Project bike: 1986 Honda CR125R

I have this book and will scan anything you need. Most, if not all the pictures shows the 86 bike. Good luck with the rest of the build!

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The crank you have looks like an 83 crank

Have you thought about having an RM rod pressed into the 86 crank ? You may be be able to use a YZ piston 56mm by doing this

Nice work and nice find on some of those HRC parts

That ring is going to hit the port roof.
Is the cylinder all way down? Dont look so in picture, looks like a gap by the gasket left cylinder stud..
Take of the rings and tighten the base nuts little more then fingertight and measure squish hight.

Maby HRC cylinder is designed for single pistonring or taller piston? (squish will tell)
Or are you using longer stroke crank?

That sucks. Is that ring end oriented in the stock location? Not sure if you could have the piston modified by a engine machinist or the piston manufacture to relocate the ring end pin for you. It would have to be done correctly so that the pin didn't "walk" out and destroy your cylinder wall. Even still, that seems like a pretty large area that the ring could try to expend out into a little on every single stroke. Not sure how it would work out long term. I guess you could try one ring, just making sure the ring end gap is/stays on the tighter end of the recommended range. Probably depends too, how much you intend to ride it.

That's a pretty interesting development. Although not available from Honda anymore, it might we worth looking into the aftermarket for the RS piston if it is really the same. If you're really lucky you might find an N.O.S. RS 125 piston, but doubtful. I would check with all of the bigger (and longer established) piston companies, Wiseco for example. You might get lucky, or they may be able to make you something. That HRC cylinder is a great find, it would be nice to take full advantage of it by getting the right piston for it.

have you thought about a yz 125 piston 56mm with a Rm rod which will give you the right size wrist pin and work with the bottom end ?

Perhaps wossner could make you a batch of single ring cr 125 pistons you'd have to have at least 10 of them made up.

I looked up 86 Honda RS125 pistons, and didn't see too much, but saw something interesting. One of the kits mentioned that where the "enlarged power port" is, that it must be chamfered so the ring doesn't snag on the upstroke... Something to that effect. I'll see if I can post a screenshot in a minute here if I can find it again. Doesn't really seem like something Honda would do, but then again these weren't production parts made to last really long amounts of time... Maybe it can last a while like that, but guessing that they intended to get a little more power at the cost of ring life, being that only pros really had access to this stuff. Of course, they have the luxury of intensive maintenance and short service intervals.

I'd be tempted to try riding it with just the top ring, given that all of the tolerances are good (ring end gap and cylinder wall condition). I think the worst case scenario, as long as the top ring end falls on continuous cylinder wall, is slightly less power if you have a little leakage. I doubt that though, if you keep it fresh and tight.

Make 1980's CRs Great Again!

A guy I ride with gave me a 85 CR125 and my son and are going to restore for his vintage bike/project. It's all there but the crank is seized up. As soon as I finish my 80 RM125 we'll crack it open get started but this thread is a great inspiration in the meantime!