[Chitty]

Fairly new to the r's in general. Last week i went down to Little Sahara from the Omaha area and didnt bother checking my jetting when i got there which caused me to run a little to lean and burn up some rings. Still on stock jug and bore. Just looking for the piston size i need to order for my first bore. I am thinking i want to move up to a 265 but dont know the correct piston size in mm. Went to a local machine shop and they say they can bore and fit my cylinder with the piston i bring them for $75. Doesnt seem outrageous. Please help.

 

[motofool250r]

75$ is a decent price for a bore, the question is are they going to chamfer the ports also are they a shop familar with two strokes?

stock bore is 66mm 265 is a good setup but if your looking for some more power you would be better off porting it.

Neil (C-Leigh)on here does a killer port job and can bore your cylinder to match the piston also.

 

[Chitty]

What does chambering the porting consist of? I'll ask but i dont think that that was included. Do you know the turn around time for C-Leigh's shop? Also for a 265 is it .020 over? All i see for ordering pistons is stock plus .010 .020 and .030. I dont want to go to big because i am content with the power i have i just didnt know the best route to get back on the trails again. New to all of this.

 

[motofool250r]

you can go pretty big on a stock cylinder as far as bore goes but the power increase will be minimal. if you do some porting a cylinder head mod like the cr250 head gasket and a new dome shape you'll see a much bigger improvement.

what is your time frame to get back to riding?

i dont know what Neil's time frame is but ill clue him into the thread for ya.

 

[C-Leigh Racing]

Going from a stock STD bore up to a 265cc bore, you would need a 68.00mm piston, but if you go that big, that is your last bore on that cylinder if it is a 87/88 or 89 year cylinder & crank.
Now you can get a larger piston than the 68.00mm. The old model 85/86 year ATC/TRX engine, went up to a 69.50mm, but to use that year piston with the 87/88/89 year long rod crankshaft, you would need a 3.0mm cylinder spacer plate to go under the cylinder.

The other stuff you were asking questions about, I'll pm you with the details.
Neil

 

[89 250r]

c-leigh racing i'm using the 89 long rod engine and 89 pistion with the 85 atc open intake cylinder and i'm at 67.50mm. so your saying i can go to 69.50mm on the 85 atc cylinder, what cc is the motor at 69.50mm bore? also if i mind right i read somewhere that someone was running a bore of 70.00mm with a yz250 piston is this correct?

kevin

 

[Chitty]

Ok I have the short crank but the 88 cylinder. I just want to go to the next bore size to save the life of the sleeve. What size piston would I need for the smallest bore from stock? I know I need the 86 piston just don't know what size. Thanks

 

[Chitty]

bump up

 

[acecarlos]

Chitty: 66.25MM is the next bore over.

89 250R: 69.50MM is the last 'standard' piston size for a 85/86 250R piston kit. at 69.50MM x 72MM stroke, you have a 273CC engine, a 68.50MM bore x 72MM would be a 265CC engine...I would go to 68.50MM first, save the extra bores for later.

YZ250 pistons will get you to 69.75MM & 70MM.

Carlos

 

[Chitty]

Carlos, Thanks bro just orderd a 68.5 and i am hoping to be back on the trails again by saturday. Once again i appreciate it and thank you.