cylinder.

Iím thinking itís a piston to bore tolerance issue... what say ye?

FD2C1516-F293-4AA5-93E0-72A3EEE605C1.jpeg

Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
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Location: So Cal
Tue Sep 10, 2019 11:50 am quote
Another pic...

32CF0B48-1C87-4DF6-AD32-BA1632B137F1.jpeg

Molto Verboso
Joined: 26 Oct 2015
Posts: 1824

Tue Sep 10, 2019 2:07 pm quote
I read "nearly new" GS piston and "WOT" up a incline. I'd venture to guess you were running lean, and the piston and rings weren't broken in yet. What's your carb and cylinder setup?
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
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Location: So Cal
Tue Sep 10, 2019 2:38 pm quote
Polini 177, 20/20 with bellmouth, autolube, 160/BE4/125, 50/160 pilot. Had at least 800 miles on piston & rings, put them through plenty of heat cycles, this was their first torture test. Didn't expect to have to baby a so-called race piston.
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Old douglas ..smallie with polini 115..super with nasco 177
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Tue Sep 10, 2019 2:47 pm quote
exactly same
Same pattern I had when the GS ipston did this inside a DR 177 barrel

same place after a hill climb

the bit in you story that got me was the short stop......I did the .same not long before it seized.....ie after the hill we stopped (3 to 5mins) for bit to let it cool down..

Im now thinking this brief stop caused the issue more than the set up

like you, it was on tested engine tat had done a good 1200km.

Ps the GS piston went into the DR barrel with 6thou bore clear...no other mods to anything...and it runs so well Im going to repeat it on the next scoot
* 24, sip road.22 tooth clutch, gs piston dr177
Molto Verboso
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Tue Sep 10, 2019 3:49 pm quote
SoCalGuy wrote:
Polini 177, 20/20 with bellmouth, autolube, 160/BE4/125, 50/160 pilot. Had at least 800 miles on piston & rings, put them through plenty of heat cycles, this was their first torture test. Didn't expect to have to baby a so-called race piston.
I would consider 800 miles run in. Even "race" pistons can easily melt down very quickly! I'd still guess that it's jetting, if this was it's first torture test. Which exhaust do you have?


I too am also part of the "I melted down my GS piston" club. I've did it earlier this last Spring on my stock 200 cylinder. Normally I know exactly how to jet a stock 200, but I was trying to figure it out with a variable Vape ignition installed. I suffered a hole in the top of my piston while riding up and down two short hills that were each about 400 meters long.
Hooked
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Tue Sep 10, 2019 3:52 pm quote
Melt down
SoCal: only two things can cause that much damage.
1. Cylinder bore too small
2. Lack of oil
3. Combination of both
Did I say two or three....whatever. From the looks of that piston you could have both. Scott
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
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Location: So Cal
Tue Sep 10, 2019 4:20 pm quote
Blucati: Sounds exactly like my situation. Are you running premix or auto lube?

Iíve since been told the GS piston has an expansion issue, and that .006Ē is the minimum clearance. In an iron barrel maybe even more is needed.

Scott: Itís also possible that Iím reaching the limits of the autolube system. Donít have a tach, but scoot was GPSíing 72mph, which was probably somewhere around 8000 RPM on the 8Ē wheels.

whodats: I was running a BGM Big Box exhaust. So you think not enough main jet? Too big air corrector? I ran the identical set up with a Polini piston for thousands of miles.

It ran like a madman while it ran. Iíd like to bore out the barrel and use another GS piston, but obviously need to figure out how to keep it cool before giving it another go.
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Old douglas ..smallie with polini 115..super with nasco 177
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Tue Sep 10, 2019 5:02 pm quote
Im on 8 inch premix with that dr 177 set up..checked my reords and it was bored to 7thou
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Honda elite
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Tue Sep 10, 2019 6:33 pm quote
SoCalGuy wrote:
Scott: Itís also possible that Iím reaching the limits of the autolube system. Donít have a tach, but scoot was GPSíing 72mph, which was probably somewhere around 8000 RPM on the 8Ē wheels.
Wow
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
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Location: Victoria, Australia
Tue Sep 10, 2019 7:09 pm quote
Well that sucks. I think the difficulty is you never quite know exactly how close to the limit you are... ie you're riding around at just 2% away from a seize but it feels like it's running fine - maybe for years. Then one day you're tipped over by a combination of stuff that puts you into the 100.1% zone.
The similarities between yours and Bluecati's are very interesting, not precisely sure what we draw from it though.

Can't really help other than to ask what your timing was set at?

Not sure about the part we can't see in the photos, but I'd be cleaning that up (and any smearing on the bore of course) with acid and re-using it. Done it before with fairly similar damage and the scooter ran well for thousands more km.
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Tue Sep 10, 2019 10:10 pm quote
SoCal -First off - that sucks.
Glad there was no drama when it died.
(You also shoulda shared the diving save of a rebuild you did to complete your ride after this happened!)

Based on what you guys have posted - I suppose I will be posting pics like this myself at some point.
As you know - I have a super similar set up (main is 128, and I am on a BE3 rather than BE4).

I failed to measure my bore and piston when purchased - hey - you just slip it in the hole and everything works right?
Knew I forgot something.
Damn.

Anyway - two things strike me about the failure mode.
1. Looks to me like the piston expanded in the perpendicular to the piston pin - growing oblong. That makes sense as the piston has a lot of additional structure holding it from expansion on the pin axis. In fact - as it heats - there is really no place else for it to expand - the relative stiffness of the pin axis forces expansion to occur in the perpendicular axis more than it would if it were balanced in structure I would think.

2. I hadn't realized you pulled over. That's an interesting tidbit. As did Blucatti. That might be related. The larger the mass - the longer it takes to change temp. Even if you saw lower temps at the cylinder head - the piston mass may have been in there soaking up the heat and getting hotter - not cooler - with no cooling from the fan or fuel. In this way, the stop may have actually been part of the killer - taking you to that 100.1% we all hate to reach.

Just a theory - but it's interesting how you guys both had melt down after a "cooling off" period of not very long. I know on my bike when I shut down - I see temps rise for about the first 5 minutes. But now thinking about it - that is at the head - which has fins and a chance to dissipate. If the piston was down at BDC when stopped - it would likely be going up in temp during that same window - and longer.

*Edit - left one thing out. That oblong expansion really shoved the rings right in the cupcake hole of your exhaust port. Hard to see from pics - I think they were broken on that side? When I pull mine for re-build - I am going to look at that top chamfer on my exhaust port and make sure its a generous ramp for re-entry. I wonder if you would have gotten away with nothing more than a soft seize if the rings hadn't given up the ghost?
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
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Location: London UK
Wed Sep 11, 2019 4:02 am quote
Sad to see. Can take a while to run in but suspect that was done by 800 miles.

The general colour and detonation melting says it is running lean. Piston crown should be mostly black when running well. At open throttle and lower rpm (like on a hill) the pilot jet or atomiser are most likely the issue. If the timing is way over it could be that too.
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Wed Sep 11, 2019 6:38 am quote
if there was too little clearance it would have shown up already.

What was the EGT at?
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4884
Location: So Cal
Wed Sep 11, 2019 7:09 am quote
Patrick- EGT unknown. Donít have an EGT gauge.

Ginch - Timing was dead on 18į, I checked and rechecked it several times. The piston is beyond repair. Itís a mess. The pics donít do it justice. Yeah, it sucks.

charlieman - After hearing Bluecatiís story, I do think the shut down had something to do with it. I wouldíve been better off leaving it idling and cooling it down by blipping the throttle and dumping nice cool gas on it.

Jack - Could be lean, but like I said I ran this exact same set up ... and ran it hard ... with the Polini piston for many miles. I tried a lot of different jetting optionsÖ smaller air corrector, different atomizers, fatter main jets. This particular combination ran well and the temps never exceeded 320į. One interesting difference is that I did not cut a window in the GS - perhaps that affected the cooling? Iíve been told that some people pull the choke when taking on a long hill full throttle. Itís actually a smart idea although not an optimal solution.

hibbert - hahaha. I donít always GPS that speed, but when I do, itís a hoot...
Ossessionato
2 matching N.Z. '69 VBC Super, 177cc Racer, VespaCross Bodge, Puch SRA150, Piaggio Zip 100! & others
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Thu Sep 12, 2019 3:08 am quote
Been there, done that...
Hmmm.... interesting...

the bit in you story that got me was the short stop......I did the .same not long before it seized.....ie after the hill we stopped (3 to 5mins) for bit to let it cool down..
...
Just a theory - but it's interesting how you guys both had melt down after a "cooling off" period of not very long. I know on my bike when I shut down - I see temps rise for about the first 5 minutes. But now thinking about it - that is at the head - which has fins and a chance to dissipate. If the piston was down at BDC when stopped - it would likely be going up in temp during that same window - and longer.
...
I do think the shut down had something to do with it. I wouldíve been better off leaving it idling and cooling it down by blipping the throttle and dumping nice cool gas on it.



That got me a thunkin... ive had a GS do much the same...
considerations; its a very light piston, with thin rings...
When motor has been running the barrel is HOT (& fins are like heat-sink, with air flowing past to help dissipate heat, blah, blah)...
when the engine stops, the barrel is STILL HOT, but there is no longer airflow to cool things down ...
When the piston stops, it will stop in the same place most times, which is when compression is higher than inertia... that will mean its GOING to stop when the piston is going UP, & the top of the piston (& obviously the rings) WILL BE above the exhaust (otherwise it will compress past TDC & the piston starts going back down again.
To confirm this theory... if you've got a GS Piston, you have higher than normal compression.
Soooo... piston has stopped (see green in pic below).... barrel is hot, & all 360 degrees around the barrel will pretty quickly become an even temp (as the heat dissipates)...
The RINGS are THIN, therefore they WILL heat up QUICKLY (compared to a standard thickness ring ... 1.0mm vs 2.5mm ?), so HEAT will quickly transfer from the barrel, thru the rings & into the thinner/lighter than "standard " piston...
There's nothing else THE PISTON can do besides EXPAND OUTWARDS, plus, as it has become a heatsink & is "sucking" the heat INWARDS from the barrel as the fins are no longer doing much (they have reached their capacity as there is no airflow) & the thickest/hottest metal is right next to the piston rings...
above the piston crown is an open space/void & below it is exhaust/ports & a void...
Therefore the HEAT at the area where the very thin rings are will ALSO be causing the BARREL to expand INWARDS...
This is causing the gap between the inwardly expanding barrel & outwardly expanding piston is heaps smaller...
the very thin rings are by now stupidly hot .... & possibly "sticky in their grooves due to the growth of the ring (in width) & growth of the piston (causing the slot to get thinner)
So things get all sticky like when its started again.

But don't they rev ohh so well!

44CC40CB-A2E6-4EE9-A234-89FD20E345AC.jpeg
The GREEN is likely where the RINGS will stop whenever the engine stops (above the exhaust port, where/when the compression rises, causing the piston/engine to stop ... well, it ain't gunna stop on the down stroke, nor where theres a big exhaust opening.

B44E46F3-ABA1-4DCB-ADB7-14C3FEBAE69F.jpeg

Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4884
Location: So Cal
Thu Sep 12, 2019 9:03 am quote
SEB: Brilliant explanation of what happened and why. Looking more closely at the piston, it looks like those thin steel GS rings (the top one in particular) may have absorbed so much heat from the barrel during the shutdown that they expanded in their grooves, got sticky and caused the piston to balloon out.

The piston didnít lock up, the engine just lost compression.

What still confuses me is that the CHT never exceeded 320į. Was the barrel hotter than the head?

If it wasnít a tolerance issue, how to keep it from happening in the future?

Last edited by SoCalGuy on Thu Sep 12, 2019 2:38 pm; edited 1 time in total
Hooked
2001 LML 150
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Thu Sep 12, 2019 12:57 pm quote
Interesting reading here.

Just one thing which heat and expansion. All dimensions increase as temp rises.

Of course thereís always the exception to the rule ... which is water.
Molto Verboso
1980 P200E project, 2005 Stella Mal 177 MKIII in pieces
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Location: Staten Island, NY
Thu Sep 12, 2019 1:13 pm quote
This sounds a lot like what happened to me recently. I believe mine was from an air leak at the head from not being able to torque down on one stud.

Piston never got stuck, rings never broke but got locked in place by melted piston parts which caused a loss of compression. Also had lots of pitting on the piston top and head.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
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Location: So Cal
Thu Sep 12, 2019 4:09 pm quote
Interesting ... when I pulled the head, two studs felt a tad shaky. Not loose, just not as tight as the other two. There was no evidence the head was leaking.
Molto Verboso
1980 P200E project, 2005 Stella Mal 177 MKIII in pieces
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Thu Sep 12, 2019 4:19 pm quote
Mine was like that. No oil leaking out onto the head. But one stud nut came off with relative ease compared to the rest.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
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Location: So Cal
Sat Sep 14, 2019 5:52 am quote
More meltdown pics...

7437EA97-8497-4F63-8D4A-963C50D99C88.jpeg

Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
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Location: So Cal
Sat Sep 14, 2019 6:43 am quote
Unusual burn pattern inside head... leak?

A7345190-6FDF-49A6-9A59-688C7AC08D86.jpeg

Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4884
Location: So Cal
Sat Sep 14, 2019 7:01 am quote
Underside of piston

343281B0-E9FD-4DB2-94C4-2C1F4824D597.jpeg

Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
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Location: So Cal
Mon Sep 23, 2019 6:07 am quote
Quote:
Not sure about the part we can't see in the photos, but I'd be cleaning that up (and any smearing on the bore of course) with acid and re-using it.
Good call, Ginch. The GS cleaned right up. Itís absolutely re-useable.

D0C3150E-E907-4FEC-9DE4-9B8FAFBDD285.jpeg

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
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Location: Victoria, Australia
Tue Sep 24, 2019 12:52 pm quote
SoCalGuy wrote:
Quote:
Not sure about the part we can't see in the photos, but I'd be cleaning that up (and any smearing on the bore of course) with acid and re-using it.
Good call, Ginch. The GS cleaned right up. Itís absolutely re-useable.
So glad it did! I broke a few rings before finding the acid method to ease them out of the slot. I was also surprised how well it worked to remove smears, yet without appearing to do any harm inside the exhaust port.
Molto Verboso
Joined: 26 Oct 2015
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Tue Sep 24, 2019 1:29 pm quote
Ginch wrote:
SoCalGuy wrote:
Quote:
Not sure about the part we can't see in the photos, but I'd be cleaning that up (and any smearing on the bore of course) with acid and re-using it.
Good call, Ginch. The GS cleaned right up. Itís absolutely re-useable.
So glad it did! I broke a few rings before finding the acid method to ease them out of the slot. I was also surprised how well it worked to remove smears, yet without appearing to do any harm inside the exhaust port.
Just make sure to wear a respirator when playing with muratic acid. The fumes will harm the inside of both your intake and exhaust port!
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7096
Location: Victoria, Australia
Tue Sep 24, 2019 6:51 pm quote
Ha ha! Funny but true! I should have mentioned to only do it outdoors, it's pretty wild stuff.

My motto as always is Safety something something.
Molto Verboso
Joined: 26 Oct 2015
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Tue Sep 24, 2019 8:47 pm quote
Ginch wrote:
My motto as always is Safety something something.
Words to live by!
Veni, Vidi, Posti
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Tue Sep 24, 2019 9:08 pm quote
butt in
Protect what you;d like to preserve, riding or working on these tempramental contraptions. Eye's, ears, nose, hands, legs & feet.

I'm going a bit deaf and now need aids, I've hurt my naked hands & burnt exposed leg & foot while welding, luckily not bad.

Then I nearly lost an eye to a steel sliver under 1mm that bounced offa my not safety glasses & into my cornea. Serious surgery required! Two weeks I'm one eyed & that only sux less than zero eyes. NEVER again, I now grind FULLY protected, it's very little extra hassle & excellent insurance, The ear muffs make the job a lot more pleasant as well.

/jack
Learning a LOT about pistons here, tnx.
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Wed Sep 25, 2019 7:46 am quote
Re: butt in
V oodoo wrote:
Protect what you;d like to preserve, riding or working on these tempramental contraptions. Eye's, ears, nose, hands, legs & feet.

I'm going a bit deaf and now need aids, I've hurt my naked hands & burnt exposed leg & foot while welding, luckily not bad.

Then I nearly lost an eye to a steel sliver under 1mm that bounced offa my not safety glasses & into my cornea. Serious surgery required! Two weeks I'm one eyed & that only sux less than zero eyes. NEVER again, I now grind FULLY protected, it's very little extra hassle & excellent insurance, The ear muffs make the job a lot more pleasant as well.

/jack
Learning a LOT about pistons here, tnx.
yikes
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Location: california
Thu Sep 26, 2019 7:04 am quote
After break in and before tear down, I had a soft seize with my GS.
SoCal's and my set ups are similar.
I have the MMW head and slightly different jetting.

Mine occurred differently.
I was running it hard up a hill and working the bike through corners.
Came over the last rise and corner on the power, and then let off as
I coasted in gear and started the decline.

So I had throttle pretty well shut and revs probably about 5K.
Then the seize... (editors note - I don't know why they call it soft seize - both the engine and my sphincter were squeezed extra hard).

SoCal's meltdown happened at speed while on the throttle - and just after the pause/rest they took - so conditions were a bit different.

Couple things I saw when I tore down for the gear box refurb thought I would share:
- Piston expanded in same direction as SoCal's - front and rear.
- Also - four "corners" where studs are. Interestingly - the one stud I have that is a little wobbly and on the list of fix it items did not grab
- Burn patter on head looks similar on mine to SoCal's. I suspect this is the norm related to exhaust port - but would be interested to hear what other's think.

a7345190_6fdf_49a6_9a59_688c7ac08d86_77690.jpeg
This was SoCal's head after meltdown.

IMG_1866.jpeg
Here's mine. Looks like if left to run - it would be similar? Normal when using GS piston/Polini cyl? Note tho - mine looks a bit more symmetrical then SoCal's. Thoughts?

IMG_1881.jpeg
My GS piston post soft seize. No noticeable change in power/performance/compression afterwards - continued to run strong/problem free. Upped my primary jet and took to blipping throttle when coming off it now. These pistons seam prone to expansion in u

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