How did this happen?
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Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 167
Location: George, South Africa
Sun May 19, 2019 10:14 am quote
This Sprint engine was redone more than 5 years ago, but last week I got it back with this comment: it won't start any more....

I opened it up, and this was what I found. The gudgeon pin was broken in 3 pieces (one piece is still in the connecting rod), the rings and top part of the piston, as well as the cylinder head, are still intact, and the cylinder does not have a mark in it.

Does anyone wants to speculate on what could have happened?

292.JPG

Hooked
1955 Allstate 1958 lambretta 1965 Allstate 1968 sprint
Joined: 20 Jul 2017
Posts: 312
Location: Central california
Sun May 19, 2019 10:47 am quote
Broken piston
Well my experience with two strokes in general....wrong oil ratio, over rev the engine. I donít see any small end bearing pieces. Could have been parts failure, but my guess is that it was abused in some way. Scott
Hooked
59" Allstate
Joined: 03 Aug 2018
Posts: 169
Location: TC Florida
Sun May 19, 2019 11:01 am quote
Wow - Ive seen catastrophic damage like that before but never from a Vespa. Someone must have been pouring the coals to that thing when it let go.

Hooked
2010 GTS 300
Joined: 15 Jan 2019
Posts: 241
Location: Texas
Sun May 19, 2019 12:42 pm quote
Oil/Seized
Looks like vertical scratches on the side of the broken piston skirt, a tell tale of contact with the cylinder wall. What does the cylinder wall look like on that side?

If the piston seized mid stroke, that would explain the broken pin and gouges (from the conrod) in the underside of the piston.

I'd suspect either your borrower used the wrong oil, wrong amount of oil, or perhaps no oil at all in the premix.

Lots of people today don't understand how two strokes lubricate.

At least you'll get a fresh engine rebuild out of it.

Ossessionato
73 Rally, 74 Rally, 76 ET3, 80 P200, 63 Li125, 65 Li225 Silver Special, 86 Elite 80, 2015 HD Road Glide Special, 2011 Ural Tourist
Joined: 18 Jan 2012
Posts: 3184
Location: Oceanside/ SF
Sun May 19, 2019 12:53 pm quote
Re: Oil/Seized
25BIKEZ wrote:
Looks like vertical scratches on the side of the broken piston skirt, a tell tale of contact with the cylinder wall.
I would say this. Itís too late now to measure, but the piston to cylinder wall clearance was too large and it was slapping. That probably cracked the piston and then everything else is collateral damage from aluminum flying.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4682
Location: So Cal
Sun May 19, 2019 1:39 pm quote
Wait a sec, it ran for 5 years, and only comment was ďIt wonít start anymoreĒ?

I think thereís more to the storyÖ
Hooked
bare metal cafe racer
Joined: 01 Sep 2017
Posts: 362
Location: Aotearoa (New Zealand)
Sun May 19, 2019 1:47 pm quote
Yeah... the bit when something went bang. And the bit just before that.
Hooked
bare metal cafe racer
Joined: 01 Sep 2017
Posts: 362
Location: Aotearoa (New Zealand)
Sun May 19, 2019 1:48 pm quote
Guessing it will line up pretty well with the end of the ride prior to ďit wonít start anymoreĒ
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 167
Location: George, South Africa
Mon May 20, 2019 10:09 am quote
Just to clarify the history of the Sprint: the owner used it for 5 years without any problems, and then he lent it to his brother. And I think this guy did not know what he was doing.

After reading the comments, I tend to agree with the verdict that the piston got stuck because of a lubrication problem and that the conrod then broke the piston apart.

A stupid question: can a Vespa engine overrev? I was under the impression that at one stage the import mixture balances with the exhaust gasses, forcing you to change gears. Because in the past I have found that you can only go to a certain speed (revs) in a gear, and then it won't go any faster unless you change gears. Or am I totally wrong?
Molto Verboso
Joined: 26 Oct 2015
Posts: 1468

Mon May 20, 2019 10:20 am quote
danie wrote:
Just to clarify the history of the Sprint: the owner used it for 5 years without any problems, and then he lent it to his brother. And I think this guy did not know what he was doing.

After reading the comments, I tend to agree with the verdict that the piston got stuck because of a lubrication problem and that the conrod then broke the piston apart.

A stupid question: can a Vespa engine overrev? I was under the impression that at one stage the import mixture balances with the exhaust gasses, forcing you to change gears. Because in the past I have found that you can only go to a certain speed (revs) in a gear, and then it won't go any faster unless you change gears. Or am I totally wrong?
You canít over rev a stock engine. What it possible though, the cylinder port timings can be changed enough to make it so that the engine can reach really high RPMs, but only if a particular exhaust system will allow it to do so. Once you get to really high RPMs, such as over 9k or so, a piston can crack.

Iím pretty sure that wasnít the case here. It probably happened on the way to church Sunday morning. Iím guessing that the sinner didnít add enough oil to the mix. Now itís time to repent.
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 167
Location: George, South Africa
Mon May 20, 2019 10:40 am quote
Thanks for confirming my thoughts on over revving a stock engine. I have a Malossi 166, and that does quite high revs, but I had to fit a SIP exhaust to achieve that revs. With a normal exhaust it performed like a stock engine.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4682
Location: So Cal
Mon May 20, 2019 10:53 am quote
Not sure it was lack of oil. Did you check if there was premix in the tank?
If it was a lube issue I think you'd see some scoring on what's left of the piston and some smears of melted aluminum on the cylinder. The rings would probably be toast too. But you said the rings are still intact and there are no marks on the cylinder. My guess is that the pin failed or the piston cracked, both of which are pretty unusual events.

I've never been able to over rev a stock engine. I've tried, trust me.
Veni, Vidi, Posti
a not so normal vbb2 '64, a weirdo vbx '86 and a not so normal pts100 '82
Joined: 15 Jun 2009
Posts: 5409
Location: Indo
Mon May 20, 2019 12:54 pm quote
Seeing this happen here due to bad oil, remember the no oil nephew, this is why he use my smallie, cause hes piston shatter due to bad oil and i forbiden him from using the oil to my smallie, althouht he ended didnt put oil at all thats why Brother Mike aka Waspmike call him the no oil guy
Ossessionato
Joined: 22 Oct 2011
Posts: 3842
Location: Tega Cay, SC
Mon May 20, 2019 6:51 pm quote
My 2 cents: Some one forgot to put the correct amount of oil in the gas. Or forgot to put any oil in the gas.
Ossessionato
Joined: 22 Oct 2011
Posts: 3842
Location: Tega Cay, SC
Mon May 20, 2019 6:52 pm quote
Or a crazy amount of detonation.
Hooked
1974 200 Rally, 1970 180 Rally, 1980 P150X and P200E
Joined: 22 May 2017
Posts: 167
Location: George, South Africa
Tue May 21, 2019 6:02 am quote
Sorry, I did not mean a lubrication problem, I meant a premix problem. The petrol that came from the tank looked terrible; I do not know what oil (if any) was mixed in.
Molto Verboso
Joined: 26 Oct 2015
Posts: 1468

Tue May 21, 2019 6:48 am quote
I wonder if the small end bearing in the crank failed? Maybe it partially seized up in the con-rod, causing the gudgeon pin to spin in the piston, which in turn broke the bottom of the piston out by way of a fulcrum type of force?

We can't see the whole piston in the picture, so it's difficult to say what sort of seizure marks are on the side of it.
Addicted
85 Vepsa PX150E with sidecar, 80 P200 with sidecar, 96 Goldwing with Hannigan Sidecar, LR87aj5, LR91aj3
Joined: 11 Oct 2011
Posts: 548
Location: Central Washington State
Tue May 21, 2019 7:50 am quote
OR maybe " after 5 years and 200,000 miles"
Addicted
'64 Motovespa 150S (177) , '64 V90
Joined: 27 Mar 2013
Posts: 547
Location: S.Salem, NY
Tue May 21, 2019 8:09 am quote
[quote="whodatschrome"]I wonder if the small end bearing in the crank failed? Maybe it partially seized up in the con-rod, causing the gudgeon pin to spin in the piston, which in turn broke the bottom of the piston out by way of a fulcrum type of force?



My take as well. It seized and the friction and heat snapped the piston bushings. The whole thing may have been way too hot. I think the inertia hammered the pin into the piston and snapped the ends off the pin . Must have been some noise....
Hooked
2006 px125
Joined: 28 Sep 2016
Posts: 106
Location: Not Sunny Southport
Tue May 21, 2019 2:26 pm quote
Where's the small end bearing ? Is the crank goosed ? Could big end/crank of seized after ignition ?
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