G1 smashed piston

[Hally]

Hi guys,

First post here I think. A little while back I purchased a 1980 G1 and gave it a custom paint job, new seats and had the engine completely rebuilt. Ran well for a little while, but then strange noises and loss of power, especially up hills.
Compression test showed 55psi, so obvious problems. I have removed the engine and looking through the exhaust outlet, the piston was shattered and the jug badly scored (more than 1ml). The whole bottom of the piston was smashed. When taking the top end apart, there was only one circlip holding the piston rod. Would that have caused the piston to shatter ? Also, the engine is covered in oil and muck around the oil pump (disconnected and I pre-mix) and underneath the crank case, so I asumed the seal on that side was shot. However, now I have removed the clutch and oil pump, but the seal looks Ok, but there is a small amount of oil seeping from around the crank shaft when I crank it by hand. Could the seal still be bad even though it looks fine ? It actually looks like the oil was coming from where the two sides of the case join, so I have removed the flywheel side and the seal is fine, but one of the crank case tensioning bolts was a little loose and tightened up nearly a full turn. Could oil have been leaking from there ? and would this tightening fix it ? There are no cracks in the crank case that I can see.

Would appreciate your help before I put this back together. I have found another cylinder in good condition and have ordered a new piston and rings, circlips, rod and seals. Anything else I should check ?

Regards
Ron

[TheNewGuy]

Welcome to BGW Ron!

Not 100% sure about the piston, but I would think the wrist pin missing a retainer will cause problems. Yes, a seal could look OK and be worn out. Not sure witch bolt you tightened, but sure could be a problem. That far torn down, I would replace both seals as well.

[tonyton]

I can answer a couple of your questions. If your motor was running when the clip came out, it would take out the cylinder and piston before leaving big gouges. It would have exited the exhaust port or is still in the crankcase! The broke piston can be caused by a lower rod bearing. It would allow the skirts to hit the crank on the down stroke. The other cause is a ring catching on the edge of the ports. (not common) Foriegn debris (a clip could cause it to bust also.)
The crank seal can look good and be bad. If it leaks, the results are devestating to a motor. Lean out and lock up!
In this case I would inspect the lower end for damage, crank bearings, lower rod bearing/ play. I would replace BOTH crank seals. I would test the block for leaks by pouring deisel fuel in it. wait overnight. Any leakage would require it to be opened up and re'sealed.
Reassemble as required, be sure there releif cuts around the ports.(no sharp edges) They usually come OK but needs to be done if recently bored, and not done by the machinist. (4 cycles don't have ports, of course)
If all checks out there, you should be OK. Hope that helps....Ron beat me to it. I type/talk too much, slowly... Welcome to BGW!

[TheNewGuy]

I didn't think of the clip coming out in the engine That would cause the damage. Thanks Tony!!

[tonyton]

I've seen it happen. The hard way....

[tonyton]

One other thing. well, a couple. When your piston came apart, chunks could have wedged between the crank, and the bottom of the case. You can guess the rest on that... Test it for a leak, then turn it upside down and get all the debris out. You might find a clip...
Sorry about that.....

[toadady]

not surprising on a 29 year two stroke to have the piston skirts break off from just general metal fatigue

[tonyton]

Originally Posted by Hally

Hi guys,

First post here I think. A little while back I purchased a 1980 G1 and gave it a custom paint job, new seats and had the engine completely rebuilt. Ran well for a little while, but then strange noises and loss of power, especially up hills.
Compression test showed 55psi, so obvious problems. I have removed the engine and looking through the exhaust outlet, the piston was shattered and the jug badly scored (more than 1ml). The whole bottom of the piston was smashed. When taking the top end apart, there was only one circlip holding the piston rod. Would that have caused the piston to shatter ? Also, the engine is covered in oil and muck around the oil pump (disconnected and I pre-mix) and underneath the crank case, so I asumed the seal on that side was shot. However, now I have removed the clutch and oil pump, but the seal looks Ok, but there is a small amount of oil seeping from around the crank shaft when I crank it by hand. Could the seal still be bad even though it looks fine ? It actually looks like the oil was coming from where the two sides of the case join, so I have removed the flywheel side and the seal is fine, but one of the crank case tensioning bolts was a little loose and tightened up nearly a full turn. Could oil have been leaking from there ? and would this tightening fix it ? There are no cracks in the crank case that I can see.

Would appreciate your help before I put this back together. I have found another cylinder in good condition and have ordered a new piston and rings, circlips, rod and seals. Anything else I should check ?

Regards
Ron

Read this! This was a fresh, completely rebuilt motor.????????? ....and I'm a nice guy

[Hally]

Thank you guys. What a great site.

I have removed both seals and the main, clutch side seal was damaged inside (the rubber seal on the inside rim was broken, causing a small leak, and there is a crack in the crankcase, causing the oil leak. This can be repaired.

I think you are right about the clip coming loose and causing my piston breakage. I did find soime bits of metal in the crankcase, including a thin 2 " long sliver straight off the cylinder.

Looks like I'll be waiting a couple of weeks for all the parts. I'll re-assemble and let you know how it goes.

Thank you for your responses.
Cheers
Ron

[Flintrock]

Hally, I think that a pretty good argument could be made from what I read here that a piston circlip let go and caused your problems....... If you had it rebuilt by someone else I think I would be talking to them about the problem before spending more money on it...