2007 CR250 new "B" cylinder , piston size

mxracer666
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mxracer666
What is the correct aftermarket (Pro-X, Vertex) piston size to run? I believe the "A" cylinder would run a 66.34mm, but not sure on the "B".
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Paw Paw
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Paw Paw 11/7/2020 5:39pm
11/7/2020 5:39pm
In reality all three sizes will run just fine in your bike.

Paw Paw
mxracer666
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mxracer666 11/7/2020 5:57pm
11/7/2020 5:57pm
Thanks! Am I correct that the "A" cylinder measures out at 66.40? ( so the "new" clearance is .06mm) What should the "B" cylinder measure out at?
captmoto
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captmoto 11/7/2020 10:15pm
11/7/2020 10:15pm
I might be talking out my ass but I seem to remember the only difference for OE pistons was 2 rings vs.1 ring. The 2 ring piston is discontinued. Have you had your cylinder properly mic'ed so you know the exact bore?
40
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40 11/8/2020 5:23am
11/8/2020 5:23am
A Piston OD: 66.340 - 66.348 mm (2.6118 - 2.6121 in)
B Piston OD: 66.333 - 66.340 mm (2.6115 - 2.6118 in)

The A pistons are larger than the B pistons per Honda's sizing system.
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jc34
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jc34 11/8/2020 7:10pm
11/8/2020 7:10pm
Doesnt matter, recommend you run a wiseco in it. I ran a vertex for a while and noticed the forged Wiseco has much better performance.
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FU2
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FU2 11/11/2020 11:08pm
11/11/2020 11:08pm
jc34 wrote:
Doesnt matter, recommend you run a wiseco in it. I ran a vertex for a while and noticed the forged Wiseco has much better performance.
Maybe next time get the right vertex piston for your cylinder. Or don't try to run 100 hours on a piston.

Vertex has different sizes
A 66.34
B 66.35
C 66.36
D 66.37
And in the past you had E and F.
Once you put a D in a A size cylinder then there is no turning back.

Great piston a vertex, run it for 15 hours then it's done. 14 years now running the same size piston in the still original cylinder, a size B vertex. Prox for a honda is a shit piston ,same as a honda piston.
hypermoto
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hypermoto 11/12/2020 5:48am
11/12/2020 5:48am
jc34 wrote:
Doesnt matter, recommend you run a wiseco in it. I ran a vertex for a while and noticed the forged Wiseco has much better performance.
FU2 wrote:
Maybe next time get the right vertex piston for your cylinder. Or don't try to run 100 hours on a piston. Vertex has different sizes A...
Maybe next time get the right vertex piston for your cylinder. Or don't try to run 100 hours on a piston.

Vertex has different sizes
A 66.34
B 66.35
C 66.36
D 66.37
And in the past you had E and F.
Once you put a D in a A size cylinder then there is no turning back.

Great piston a vertex, run it for 15 hours then it's done. 14 years now running the same size piston in the still original cylinder, a size B vertex. Prox for a honda is a shit piston ,same as a honda piston.
You can absolutely to from a D piston in an A cylinder back to a A piston. It’s such a tiny difference, it’s not actually doing any additional wear..

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