Newbie doing a YZ400 rebuild, I have a question?

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,





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

That's a Wiseco 2mm over piston. Any Parts Unlimited dealer that sells Wiseco can get replacement parts for it.

Wiseco standard ring end gap is .004" for every 25mm of bore size which would put you at .015" .

Thanks for the info Rich

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