I have a 1998 WR400F.
Is there any company that manufactures big bore kits?
search the web for "wr big bore kit". There are two or three companies that sell them for approx. $400 to $500 plus core of your sleeve. I think you are limited to a 417 to 420 for that year but you can also find a stock 426 sleeve, piston and rod and go that route. Its better cause the 426 rod is a lot stronger and the conversion is a bit more reliable and you get the extra 10 or so cc's from what I have heard and read. Good luck and keep us posted on you finds. I have a 1999 wr400 and want to do the same!!!
here is one link
athena makes most the kits sold.
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