Difference between A and B cylinder?

Can someone tell me the difference between an A and B cylinder, specially for the 3rd gen cr250's? Its something I've never known.

02 & 03 A-B Bore sizes below: 

A = 66.398 - 66.405 

B = 66.390 - 66.398 

I don't know the exact measurements, but A would be the smallest with very small increases up to C cylinder. Also I'm not sure where you find what size your cylinder is, I just had my bike apart a month or 2 ago and searched from top to bottom trying to find what size it was.

These are the specs from my service manual

 

A piston - 66.330 - 66.338 , 2.6114 - 2.6117"
B piston - 66.323 - 66.330 , 2.6111 - 2.6114"

 

A cylinder - 66.398 - 66.405 mm , 2.6141 - 2.6144"

B cylinder - 66.390 - 66.398 mm , 2.6138 - 2.6141"

 

There is a mark on the cylinder with either an A or B.

Edited by Kawikx450

just follow whats the letters. A cylinder get A piston.

 

those numbers above are just for OEM manual. if you bought a Wössner or Wiseco or Prox, you would just 100% use thesame lettering aslong it is within spec.

 

New cylinders, you have to use the SPECIFIC piston or youll be too big and scourge the cylinder.

used cylinders can use either B-D pistons.

 

I have never had to measure the bore on any of my cylinders. i just make sure there are no damages and obviius wears on the nikasil. if i see crosshatch, im good. i have never had any compression problems whether the cylinder is out of round or not. bike feels sounds healthy after top end.

 

also, nikasil is a very hard plating, they use it in M16 barrels.

 

forget about honing your cylinders, just clean it with sponge and some dish soap and wipe dry.

I don't know the exact measurements, but A would be the smallest with very small increases up to C cylinder. Also I'm not sure where you find what size your cylinder is, I just had my bike apart a month or 2 ago and searched from top to bottom trying to find what size it was.

 

Honda does it backwards. A is the largest and B is smaller.

I guess I have been lucky, I have always just told the parts guy to send me a complete top end kit for my CR250's.   Since '86, never had a problem and bikes always ran perfectly or seemed to.  Current 01 CR shows to have an A cylinder, one oem whatever replacement and a current Wiseco Pro Light. I wonder if they automatically send the A unless otherwise specified.

Well that is strange, Honda makes the B cylinder/ piston smaller?  Wouldve never known, thanks.

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