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what is the difference between Honda's A & B piston

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Hey guys I got a 1998 CR 250 dirt bike, im gonna do a top end this weekend but I have a question. My bike has 2 pistons OEM from honda, A & B. on my 125 its the same, the 125 is cylinder A so im pretty sure my 250 is A also but i will check. what is the difference between the 2?

also should I use OEM or just get a wiseco piston.

Also one more question, aftermarket top end gasket set from royal distributing or OEM Honda top end gasket set?

Thanks

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The B piston is slightly larger than the A piston. The rear of the cylinder should be stamped with the corresponding letter. I almost always recommend OEM pistons, but the '98 CR250 is one exception. Because the '98 exhaust port is really too wide, it accelerates piston wear which can lead to piston skirt cracking if the service intervals are exceeded. The forged Wiseco can better tolerate the hammering that occurs when the piston tolerances exceed spec.

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What about top end gasket set, should I get OEM or aftermarket?

Oem and cometic make great gasket kits. If you are going to go with the wiseco piston though, there is a top end rebuild kit they offer which includes piston, ring, top end bearing, piston pin, and top end gasket kit. Here is a link to a great place to buy it. I have used oemcycle for a bunch of stuff and they are great to deal with.

http://www.oemcycle.com/Item/product/900062171

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What about top end gasket set, should I get OEM or aftermarket?

OEM gaskets are the best. I've had isolated problems with every aftermarket gasket maker, but the OEM stuff always fits perfectly.

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The B piston is slightly larger than the A piston. The rear of the cylinder should be stamped with the corresponding letter. I almost always recommend OEM pistons, but the '98 CR250 is one exception. Because the '98 exhaust port is really too wide, it accelerates piston wear which can lead to piston skirt cracking if the service intervals are exceeded. The forged Wiseco can better tolerate the hammering that occurs when the piston tolerances exceed spec.

I could be wrong but I think with Honda the piston sizes are ass backwards compared to the others, so the Honda A piston is larger than the B piston. At least thats the way it appears in my 01' manual.

A= 66.330-66.338

B=66.323-66.330

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I could be wrong but I think with Honda the piston sizes are ass backwards compared to the others, so the Honda A piston is larger than the B piston. At least thats the way it appears in my 01' manual.

A= 66.330-66.338

B=66.323-66.330

You may be correct about that and seem to remember hearing something about it before. I have a 92-96 service manual but in those years Honda only offered one piston. The times I worked on later CR's, I just used the matching piston for the stamping on the cylinder and called it a day.

I'm picking up a new '07 CR250 tomorrow so I need to get a new shop manual for it.

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You may be correct about that and seem to remember hearing something about it before. I have a 92-96 service manual but in those years Honda only offered one piston. The times I worked on later CR's, I just used the matching piston for the stamping on the cylinder and called it a day.

I'm picking up a new '07 CR250 tomorrow so I need to get a new shop manual for it.

You ready to sell me that worn out, doggy '95?

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You ready to sell me that worn out, doggy '95?

I am truly out of garage space, and need to purge a bike to make room for the new CR, but I plan on keeping the '95 for a bit longer. Any interest in a unrestored 10 hour '83 RM125?

83RM125002-1.jpg

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The B piston is slightly larger than the A piston. The rear of the cylinder should be stamped with the corresponding letter. I almost always recommend OEM pistons, but the '98 CR250 is one exception. Because the '98 exhaust port is really too wide, it accelerates piston wear which can lead to piston skirt cracking if the service intervals are exceeded. The forged Wiseco can better tolerate the hammering that occurs when the piston tolerances exceed spec.

Would you say that with this model the service interval on a cast piston is shorter than other models?

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Would you say that with this model the service interval on a cast piston is shorter than other models?

Yes, for example, the 99 cylinder has much smaller ports and pistons should last longer with it.

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I am truly out of garage space, and need to purge a bike to make room for the new CR, but I plan on keeping the '95 for a bit longer. Any interest in a unrestored 10 hour '83 RM125?

83RM125002-1.jpg

YES!, if you have a great way to get this thing to Hawaii!

the only RM125 ive had was an 84, but i did have an 78.5,80,82,83 and 85 250 when i was racing.

go ole days eh CamP.

Edited by HappyG

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I am truly out of garage space, and need to purge a bike to make room for the new CR, but I plan on keeping the '95 for a bit longer. Any interest in a unrestored 10 hour '83 RM125?

83RM125002-1.jpg

If I had that I would build another garage. Paul

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I am truly out of garage space, and need to purge a bike to make room for the new CR, but I plan on keeping the '95 for a bit longer. Any interest in a unrestored 10 hour '83 RM125?

83RM125002-1.jpg

Holy crap camp. wow.

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