WR400 Rebuild


I just picked up a 1998 WR400 for a "cheap as free" price, the only catch is the motor is rooted :banghead: , Its dead stock afaik apart from the later model headlight (busted), rear guard from somewhere and a staintune exhaust


Im going to rebuild it my self, but i was wondering, while its apart is there anything i should look at upgrading or should i just rebuild to stock, any suggestions are welcome

Its currently running fine but its got some nasty rattling comming from the piston, last owner reved the shit out of it and didnt look after it enough, rest of the bike is in perfect condition

Any thoughts?


from what i am told the 400 bottom end is the same as the 426. if it is, as you rebuild, use the 426 parts {piston+cyl} . or even a 440-450cc kit.

if you have the skills. take the bottom end apart and take a look before you even order any parts.

If I remember correctly, the connecting rod is different from the 400 to the 426 and would have to be changed. An easier modification if you do not want to split the case is a 420 kit. Make sure you look at the timing chain while you are in there. Mine was so worn that it tore a whole through the cylinder.


That rattle is likely the counter balance key. fairly easy fix just search this forum, lots of posts on it.

You can test for play in the crank easily when you take the head off, and check for slop in the stator-side bearing easily enough. A lot and youll want to replace that also. You can get a new wr400 oem crank for around 300$ online. You will probably want to get a rod kit instead, or a rod with a wider small end ( I forget what year WR did this) to aid in preventing the piston from rocking around on the pin and small end.

Check the valve buckets too, these bikes can inhale sand and it removes metal around the buckets and they can slap around in the pockets milled into the head. If theyre bad I think I saw what appeared to be slightly oversize buckets on the fiche. if theyre really bad youd need a new head, around 500$ if you look online. A complete head with everything is a little over 700$.

Good idea to check the chain while youre in there. as well as the gear on the crank. Mine was worn to junk. Chains are cheap at 35$ (easy to replace too).

if the bike was cheap, it might be worthwhile as you end up with basically a new motor.

yeah it will be worth while :banghead:

If when u speak of the Counter balancer, you mean the flywheel... it was just done, This particular bike coped a hit on the bashplate and knocked it into the casing and damaged the flywheel, dislodging the key (i think) then wearing out the crank etc, its all just been remachined etc. Im not exactly sure as im not the one who did all the work

Ive rebuild a couple of engines now so im pretty sure im ready to tackle splitting the cases if it comes to that, i might run a compression test on it to see what i find, short of that i wont know much till i pull it down

Thanks for the help


hey hey

finally got around to pulling the motor out to start work on it, ill rip the head off tomorow to have a look.

with the 426 piston with the different smallend, can i use a 426 rod kit with the stock wr400 crank?



keep us posted on your progress, ie what you end up doing and what it costs.

got a feeling i might need to do the same.

good luck

Well i can do a picture thing as well cause i generally take pictures of everything i do anyway :applause:

Did some more research tonight, and looks like i will be going for the Wiseco 13.5:1 426 piston in either a 426 cylinder or overbored stock one. Using 426 rod kit in the wr400 crank. But depending on teh damage might go to 426 crank as well so i dont have to worry about that counterballance gear.

Here are the picks from tonights work:





From what i can read the high comp 426 piston seems the way to go without going overboard :ride:

Now is the perfect time to make a 426. The WR's in that time frame used the same rod. YZ rods have smaller end. I used a stock WR head, an 01 WR cyl. and a Wiesco 95mm., 13.5 to 1 piston. Been making great power for three years now, no problems, 3000 miles. Stay on top of your valve lash and you'll be fine. :applause:

might want to change out the exhaust cam with 2003 cam so you have auto decomp. I haven't done it yet, but it's on my to-do list. The replacement cam is about $230 according to my dealer.

Hot Cams NEW exhaust cam DOES have the auto decompression. You can use the 450 YZF cam, but you will have to do YZF timing and the pitch of the sprocket teeth are a hair off.

It IS do-able (I have the YZF OEM cam) but if I had to do it again, I'd go w/ the Hot Cam.

You will be hard pressed to bore the 400 cylinder to use a 426 piston. The cylinder walls are approaching the Dangerously Thin region.

Go with a 426 crank and cylinder.

This is rather confusing the 400 bore issue. Some people say you can take the 400 bore out to 444cc, then you say you can barely take it out to 426cc.

I like the idea of boring the 400 as i can get the piston/bore clearance spot on with what ever piston i end up choosing rather than using the 426 cyl and hoping for the best

as for the cams, auto decomp would be nice but its not THAT hard to start it :applause:

Got the head and cyl off tonight and found a few probs which i was expecting anyway. First off the head is fine, valves all look seated properly etc. But the piston seemed to move around alot in the bore. Pull the Cyl off and we find the bore doesnt look toooo bad but in places there is light scoring from id say dirt. Plenty of carbon everywhere but generally ok.


Piston has a small chunk busted off id say thats from the piston knock. Rings are all in once piece and look ok but i suspect they arent that old anyway.


Small end bearing doesnt appear to have any give but the big end does, slight side to side movement and there has been plenty of heat going into the crank around the bearing as its a nice blue tinge.


So there ya go, keep an eye on those rattles ;D

What do people think a WR400 would go with a bore or stock 426 Cylinder, with 426 Rod kit, and the wiseco 13.5:1 426 Piston jetted to suit, would go like? Also has freed up exhaust as you can see from the pics. Kinda hoping itll keep up with my uncles WR450 and a mates new CRF450

thanks for the detailed pics and info gearbox.

you are making me nervous, as all i know is that when you see those bits in the pics, it's serious wallet time.

good luck keep it coming

Hey guys, Its been a while since i touched the bike as I've been busy with other projects. At the moment its like everyone i know is buying bikes so now i got some motivation :lol:

yesterday i split the casings, you can see all the pictures at the link bellow.


It didn't seem like the con rod bearing was shot, but i took it to local mechanic to confirm. He said old oil will take up the slack and make it seem less dead than it really is, after he degreased the bearing the truth revealed it self. So the con rod bearing is shot, the piston has a chip in it and the cylinder has some light scoring. There was light pitting in the piston pin as well. The mech said if I'm not going crazy with it ill be able to brush hone the cylinder and get away with it. I mentioned earlier in the thread that it had some woodruff key issues. After pulling the bike down its actually the fly wheel. The woodruff key was sheared in half and the thread on the end of the crankshaft is chewed out. The guys that fixed it just put it back together with some locktite or something just to make it work. So i may buy a whole crankshaft assembly instead of a rod kit.

Im in the process of deciding what parts to get. I wanted to go a high compression piston but after some research i think ill give it a miss. Since im replacing everything except for the cylinder i was thinking i might stretch it and get a new cylinder and go the 426 route. That should give me more power with more reliability than stock :lol:

Hopefully should be plenty of progress over the coming weeks. I paid for a full years rego while it was pulled apart :applause:



Doing a great job so far, your really going to like the increased power that your moving up to. Going the 426 route is the smartest because OEM parts are right there for you. Jetting is a piece of cake and you'll have many miles of service. I'm around 6000 miles now and it runs new. Good Day


Doing a great job so far, your really going to like the increased power that your moving up to. Going the 426 route is the smartest because OEM parts are right there for you. Jetting is a piece of cake and you'll have many miles of service. I'm around 6000 miles now and it runs new. Good Day

Hey i read else where that you are running 426 bottom end with a 13.5:1 piston. I'm thinking of staying away from the higher comp piston as it can get fairly hot here and i don't want to have to worry about heat issues and/or upgrade the radiators etc.

But how does your bike compare in straight line performance to 450's? are you running hot cams or anything? I'm considering the hot cams as yamaha of troy are selling them fairly cheap on ebay delivered to my door (in australia)


My bottom end is the stock 400 with a 426 jug and stock 2000 400 cams. 13.5 to 1 piston makes tons of power down low and on high revs. I ride with a KTM 525, another WR400, dual sport Honda 650, two CRF 450's and quads and have no problem taking the front or just cruise mid pack. Straight line power is big, that's why I changed the rear sprocket from a 50 to a 48 to keep the front tire in the dirt.

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