Thread: Breaking piston rings - Mechanic Mags Help!

OK - Long story short, I'm trying to refurbish a 1974 Harley-Davidson golf cart. We dove into a top-end rebuild last night, and we were almost to the finish line, I could taste the f*&$*^$ beers, and as we were slipping the cylinder down over the piston and new rings, we had misaligned the cylinder bolt holes and we stupidly tried to rotate the cylinder and the top ring caught and broke.

Is there a trick to this? The new rings seem HUGE, but I think they are just new...they do compress down pretty well around the little stopper pegs in the ring grooves. I guess we just need to go more slowly and carefully.

New rings are in the mail with a new gasket (ruined that too and got Permatex #2 all over the fing rings and the piston, that was fun) so I don't wanna f it up again when I go to finish the job. At least it didn't fall down into the crankcase.

it doesn't sound like you used a piston ring compressor ?

A ring compressor would hold in the rings until you slipped the cylinder down over the piston

this is based on dirtbike top end work but should apply...

First - New Cylinder or old? If old - was it bored? Mic your cylinder if you have not already done so... if the cylinder is worn the rings will spread too diw and come out of the ring groove too far and this can cause breakage.

If a rebore -roDid you hone the cylinder? If so, are the cross hatchs too aggresive? I've never seen these catch a ring end but it theoretically possilbe.

Rings - did you happen to get a set of .020 overs and use them on a non overbored cylinder? I'm guessing no but... : )

Making sure the piston is not at top dead center when reassmbling is helpful - that was you do not need to guide the piston all the way in...

Beyond that, a "third hand" to keep the ring compressed as you slide the clinder over the piston comes in handy as does liberally lubing both surfaces w/ appropriate motor oil - go deep enough into the cylinder to insure lube is beyond the depth the piston will seat to.

Also - we used to cut a piece of cardboard or use masking tape to make a removeable cover for the crank opening to prevent anything from getting in there by mistake - if you do it with card board (or even sturdy paper) simply cut a hole for the crankshaft and a straight line the edge of the "cover" - flex on flex off.

All in all I think you just had some bad luck.

Nope, didn't use a compressor. I'm new to this, so I didn't think to look for one. but I now know why they are important and I'm heading to Napa to get one:


The cylinder and piston are both new, and matched, and standard bore. Specs say 2.75", but the piston measures 2.73 with my micrometer. The piston and new cylinder are definitely matched though.

Yeah it made sense to me that we would not want it at top dead center, to have the bottom half of it residing in the crankcase would definitely aid in seating it...it was impossible to get two hands around the rings with the piston at bottom center, so we were going with top dead center - but now that I know of the tool referenced above, should be easier to give it a try with the piston at bottom center....

Great idea with the cardboard. I used a rag and copious amounts of masking tape. We also had a shop vac for suction when I cleaned the old gasket and dirt from around the cylinder base side of the crankcase.

One things for sure - I'm sure learning a lot, and this project sure has been fun.

Originally Posted by Lane Meyer

Nope, didn't use a compressor. I'm new to this, so I didn't think to look for one. but I now know why they are important and I'm heading to Napa to get one:

That will help a lot.

If you didn't yet, before you put the new rings on the piston, carefully slip them (one at a time) into the cylinder to check the gap. It should be aligned in the plane perpendicular to the bore (like it will be when it is on the piston). A good way to align it is by pushing the piston in after it (if it is a flat top piston). If the piston is not available, you can probably find some other way. I can't tell you what the gap should be, hopefully you have that info. A good machine shop can tell you a general figure for gaps based on the bore (it will likely be in the range of .010" to .015"). If it is too small, carefully file it until it is right. A good way to do that is clamp the file in a vise and run the end of the ring over it. If it is much too big, there is a problem like wrong rings for that bore.

Good luck.

Edit: re-read your post about the pins to locate the rings in the piston. That may render some of my comments useless, or wrong. Checking the gap is always a good thing, though. In this case, the correct gap may be different from what I said.

This comes from many a rebuild...
1. Make sure your pistons are installed in the correct direction. There should be a marking on the top of the piston indicating what way they are to be installed. Ring gap should not pass over any exhaust or intake ports.
2. Check your ring end gap.
3. Make sure you rings are aligned correctly on the ring pin.
4. Large hose clamps can work instead of a ring compressor tool if you can con a buddy into helping you with mass quanities of beer.
5. Dont drink heavily while installing...wait till you f it all up for some sober reason and then drink heavily.