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I'm going to be rebuilding my 400ex I want to put the highest compression piston possible in it and the largest diameter possible. what brand and where do I get it?
 

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89mm 12.5 to 1 compression ratio Wiseco = 435cc

If you can believe everything you read, Rocky Mountain ATV offers a 94.5mm 12.5-1 Wiseco 440 kit, only thing 94.5mm bore with a 70mm stroke = 491cc. Check it out here https://www.rockymountainatvmc.com/p/921/3783/Wiseco-Big-Bore-440-Kit?term=440+big+bore+kit+2000+400ex+trx

It's probably a mis-print (unless RMATV is having these specially made for them, which I doubt)

The biggest piston that I am aware of is 89mm and the cylinder has to be sleeved to install it. If it were possible to go to 94.5mm, the cases would have to be machined for the larger sleeve and that would get into the bolt holes that hold the cylinder down.

In my experience it is not possible to take the 400 to 491cc unless you were starting with at least a 450
 
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