Hi guys, so the saga continues...  As some of you know I bought this maybe 3 months ago, figured out that while it ran ok and shifted through all gears the crank seals needed replacing.  While changing the stator side seal I found the inner part of the crank bearing would turn on the crank.  It was suggested to used red locktite on it so I bought some high heat red and let it cure for a few days.  Started the bike up and while it ran better and was able to get it to idle below 2K RPM, it was blowing white/grey smoke so I figured the right side seal was bad and burning transmission oil.  So, when I pulled the clutch cover off I see needle bearings laying in the bottom - well crap!!!  So now I've pulled the motor out and have ordered all new bearings and seals to replace them all.  When I pulled the cylinder off I noticed it looked like it had some minor scoring and spots where it looked like it had some rust at some point, so time to bore it while I'm at it.  The piston has these numbers on top 514 and on the bottom (of the top of piston) 964, can anyone tell me if this a standard bore piston?  As always, thank you for the help! 

Evening Tony.

Oversize pistons, original or pattern, often have the oversize shown on top - eg 0.25, 0.5 etc. You could measure the diameter of the piston and see how this compares to the size of the standard bore.

Have a close look and see if there are any letters on the inside of the piston - you'll likely need a torch and to view it from different angles.

Hey Alan, good evening to you too! Thanks for the reply, I did measure the piston and cylinder but am not sure how much I trust the cheap harborfreight caliper and equally as cheap Amazon telescoping bore gauge (or that I'm using it correctly). I'll have a look at the underside of the piston to see what I can see. Thanks again.

There are images of a standard CT2 piston on CMSNL, and this piston also has 514 on the top, so yours is probably genuine. 

PISTON (STD) 0.96 for Yamaha - order at CMSNL

A company called ART made genuine Yamaha pistons at one point, so these letters might be cast into the inside.

Thanks Alan, yep those are the numbers stamped and in the same place. The measurement I got with my cheap calipers seem to show it as stock size. Guess I'll order a first over piston.

Maybe get your chosen rebore engineer to check over the scoring first - you might need to go to second (third?!) oversize, if the scoring is bad.

The scoring really isn't bad at all but it wouldn't be a bad idea to have the machine shop take a look at it anyway.