piston ring trouble...no compression

ok so about 2 months ago my piston cracked and i just got a replacement...so i installed it with the rings lined up and everything and it all ran fine for about a day. but when i was slowing down to stop it, i heard a crack and im pretty sure a piston ring cracked. what could have caused this?? i tried starting it up again and i did but it only ran at like a really weak 10mph with no compression at all. also, i have the carb set at a fairly lean setting but nothing too drastic. i need some advice please.

Re: piston ring trouble...no compression

you are pretty sure a ring broke? Is something preventing you from removing the cylinder and looking?

New pistons and rings have to be the right size..

A new ring needs to be "gapped". The ring is slipped into the cylinder near the top and the gap is measured. If the gap is too small the ring can break or do other things when it and the piston expands from heat.

Too small ring gaps are filed until they are the correct width according to manufacturer's specs.

Give more details.. you have evidently had a lot of piston related trouble and i'm wondering if there's some common cause.

Re: piston ring trouble...no compression

One other thing is how worn the cylinder was. The ring does not go to the top of the cylinder. Over time there is what is called a ridge that forms where the ring stops in the cylinder. Generally you have to use telescopic gauges to measure the cylinder to see how much of a rige the cylinder has. This ridge can break a new ring f it is more than a thousand.

Re: piston ring trouble...no compression

here's what i use.. quality and accuracy are quite acceptable enough for a tool that's only used once in a while..


I'm fortunate to have one of the retail stores nearby.. sometimes the cheap prices cannot be believed.

Re: piston ring trouble...no compression

Yes, those are the tools. Measure right at the top of the cylinder and then measure 1/4 inch down. They sell a ridge reamer to remove the ridge. They are expensive. For this small cylinder, get a wood dowel, 1.250" measure the cylinder length and cut the dowel 3/16 - 1/4 inch shorter. Place that dowel in the cylinder. Take another piece of wood dowel and tape a fine grit waterproof sand paper to it, ( I use 400 - 600 grit). Set it in the hole resting on the wood dowel, with the crown you should just be at 1/8 of an inch in the cylinder. rotate the cylinder a few times to clean the ridge off.

If you really want to be elaborate you can drill a hole in the bottom of the wood dowel and add a threaded insert and bolt so you can adjust the height. If you have some old fork head bearings you can cut a groove in the top of the two dowels and oil it and put the bearings in there for the dowels to roll on. Add tape to the side of the dowel to make it fit snugger in the cylinder. My homemade tool!

I use this to clean up cylinders where people have beaten the piston out of the cylinder and distorted the top of the cylinder. wish they would use liquid wrench and let it sit overnight.

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