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Need some advice from the LC4 experts

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I have a 2003 KTM 625 SXC that I took the top end apart for lack of compression and found the top ring had been chewed up and spit out. I need to replace the piston and have the cylinder plated so...

My question is can I replace the piston with a bigger one from like a 640 or are all the big LC4 6XX motors the same bore but have different model numbers?

Reason I ask is because I rebuilt my 99 Yamaha WR400 as a 400 only later to find out I could have turned it into a 426 for a few buck more.

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You need to find out which piston was in it first. Even though the pistons are the same basic size there is an A, B, and C size option. The A is in it stock. When you do a normal rebuild the B size is 1/100th of a millimeter larger to make up for deglazing the cylinder and the C size is 1/100th of a millimeter larger still. If you are repairing your cylinder it will probably go back to stock so the A size would be fine. I found that ProX makes replacement piston/ring sets in all three sizes and very much less than OEM cost. Wiseco sells them online. Good luck.

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