Need Help With Bore

[CODY_M11]

OK SO I POSTED A PREVIOUS THREAD ABOUT TRADING MY KX250 FOR A 250R. WELL I DID IT.

IT NEEDS A PISTON BUT MY ONLY PROBLEM IS HE TOLD ME THE CYLINDER WAS ALREADY BORED 50 OVER.

MY ? IS WHAT IS THE STOCK BORE ON THESE? I CALLED A DEALERSHIP AND THEY TOLD ME 2.6 INCHES OR 66 MM.

I CHECKED THE CYLINDER WITH A MICROMETER AND IT SAYS 2.648 INCHES OR 67.2592 MM.

THE DEALERSHIP TOLD ME THE MAX BORE COULD ONLY BE 66.07 MM.
HOW IS THAT POSSIBLE WITH MINE BEING AT 67.2592?

THE ORIGINAL PISTON ALSO MEASURED AT 2.610 INCHES OR 66.294 MM.

ON TOP OF ALL THAT BS I HAVE A ? ABOUT JETTINGS. ILL BET ITS STOCK JETS BUT HAVENT CHECKED YET.

SINCE THE CYLINDER IS ALREADY BORED IM GOING TO PUNCH IT OVER TO SIXTY CAUSE IVE NEVER HEARD OF ANYTHING BEING BORED 50 OVER.

SO WHAT JETS SHOULD I RUN WITH A LRD PIPE AND SIXTY OVER BORE?

ONE REASON I WANT TO FIGURE JETS OUT IS BECAUSE THE ORIGINAL PISTON ( PIC INCLUDED) HAS A HUGE HOLE IN TOP OF IT. I WAS TOLD BY SOME FRIENDS IT WAS PROB RUNNING TO LEAN AND NOT GETTING ENOUGH FUEL CAUSING A HOLE TO MELT IN THE PISTON.

WELL IF EVERY ONE KEPT UP ANY ANSWERS WOULD BE APPRICIATED.

?S WERE
1- WHAT IS STOCK BORE?
2- WHAT JETS TO RUN ON SIXTY OVER BORE AND A LRD PIPE?

THANKS EVERYONE AND SORRY IM NEW TO THE 2 STROKE WORLD BUT LOVE IT ALREADY AND CANT WAIT TO HAVE MY R LOOK LIKE SOME IVE SEEN ON HERE

[CODY_M11]

MY NEW R

[CODY_M11]

SORRY THE PIC OF THE PISTON WAS TOO LARGE TO POST

[86 Quad R]

66mm is stock bore for all year 250r's. what year cylinder do you have? piston sizes are avail(from wiseco) up to a 69.5 for the 85-86 atc250r and 86 trx250r and 68mm for the 87-89 trx250r. what i'd suggest is to carry the cylinder to a local reputable 2-stroke builder/machinist and have him varify the trueness of the existing bore and go from there as to whether or not it needs a bore/freshen up and order the coresponding piston for that particular year cylinder. hope this helps.

here (http://www.wiseco.com/ProductDetail.aspx?ItemID=526M06600&ModelID=356&ModelYear=1986&AppID=1112) is reference for the 85-86 year

here (http://www.wiseco.com/ProductDetail.aspx?ItemID=562M06600&ModelID=356&ModelYear=1988&AppID=1142) is a reference for the 87-89 year

[Honda 250r 001]

Buy a wiseco piston, size 67.50 and have the cylinder rebored for that piston.

[tt racer]

Ive seen alot of cylinders go .080 over but you only need to go .060 over. Get the piston first then let a machinest bore it to that piston. The jetting start a little fat then come down. Please do the proper break in procedures, dont let people tell you break it in like you ride it. I hate that when i hear that. Anyway good luck!

[Honda 250r 001]

Originally posted by tt racer
Ive seen alot of cylinders go .080 over but you only need to go .060 over. Get the piston first then let a machinest bore it to that piston. The jetting start a little fat then come down. Please do the proper break in procedures, dont let people tell you break it in like you ride it. I hate that when i hear that. Anyway good luck!

I completely agree..

breaking in it essential.

[1promodfan]

If you have a '85-'86 cylinder you still have a few bores left. I wouldn't have any problem boring it .60 over. I know of guys that have gone a lot further than that.

[RyanWsly]

I've punched the old ones out as big as wiseco sells .140 and had no problems, if it's a long rod, you can add the spacer plate and run the old piston as well. This one is a little vague, but a friend had a friend who worked at a Honda Dealership and knew what Yamaha piston would work with a long rod engine and give you more bores minus the spacer plate, but I have no idea what it was, someone out there knows because I've seen it done. Like they said though, measure it up, may be able hone it and use it as is.

[mxduner]

Originally posted by 86 Quad R
what i'd suggest is to carry the cylinder to a local reputable 2-stroke builder/machinist and have him varify the trueness of the existing bore and go from there as to whether or not it needs a bore/freshen up
x2, or you can do it as well seems you have the mic already, and you measure it properly. Also verify porting type to help with your jetting and tuning your lrd pipe(if it's adjustable)

[1promodfan]

Originally posted by RyanWsly
I've punched the old ones out as big as wiseco sells .140 and had no problems, if it's a long rod, you can add the spacer plate and run the old piston as well. This one is a little vague, but a friend had a friend who worked at a Honda Dealership and knew what Yamaha piston would work with a long rod engine and give you more bores minus the spacer plate, but I have no idea what it was, someone out there knows because I've seen it done. Like they said though, measure it up, may be able hone it and use it as is.

Yea thats a YZ piston.........I think its a 71 or 72mm. Neil from C-Leigh racing can verify.