Need some advice from the LC4 experts

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.

The 625 and 640 are the same 101MM bore...

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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