how to id big bore kit?

[RACER557]

i just traded for a 04 yz 125. as i was going through the bike i found some numbers hand scribed in the cyl on the carb side. i was told it is a 139cc motor. uses stock piston with a stroked crank. the numbers are 03/124 or 031124. i realize it is probably the year and size but just wanted to know. thanks

 

[dirt bike dave]

Well, you can pull the head and measure the bore and stroke yourself to see which (if either) is stock.

I think 139 big bore pistons were pretty common, so it would seem odd to stroke it and not bore it too. One guy that offered stroker kits was Rick Peterson at RPM (Rick Peterson Motorsports), but not sure if there was a 139 stroker/stock piston combo. I think RPM got a lot of magazine ink and made a fair amount of sales, but his high prices and some of his claims were a bit questionable, if memory serves me correctly.

 

[RACER557]

thanks for the info. havent taken motor apart yet. i know the guy who rebuilt it last time and he told me it has a stock piston with mods to the base??? just wondering what am i getting into here?!