Piston question, stock?

[rjs04]

I have searched for this for a couple of days and haven't had any luck. Anyway I am working on a blaster I just got last week. I am rebuilding the bottom end bc the rod bearing on the crank was shot, the piston looks pretty good but I figure while I am in there I might as well do it right and replace the top end too. I haven't had a chance to measure this piston yet but I was wondering if anyone could tell me if it is stock or not.
The top of it has an "x" on the stator side of the top of the piston, an arrow pointing forward and the number 50 toward the back of the top of the piston.
The inside says 37F01 on it.
The letters ART is on the concave near where the wristpin goes through.
It all seems like stock markings but the 50 is throwing me off?
Thanks for any help

 

[Wifesblaster]

I would not rely on the numbers. If you are taking it to the shop to get the top end done then they can measure it. It will most likely need to be bored. Then you can oreder the piston BEFORE you get it bored. Pistons are slightly different in size, this is why the shop needs the piston before they can bore it.

 

[JBRoc01blaster]

That is a very good point, piston sizes can vary depending on the company that makes them. Also it is best for the machinist/shop to know if your going to be fitting the cylinder with a forged or cast piston as forged pistons expand more than cast once they get hot. A forged piston with too little piston to cylinder clearance can, and will cause problems !

 

[Sruiz67]

I am a newbie I the sight but don't make my mistake. I bought a blaster and thinking it should be a stock piston i ordered a 66mm piston. Put it all together and all sorts of piston slap and 80 psi compression. Took it back apart and this time had a friend mic the bore and it was a 66.50mm. I threw 125 dollars away on the wrong size piston. Now have to order another one this time the correct size. Save your money and time mic the bore.

 

[Blaner]

you have 66.50mm piston (0.5mm oversize) ART is stamped on the genuine Yamaha pistons but rumour has it they are made my ProX.

Check your bore carefully to determine how much bigger you need to go to determine your next piston size.

 

[Stevee159]

Can anyone explain how to replace the piston? I'm new to this and trying to do it myself.

 

[Blaaster]

This could be of help.

There are possibly some vids you could search for.

http://www.blasterforum.com/general-support-17/downloadable-yamaha-blaster-manual-17325/

03+ supplement:
http://www.blasterforum.com/do-yourself-20/03-06-blaster-repair-manual-supplement-1916/

www.youtube.com/watch?v=dLwahcpsKEM‎

This will open a hornets nest of info.