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01 cr250 A/B cylinder question

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OK, I knew that there was an A cylinder and a B cylinder, but doesn anyone know the difference between them?

This Service Honda link lists the cylinders and pistons in a matrix, but it doesn't say what the letters mean.

Any help is appreciated.

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If I remember correctly, there is a slight difference in diameters between the two and you need to match the piston to the jug to maintain the proper clearance.

:thumbsup:

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I think the difference is .0005 thousands or 1/2 of a thousands of an inch. Prodution tolerances. You can mix A & B cyl & pistons but it's best to match them. I think the B piston is larger & some people like to run them in an A cyl for tighter fit like if your A cyl is slightly worn.

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I think the difference is .0005 thousands or 1/2 of a thousands of an inch. Prodution tolerances. You can mix A & B cyl & pistons but it's best to match them. I think the B piston is larger & some people like to run them in an A cyl for tighter fit like if your A cyl is slightly worn.

You are correct on all counts. :thumbsup: Tdub

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Sry to bring this up again but I am ordering new stuff tonite (head, cyl, piston, gaskets,ect..), should I get an A cyl or B cyl new in general, or does it matter, they cost the same ($222)...And would I be better off just getting the wiesco prolite kit with gaskets rings and all of that for 90 instead of getting the stock sets? thanks, keith

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it wont matter what cyl. you buy as ong as you get the appropriate piston to go with it. a wiseco will fit either the A or B cyl...if im not mistaken, its the same head for either cyl. :thumbsup:

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it wont matter what cyl. you buy as ong as you get the appropriate piston to go with it. a wiseco will fit either the A or B cyl...if im not mistaken, its the same head for either cyl. :thumbsup:

Yep what he said. Buy a Wiseco top end, check/adjust the ring gap and you should be good to go.

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I'd add that Honda doesn't sell "A" pistons for the '03 CR250 (and probably other years). When you order an A, they give you a B piston instead. So you might want to find out which pistons are avialable if you want a "matched set".

Functionally, A and B cylinders are identical except for bore diameter. Honda will crank out a huge number of cylinders and pistons on the production line and because of things like tool wear, etc., there will be some size variation throughout the batches. To effectively reduce the production tolerances and achieve a more uniform piston-to-cylinder clearance in the assembled engines, Honda sorts through the batches of cylinders and pistons and marks them as either A or B depending on diameter. The bigger cylinders and pistons are marked A, the smaller ones are marked B. A cylinders get A pistons, B cylinders get B pistons. The specs from the '00 - '01 CR250 service manual are:

A cylinder = 66.398 - 66.405 mm (2.6141 - 2.6144")

B cylinder = 66.390 - 66.398 mm (2.6138 - 2.6141")

A piston = 66.330 - 66.338 (2.6114 - 2.6117")

B piston = 66.323 - 66.330 (2.6111 - 2.6114")

(Note: A February 2005 revision to the service manual removes the A piston spec, leaving only the B piston spec.)

So the maximum difference between an A and B cylinder is only 0.015 mm (0.0006"). That's 0.6 thousandths of an inch maximum difference. For comparison, the recommended piston-to-cylinder clearance for a new Wiseco forged piston is 0.038 mm (0.0015"). Piston-to-cylinder clearance spec for OEM parts is 0.060 - 0.075 mm (0.0024 - 0.0029").

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Just to add that the Wiseco Pro-Lite is 66.40! I'm using it with my A cylinder with no problems at all.

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