Hello 

I am looking to restore a 72 Yamaha 175 CT2.  I have the engine apart and inspected the piston and rings.   I believe the rings need to be replaced because the end gap measurement is greater than .33mm.  I don't know which rings are the correct ones to buy.   I believe the current rings are keystone type although I am not sure.  You can see from the photos that there is the letter "N" on the both the top and bottom rings.  The piston has "68" on it beside the forward arrow on the top of it.  In addition, there is '9 67' on the opposite side of the piston.   When I use a caliper to measure the piston it looks to be 66mm diameter.  I don't have the tools to measure exact.  Also, the cylinder walls seem to be in good shape.

Any help out there to help me find the proper rings to replace old ones?  Any other advice would also be appreciated...    

Thanks...

Your piston part number is 314-11631-00-96 (standard, stock, original bore size)
The rings for that piston is 314-11610-00-00

Thanks very much for that!

How much bigger is the gap?  In my experience, original rings in a new bore are usually very near the max gap spec.  

So don't be surprised if new rings in your used bore still exceed max gap spec by a little.

The gap was more than .45mm.   I picked up a new NOS set for $30 today otherwise I might have tried to work with the old ones.  The top one was a bit rusted in place and I just thought a new set might be the right way to go.   The new set has a green coloured one and I am assuming that one goes on the bottom?  The expansion springs if I am correct has a single and double bend with the double bend for the bottom ring.  I have a buddy who is going to deglaze the cyclinder for me and double check the shape with a micrometer....hope that will all work. 

The cyclinder wall on the old bore has a bit of rust and I hope a honing will clean it up.   However, just thinking out loud here....if it doesnt clean up well with a honing is it possible to rebore a bit and I'll still be able to work with the old standard piston and the new rings?

Any other thoughts to help with this is much appreciated.  

 

It might have been wiser to wait and do your measurements before purchasing the rings .
If your measurements reveal you need a rebore , then you'll also
need a slightly bigger piston ( first oversize hopefully ) and matching rings .

Ouch rust often equals pitting which would mean a first over bore, new piston and rings for that but not terribly expensive. It's amazing how damn near everything on these bikes is cheap compared to even a relatively simple part on a modern car.

I guess I might not be fine with this yet. Darn!  Any advice on how I can determine if the pitting warrants a new bore?   Will I need to have a machine shop inspect or can I diagnose myself?  

Probably best to leave it to a machine shop to check out .
We are dealing with very small incremental differences between
good and bad  .

Having measured all the critical values and compared against the service manual tolerances I determined my DT250B barrel needed a rebore. I rang my local machine shop to discuss the job and they said no problems BUT along with the barrel you must also bring the new piston, new rings AND all available tolerance data from the service manual too.  You see therefore they are taking a 'known' reference point (the new piston) and actually maching your barrel to 'fit' your piston and rings set such that all tolerances are satisfied too.
As for those standard Yam NOS rings you bought don't be too tough on yourself for buying 'em, you could always sell them on (or even keep 'em for the next project.. ).  On the positive side look at the experience/knowledge you've gained for just a few bucks, so you see your still "Lucky Luke"

Just sayin...'the glass is always half full'