Need help on figuring out whether my piston is stock or bored

[Cosman92]

I blew my Yamaha YZ 250 up the other day. I need to order a new piston, just the piston and rings. I have no history of the motor and i do not know whether the motor has been bored or not. I want to get a stock piston to put in and i want to get rid of the bike to get a 4 stroke. IS there any way to find out whether my piston is stock or not? The letters ART are on the piston and the connecting rod has the numbers 5x5 on it. The jug says 249cc. Im not trying to sound dumb on this but i just need help with this as i want to order the piston tonight.

[Freak]

find out what the STD bore size is, take your jug to the local machine shop and have them mic it. Then you will know what you have. I'm sure you can run a search on this site or the internet in general and get the std size. good luck.

[Silver_Fox]

What do you mean by "blew up". You may need new con rod as well. If it was'nt that serious do as Freak suggested and measure the bore.

[MXtras]

What year is the motor? If it's relatively modern, it's likely a standard bore as the cylinders have been plated since the late 80's I believe. Not sure exactly when, though.

Scott

[Cosman92]

the bike is a 1992, im going to run a search on the bore and i will mic it myself. As i said the engine blew up yes, but just the skirts of the piston were totally destroyed. The connecting rod is fine

[dogger315]

Don't know if you still need this info, but the bore on the 1992
YZ250 is 68mm just like the stroke. The cylinder is plated like
MXtras stated but you might be able to clean it up with a ball
hone and a drill if it is simply scored with no deep scratches.

dogger

[MXtras]

Dogger - when did Yamaha start plating their cylinders? 1987? I can't recall.

I know Kawais were plated in the early 80s - like 1981 or so.

Scott


(Hey! My 1000th post! WooHoo!)

[Cosman92]

thanks for the help guys. I found the stock bore last night and took a micrometer out and got the size. its like 68.002 or something like that so it has like no wear. I ordered a Wiseco piston set today. The only thing i found weird was there was only 2 seals on the head not a head gasket, whether the previous owner didnt put it on or something like that but i ordered a gasket kit and a head gasket comes with it

[dogger315]

[quotewhen did Yamaha start plating their cylinders? 1987?]

Scott, I'm not positive but I believe it was 87 or 88. I know Honda
started in 78, Kawasaki in 81 not sure about Suzuki and Yamaha in
the late 80s.

Congrats on the 1000th post. That should earn you a special distinction.
Maybe something like "Manufacturer dude".

Don