wr400 Power unleash

14 replies to this topic

Posted August 15, 2006 - 01:06 PM


Hi all shall i get the beers in :thumbsup:

I have nearly got the bike Looking the way want her!!!

But may go down the route of unleashing some ponies!!!

Any ideas what size i can bore it out too?
Or can a top end of a 450 go on her and use 400 crank?

I know i will lose the realiablity a tad,but i look after the bike alot and always do regular oil/filter/bearings etc etc to engine any how.

I dont ride it alot or commute,just generally have a blast for a hour now and again.

Dont wanna throw loads at it

Just wanna unleash some ponies

Many thanks in advanced


  • Bamster

Posted August 15, 2006 - 05:27 PM


What free mods have you done?
Have you YZ timed it yet?

  • redrocket190

Posted August 15, 2006 - 06:09 PM


The plan for my 1999 WR400F is to have the motor re-built by Powroll over the winter. They will exchange my WR400F rod, cylinder, piston etc. etc. for later stronger YZ426F parts. This is cheaper than over-boring the stock cylinder. Then they also stroke the motor 2mm to give you 440cc overall. Not a 450, but I am sure it will be close. It's $400 for the stroker and $584 for the Yamaha parts....

  • steve lemley

Posted August 15, 2006 - 07:40 PM


keep filling us in on the progress, can I have the number to the shop I want to do the same thing soon, my rings are starting to go so soon its going to need a rebuid anyway thanks...

  • 2wheels-drz

Posted August 16, 2006 - 10:05 AM


ha ha JC...know ya from VD, ur addicted to phils stuff.....me too lol!!! i ya can build a 460 from a 400, with some crank work and a bore..... but i think you would be better buying a 450 plant from a smashed up bike and jam it in ur frame....??

  • R_Little

Posted August 16, 2006 - 11:20 AM


I have a Lukes Racing Wr400 motor bored to a 444. It uses a 95mm bore and a 426 rod. The 400 rod small end is smaller and the weak link of the 400 motor so the 426 rod should be more reliable.

Yes there is more power...power wheelies now in 4th compared to 3rd before and even with an FMF "Q" pipe the bike is as fast as any 450 out there. I have never been pulled by any other bike in the the deep sand of South Jersey.

That being said the 400 motor was not actually slow either.

There is no more vibration with the 444, but it does require a bit more of a kick to start it and the motor will boil over a bit earlier than stock so I run engine ice and a 1.6 rad cap to prevent this.

I would NOT recomend the hi-compression piston because of the starting and heat issues.

Cost of the whole rebuild was about $1,000 delivered.

  • bluebike1999

Posted August 16, 2006 - 03:01 PM


i am gather parts right now to go from 400 - 426 (have been for about 4 months now, bike making a rattle and getting louder, just want to get to it before it gets to me)
using all genuine parts , just getting second hand barrel (426)
have found one for $275

replacing crank pin, bottom end bearings, rod, piston/rings, barrel.
am doing cam chain and guides etc while i'm there

as its reads above the 426 rod is much stronger.
mine is currently yz timed, but after swap i will revert to wr timing to increase longevity(my opinion only)

PM me if you want details as to where to get very cheap mail order genuine parts like this.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • steve lemley

Posted August 18, 2006 - 06:35 AM


exactly what parts are needed to do the power up from a 99wr400 to a 426 motor with stock parts. What has to be done and what doesent have to be done? This sounds like a better route than the big bore kits.... Or is it???

  • R_Little

Posted August 18, 2006 - 06:58 AM


exactly what parts are needed to do the power up from a 99wr400 to a 426 motor with stock parts. What has to be done and what doesent have to be done? This sounds like a better route than the big bore kits.... Or is it???

You need to replace the piston, cylinder and rod then you will have a 426.

You can usually buy a used 426 cylinder that is in good shape cheap. I spent the extra $300 or so and had a 426 cylinder bored to 444 and plated.

The big advantage of the 426 conversion is the rod which is more reliable than a 400 rod. BlueBike1999 your rattle noise is most likely a rod or a counterbalancer key issue. Don't let either go too long

If you do this make sure to replace that $2 counterbalancer key when you do the swap or better yet swap to a splined 426 crank if you can get one cheap.

Remember also that the 400 clutch is 1 plate smaller than a 426 so think about new plates and maybe a set of HD springs.

  • JJHayesIII

Posted August 18, 2006 - 10:25 AM


I opted for buying an OEM 426 crank, which saves the trouble/cost of having to rebuild your stock crank with the 426 rod...

Since it's a splined crank, as posted above, you would also need to replace the drive gear with a splined gear, instead of a keyed gear, which gives another reliability improvement

Here's the stuff I bought, with approximate prices:

5JG-11400-20-00 WR426 Crank Assembly $285.56
5JG-11536-00-00 WR426 Drive Gear $53.58
5NG-11631-10-00 WR426 Piston $105.88
5JG-11633-00-00 WR426 Piston Pin $9.44
5JG-11603-00-00 WR426 Piston Ring Set $22.43
93420-19084-00 WR426 Piston Pin Circlips $0.19
5JG-11311-00-00 WR426 Cylinder $264.40
93306-30633-00 WR426 Main Bearing $31.73
90280-03001-00 Woodruff Key $2.12

I had great luck ordering from Mike at University Motors, lowest prices I could find, fast shipping, and great service...I've ordered everything from him since then
Mike Barth
University Motors
1920 12 Ave. N
Fargo, ND 58102
FAX 701-235-6478

and here's my disclaimer: This is my first rebuild...I've never even done a top end before, my motor is still in pieces (although, I did get my bottom end together the other night)...It doesn't run yet, but it went together fine so far...good luck! Make sure to post updates with what you do...

  • bluebike1999

Posted August 18, 2006 - 02:38 PM


R little
thanks for the heads up on the clutch springs. i'll add that to my next order. i had actually forgoten about the counterbalancer, well i had just assumed it was ok but i'll make sure i replace that key. i did check it when the rattle first raised its ugly head and it felt ok with no slop, but i'll do it anyway.
Thanks again

  • jimbob69

Posted August 18, 2006 - 09:10 PM


Does anybody know if its possible to bore and replate a stock 400 cylinder to accept a 426 piston?

If not able to find a used 426 cylinder this may be the route to go.

  • bluebike1999

Posted August 19, 2006 - 04:56 PM


i got a quote from a crowd here called eletrosil

An approximate cost to get the following work done is as follows:

Electrosil Coating: $440

Big Bore Cylinder: $88

Insured Air Freight: $22

Our current turnaround time is 3-4 working days

i am a bit cautious regarding replating as i have heard mixed responses as to its long term reliability.
as you see $550 aus which should be about $22 over there
find a good 2nd hand barrel, i've found one for $275 aus and its the genuine article and in good condition. thats two news tyres ive saved.
good luck

  • R_Little

Posted August 19, 2006 - 05:15 PM


You can rebore but a good used 426 is much cheaper...like $250 cheaper.

Does anybody know if its possible to bore and replate a stock 400 cylinder to accept a 426 piston?

If not able to find a used 426 cylinder this may be the route to go.

  • hideto666

Posted October 09, 2006 - 11:33 PM


hi guys, can all these be done on a 98 WR400?

by the way, R little. i have the same rattling noice from my engine.
you mentioned that it could be the rod? some guys told me that its the
cam chain tensioner.
thanks for your advises, cheers :devil:


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