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Newbie doing a YZ400 rebuild, I have a question?

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Hi, I'm new to the forum, I just bought a well used 98' YZ400, I'm the third owner of the bike. The previous owner(the second owner) told me it had a big bore kit that the origional owner installed in it. He had no receipts or paper work on the modification. Now that I took the top end apart I'm wondering where to get a replacement piston and rings since they're not stock.

I measured the piston skirt diameter, 93.88mm

The top of the piston says,

J78G2

2.00MM

4649P8

XR

Is this a stock 2mm overbore piston? does anyone know who makes this piston and where can I get a replacement?

The cylinder bore looks pretty good but the top ring gap was at .015", way over spec.

Thanks for the help, Ed

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