How do I know if I need new rings?

Or, in this case, a ring. I just bought a bike with a:

64cc Polini Reed Valve Bore Kit

15mm Bing Carb

Perf. Air Filter

BiTurbo Exhaust

It was not running when I bought it and the previous owner thought that it needed new rings. I took off the head and the ring is intact and in one piece. So, how do I know if I need a new one? It is kind of loose on the cylinder but if I squeeze the open ends together it makes a pretty good fit with a very small gap between the two ends of the ring. The cylinder has some black stuff on it from where a little of the combustion chamber action has slipped through this hole. Thanks in advance.

Re: How do I know if I need new rings?

When rings wear thin the gap between the ring's ends gets wider and wider.. measuring ring gap is one way. A service manual will specify a minimum and maximum ring gap.

On the average 50cc 2-stroke if the ring gap is over about 0.025 inches (0.6mm) they say its too wide.

But if the problem is low compression or lots of blow-by check first for stuck rings.. also check for glazing on the cylinder wall .. Also look for damaged ring lands in the piston's ring grooves.. in otherwords check anything (aside from ring wear) that can cause rings to seal poorly before buying new ones.

Re: How do I know if I need new rings?

Thanks for the advice. I took a few shots of my stuff. There doesn't seem to be any major scratches. There's still cross hatching on the cylinder walls. The only thing that concerns me is how loose the ring is on the piston. Maybe this is normal. I've never taken one of these apart before. I'm thinking that the problem lies elsewhere. I'll probably put it back together and look elsewhere.

Re: How do I know if I need new rings?

The loose fit on the piston means nothing..

You check ring gap by removing the ring and inserting it into the cylinder .. level it out about half an inch below the top of the cylinder .. stick a feeler gauge in the gap.

Check the ring's wear surface for galling.. under magnification this looks like the ring surface is melted.. metal is smeared

If the ring and lands and piston and cylinder wall seems ok, then move on to other things that may cause low compression .. (I'm assuming here the previous owner suspected this is the reason it wont run?)

Check the head's and cylinder's surface flatness.. how about the head gasket? is there supposed to be one and if so is it intact?

check see the decompression valve (if it has one) seals well .. adjust it's cable. Pour some gasoline into the upside down head.. the valve should not leak.

might be a crack in the head or cylinder or a hole in the piston.. If compression is so low the bike wont run, the cause should be obvious.

But since this is a modified bike the reason it wont run could be anything under the sun .. people who can't tune a bike bolt strange parts onto a sick bike and then get totally lost .. maybe that carb has problems.. maybe anything .. Got any more clues?

Re: How do I know if I need new rings?


Thanks for the responses. From the short time that I've been on this board, your knowledge and helpfulness has amazed me. I think that most things look good in this area although I will check a few more things out tonight after the kids go to bed. I'll put it back together and start from square one. You're right about kitted bikes being touchy. I thought that this might be a fun project to work on and learn from so I picked it up. I think my wife is little peeved that I went from 0 to 2 mopeds in three weeks. I may be addicted!!

BTW, I see you're from Henderson. I taught in Las Vegas for a year in 1999 or so. Lived right at the base of Sunrise Mountain off of Lake Mead Blvd. Best view in town from the parking lot of the school where I taught at!

Re: How do I know if I need new rings?

im not in Henderson .. i'm in Hiding :)

phony info preserves internet anonymity is my motto..

but i do get to vegas frequently and know the area fairly well.. was 115 F yesterday?

yes, you're hooked .. so pay a bit more attention than usual to the old lady .. but she'll detect the real reason and it won't do any good unless you're sly about it.. Once the bike is running let her ride it and you'll both be hooked. Problem solved.

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