400 convert to a 426??

Is there anyway that I can make my WR400 into a WR426?

What are the parts that I need?

Can I take them straight off 426 and put them on my 400?

Please advise…

As far as stock parts go, you will need a rod, piston, and cylinder. I'm pretty sure the 426 rod will fit a 400 crank, but I could be wrong.

But it will all interchange as far as I know.

Thanks Hick

So that is what I need from the stock parts.

Rod, piston, cylinder…..

Do I need anything else?

Like a 426 carb?

Or the exhaust + header.

Like you said, the crank may be interchangeable.

But the biggest question, can it be done??

I guess its going to be my personal project to make my

400 into a 426.

I am not so techy so I need as much details as I can get.

Appreciate any help…


A 420 kit is somewhat popular. Try Eric Gore or Thumper Racing.


My mechanic said it can be done. He is actually doing one right now. I do believe you have to change the crank out, because the large end bearing on the rod if a different size. I could be wrong about that. I'll check with Ben to see how he is doing on it.

You will not need to change the carb or exhaust. They are pretty much the same thing.

Sounds interesting...are the head specs the same other than the Ti valves? Would you expect clutch problems with this setup, didn't Yamaha beef up the clutch for the 426's? While at it why not go for the 450??? Keep us poted on the possibilities/results.



as Bill said, Eric Gore is a good source for the 420 kit.

I have a 99 wr400 with a 420 kit( which required the cylinder to be bored and replated), hot cam, all yz mods, blue and grey wire disconnected, de-octapussed etc.Reliability has not been an issue.

Bill, you show a , "KL MOD", in your specs just wondering what that one is? I heard of the BK mod but read that was not for 98-99 only 2000 up please more info.


The KL/Taffy mod is the use of a sleeve, to limit the AP pump stroke. You can find a model wheel retainer at the local hobby store and place it on the AP rod. Place with about .024-032" gap between the sleeve and AP housing.

Now you have just a smidgen of fuel entering the carb when you wick the throttle. This will eliminate any bog and make jetting the pilot/needle circut much easier. It's the pre BK mod!


The '00 YZ 426 had steel valves, as far as I know there were no changes to the head or valvetrain from '99 to '00, although I wouldn't be surprised if there was a cam timing or profile change, but this wouldn't affect your ability to put the 426 parts in the bottom end.

I'm pretty sure they added an extra clutch plate for the '00 426, but I wouldn't let that stop me either. Someone asked a few days ago if you could put a 426 clutch in 400 cases, I don't know if you can or not, but I sure wouldn't bet against it. You would need the basket, hub, and probably push rod and pressure plate (and extra plates). If anybody does this I would certainly recommend '01 clutch parts...

There are so many changes to the 450 I doubt any parts will line up (the head has fewer bolts for example, valvetrain is different so will cam chain line up etc.)

yes, but one can punch a 426 out to 450 so it too can be done. it just is not the same as a new 450 the way the the 400 could be made into a 426. one thing to watch for with all this is compression. if it gets too high, gas becomes too expensive

Easiest way to convert it, maybe the cheapest way also:

1. Sell WR400

2. Buy used WR426


I wouldn't worry about too high a compression, I set up my 97mm (444cc) @14~1 and it ran fine on premium pump.

It also ran fine on pemex.

The combustion chamber is a great design and is very tolerant of lower octane.

I would also just get a complete crank for 325$ from YOT.

Save the worries of finding a good crank guy.

I had a 13.5:1 that ran good on 91 Ron/Mon (pump) octane. But I'm at 4,000 ft.

It also ran fine on pemex.

That is impressive. :D

Save the worries of finding a good crank guy.

My sentiments exactly, I paid just over $300 for a new crank (both halves, new rod bearing, new rod) last year from NCY. If the crank isn't reassembled true then all kinds of bad things could happen. :)

I think pemex is getting better!

There is a premium choice now.

It is just the stuff some local with tar stained fingers siphons out of a 5 gal bucket I worry about now.

I have seen a few fresh crank rebuilds instantly poop the bed so until I can reccommend a trusted source, or I am willing to build friends cranks, I say buy a freshy and be assured of getting a good one.

I recently did this to my YZ-400. 426 rod, weisco stock compression piston and new 426 cylinder. After having a 13.5 to 1 compression 420, and now comparing it to the stock compression 426, I'd never go back. This thing starts so easy now, I love it. The rod big end is the same, it will fit your 400 crank. The small end is what changed.

If its any intressting, In Europe they still got the WR400 produced alongside the 426, so the 01 - 02 WR400 (EU version ONLY) has TI Valves too :) but isn't there a 444 kit for the 400 ? or is that kit for the 426?

//dw :D

I think Thumper Racing had a 440 kit for the 400 but it used a sleeve. I don't know if they still offer this or not.

The stock 426 barrel can be bored to accept a 97 mm piston which will get you to 444.

Hope this helps.

Hey dirt this is easily done. My scoot is a 2000 WR400F, now a 426. I used a new 2000 YZ426 cyl.95mm. bore, refurbished my stock 400 head,seals,guides and valves. Installed stock valve springs and cams,WR timing for now. The piston is a 400 95mm. bore, 13.5 to 1 from Wiesco. Used the stock rod because the small ends are larger than the YZ426's by 1mm.

Piston p/n is 4649M09500. I measured the valve to piston clearence which is 3/16 with no head gasket in place to give me the smallest number, with gasket, it's 5/16.

Useing the stock 400 carb,162mj and stock needle,YZexhaust.I will be installing the YZ needle soon.

The bike does not ping and starts on the first kick cold or hot. Pulls great on the entire power band and does not run hot. Pulls the tire off the ground in 3rd. at will and flys with a 14 and 48 tooth gearing set-up. Parts cost $400.00 and the results were night and day. No longer a tame WR400! Enjoy the ride.

That is good info, esp. the PN on that piston since it allows use of 400 crank and rod. I had heard they were no longer selling that part. If they are I'd imagine they also have the 12.5:1 (std.) part also.


Thanks for all the suggestions and tips

I'll see what i can get over here in Singapore

Let you know what i decide

Let the suggestions come in.....

Thanks again to ALL!

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