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eatdirt97

WR400 to 426 conversion

10 posts in this topic

hey guys, a friend of mine has a 99 WR400 and is looking into swapping cylinders. Has anyone taken a 426 cylinder and piston (OEM stock yamaha parts) and put them on the WR400 motor? If so, what else had to be done to accomodate?

Any additional info would be greatly appreciated.

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The small end of the con rod is larger on the 426, so you will need the rod, barrel and piston/wrist pin.

The only problem with that is rebuilding the press-fit crank/rod assembly is a touchy affair.

You may be able to find an 95mm aftermarket piston that fits the old rod...

Anybody?

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Yes can be done. Mine is a 00' 400, with a 00' YZ426 jug and a Wiseco 95mm piston that is designed to fit the WR rod end. Stock WR cams on WR timing and grey wire cut. That's it, no radical jetting and dynos in the mid 50's hp., at 4000ft. This spring I'll change to YZ timing and needle.

I do have the Wiseco part number if you want it.

:)

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The piston is Wiseco #4649M09500 13.5 to 1 comp. Sorry, I can't find the part # for the rings. Wiseco should have that number.Hope this helps! I used the 2001 WR426 gasket set, had no problems putting the motor together. My local dirtbike shop prepped the new head as the old head was wasted when one intake broke or the keeper let loose and destroyed the 400 piston, head and intake cam. Reason I went to the 426 or 95mm piston is because a friend gave me a very new 426 jug so I wanted to see if it can be done. 1000 miles later the motor still runs like new, one kick hot or cold. Gets 90 miles on one tank in the desert, maybe more if I baby it. Naw, wide open is much more fun!!! :)

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http://ericgorr.com

420 big bore is also an option. I did it.

YZF

WISECO makes 2mm oversize kits for the older models of YZ400 and 426F plus the new 450. For those models we have kits ranging from 420,444, and 472cc with two choices of compression ratios; 12.5 and 13.5:1 We also have 2mm high compression kit for the YZF250 that makes it 262cc and 13.2:1 This kit doesn't seem like a big displacement change but it makes a big difference in throttle response and offers a "more of everything" power increase. Our kit price with boring, NICOM plating, piston and gasket kits is only $350 Compare that to RPM's $2,300 300cc package. Our kit is 40cc smaller (about the size of an RC car engine) and $1,950 less expensive

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I've got a 97MM bore on my 98 WR400, Thumper racing had wiseco make some 97mm pistons with the WR400 sized wristpin for thier kit, mine is a hybrid of this & a 426 + 2mm.

I dont think I can get replacment pistons anymore :) If anybody knows a partnumber still avalible I need it! If not I think my only option to keep this size, is have a 426/450 rod put on my 400 crank, I've got a spare crank to do this.

I have a stock 400 jug I can go back to, if I have to, the current jug is a 426 bored out 2mm to to 97mm (444cc?) Cometic makes the gasket kits for it.

I got all this info from the previous owner, just picked the bike up.

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I've got a 97MM bore on my 98 WR400, Thumper racing had wiseco make some 97mm pistons with the WR400 sized wristpin for thier kit, mine is a hybrid of this & a 426 + 2mm.

I dont think I can get replacment pistons anymore :) If anybody knows a partnumber still avalible I need it! If not I think my only option to keep this size, is have a 426/450 rod put on my 400 crank, I've got a spare crank to do this.

I have a stock 400 jug I can go back to, if I have to, the current jug is a 426 bored out 2mm to to 97mm (444cc?) Cometic makes the gasket kits for it.

I got all this info from the previous owner, just picked the bike up.

How deep are your pockets? Wiseco will build pistons to spec. Not actually sure of what they charge but might be an option.

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