My RD200b 1975 needs a top end (really low compression on right cylinder) I tore the top end off and the cylinders looked okay couldn't see the hone anymore but no scratches or chips so thats good news. I have a couple questions. First of all the pistons have numbers on top not sure if they mean anything ( 591 and 973 on left and 516 and 918 on right) I measured the cylinder and it read 52 mm which is stock. Just wondering if its a good idea to put new pistons in (12000 on the engine) or only do that if youre boring out. And if the cylinder looks okay there's no need to bore it correct? My plan right now is to hone it and put in stock size rings but let me know if there is something that is wrong with my reasoning thanks. And I know this isnt an enduro but atleast a two stroke yamaha.

Your service manual should give you the maximum wear number, a machine shop can measure that for you then you can decide if you want to bore or just hone. 

Thank you!

I looked and it gives specs for out of round and max taper is this all the machine shop should need?
 

If it was low on right side & not just a bad sealing of your gauge--is there an obvious reason why, like badly worn ring on that side or rings stuck in their grooves etc. Just be nice to know the cause before serious surgery.

You need clearance specs too.  Probably in text of manual and not in a drawing but it will be there.

I did check the right cylinder a few times and it kept coming out the same. I also checked my left cylinder which seemed low (about 90). I also checked my buddy's r200 and his were about 120 so I know the gague can show a higher reading. The rings are not stuck in their groove but I did not look for excessive wear yet. Looked in the manual for the clearance spec but couldn't find it anywhere. Guess Ill have to keep looking it over.

the RD200 is a small engine, its hard to compression test with an automotive style tester
Mine shows 90psi with one of those gauges as well with a healthy motor.
My small weedwacker compression gauge shows 120psi.
The main symptom of low compression is hard starting and wont take a low idle.
Piston clearance is .001-.002" on those.
See if you can fit a .004" feeler gauge between the piston skirt and bore. If you have a new piston and its still looser than that, time for a rebore.
Otherwise it;ll sound rattly.
If it doesnt sound rattly and it idle and starts, just ride and enjoy it.
 

Taper and out of round is important.  Piston to bore clearance is as if not more important. 

Yamaha says 0.0016" to 0.0018" is standard clearance for a RD200b with a maximum piston to cylinder clearance of 0.004" 

Manual should have measurement technique...  piston is normally measured on thrust side of skirt about 1/2" from bottom. 

With 12,000 miles and standard pistons it's likely to be worn.  If it's a bike you want to keep, probably best to get it bored by a competent shop. 

New rings and a hone would probably be a waste of money? 

If you just want to get it running, it's strange to have really low compression in one cylinder and rings that aren't broken, stuck, or damaged or some other damage to piston or bore.

Have you checked ring end gap in bore?  Procedure should be in manual.  (What manual do you have?)

Edit, I see Akara posted with similar info while I was looking up specs. 



 

Thanks, the reason for pulling the jug and head is that it was only running on one cyllinder. It has spark and I timed it (checked it many times) and it still wouldnt run on the right atleast very well. Carbs are spotless and good working order as well. All of that paired with a lower compression on the right side I figured a new top end can't hurt. Thanks for all the info. This is a bike I plan on keeping for a long time so I suppose MarkT is right why not just bore it out and know that everything is good to go. 

I have the Yamaha service manual for the RD125B-200B