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DaoRadivo

So I bought a 426 top-end for my WR400 99...

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...for $100, and the barrel and head are in perfect condition. The guy who owned it had a massive cartwheel and bent the frame and forks, so a mate bought the whole bike for a stupidly small sum, and sold me the top end. He wanted the bottom end, otherwise I would've just bought the whole engine.

Has anyone here done the 426 top-end conversion on one of these earlier 400s? When I was working in a Yamaha shop I sold a few top-ends to people that were doing them, but never really did one myself. I have the barrel, head, cams, cam caps, top cover and all the valves and everything.

Just wondering if anyone has come across any problems doing this.

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You should search this forum since this topic has been covered many times.

I belive you will need a new (426) connecting rod since the piston pin is bigger on the 426 vs. the 400.

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You should search this forum since this topic has been covered many times.

I belive you will need a new (426) connecting rod since the piston pin is bigger on the 426 vs. the 400.

That's correct...I recently did the conversion as well, but instead of just doing the con rod, I got a whole new 426 crank assembly, which required a couple of other things to work as well, but I figured the updated part numbers were worth the trouble (elminated the woodruff key on one of the gears)

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You can use the same rod, I did. Wiseco has the piston for this job. It's a bolt together upgrade. The wrist pin on the WR is 1MM larger than YZ, wiseco will have the 95mm piston your looking for. Remember to by a gasket kit for a 426 now, head gaskets are different.

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You can use the same rod, I did. Wiseco has the piston for this job. It's a bolt together upgrade. The wrist pin on the WR is 1MM larger than YZ, wiseco will have the 95mm piston your looking for. Remember to by a gasket kit for a 426 now, head gaskets are different.

The wrist pin on all the 400s (98/99 YZ/WRs and 00 WRs) are 18 mm in diameter. The later years YZ/WR 426s use a 19 mm wrist pin.

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Riddle me this.......I bought an 01 426 piston Yamy stock and my 00 WR wrist pin would not fit in. The stock 01 426 wrist pin hole is 17mm. I then bought a Wiseco and everything worked. Still have the brand new 426 Yamy piston and OEM rings, never going to use it, make an offer.

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You should search this forum since this topic has been covered many times.

I belive you will need a new (426) connecting rod since the piston pin is bigger on the 426 vs. the 400.

Hi! Yeah I did a search (I know this is a relatively common conversion), couldn't find much though, maybe I was using the wrong search terms or something. You wouldn't happen to have any links to relevant threads, by any chance?

I know the piston pin is 1mm bigger on the 426, so I'll be putting a 426 rod-kit in the thing while it's apart to accommodate. The 426 and 400 both run the same crank pin, so it should be apples. Time for a big-end rebuild anyway, especially if I'm gonna be restoring it back up to nice happy compression, unlike the flogged-out bucket of pus it is right now.

I've also got free dyno access, so I'm gonna get my good mate to dyno-run the big girl before and after, to see if it was actually worth doing! Haha I'd imagine most of the horsepower increase would be due to the restored compression, not to the extra 27cc...I'm sure it'll make a fair difference though.

I also bought a standard YZ426F 00 full exhaust system from the guy for $60AU (about $35US). Bargain! My White Bros muffler has been bent and cracked a couple times since I've had it, I'm, how should I say it, 'crash-prone'.

Thanks for all your help guys, you're all legends!

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Search Criteria:

search these headings for a good start

JJHAYESIII

426 Piston (this will get you some good stuff from the YZ forum)

426 Crank shaft

426 connecting rod bearing

This will give some reading to do

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I searched the YZ forums, and it helped me out a lot. Thanks guys, you've been a great help! I'll let you know how I go with all this.

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