'98 WR400 Rebuild options

I have had nothing but trouble with my wr since day one, and I've rarely gotten to ride it as a result. My current issue (and this has been the issue for quite some time) is that the bike is fouling plugs like crazy, and back fires and sputters horribly once it warms up to the point it is not ride-able. The head checked out ok (thanks MXTIME!) and they recommended that I check the cylinder for roundness, and that I will likely have to replace the cylinder, piston, and rings. They also said it was pretty oil fouled, and that it could possibly be a bad headgasket, and he explained that occasionally the area where the head bolts down can leak oil, and as the piston passed by, it sucks the oil into the combustion chamber.

The last shop claimed they were seeing 75% leak down and had a quote in excess of $3k to replace the head, cams, etc. If it's not the head, what could cause a leak that bad? I have reason to believe the shop was just trying to get more money out of me, but in case they were right, what could cause that?

The bike was "rebuilt" by the previous owner after it blew the motor. I can see some damage on the crank so I assume he reused the crank and just replaced the piston and cylinder. Will marring on the crank affect the rotational balance? Or would the motor blowing cause the crank to be untrue?

So best case scenario is that I have the cylinder checked and that it is ok, meaning the headgasket is the problem. Assuming the cylinder is out of round, I'll need to replace a bunch of stuff. What are my options? I want the most reliable setup, the bike was plenty fast for me, so squeezing every ounce of power out of it at the cost of reliability is not my goal. It is however a street legal bike and extra displacement would help me lug along at higher speeds. I plan on putting a 450 carb on there if things get sorted out, so the stock 98 carb isn't an issue either.

Is there some combination of 400, 426, and 450 parts that I can mix together to get this thing reliable and bulletproof? What all should I replace if I have to crack the cases(crank), or SHOULD I crack the cases to replace parts/bearings.

Finally, what should I expect as far as cost? This bike has had me so frustrated I was planning on parting it out and purchasing a 2-stroke trail bike to put around in the woods, but if I can get my wr in tip top condition for half the price, I'd rather do that.

Thank you for your time,


If you do have to replace the crank or connecting rod and the cylinder, go for a 426 rod, piston, and cylinder. Direct bolt on, and the small end of the rod is a bit bigger, o more durable. You already have stainless valves. You should be sure to mic the valve guides, as they can weart and are often overlooked as sources of oil into the combustion chamber. Surface the head and deck the cylinder to be sure everything is straight and true, and use an OEM head gasket, and you should be good to go.

I had a leak down problem on my 03 that was the result of negative valve clearance on the intakes. In other words, the intake valves were slightly open on compression. The bike would run when started, but was very difficult to start. Correct shimming solved the problem. I also did much more than that as the engine was due for some serious work, but that is the gist of it.

Thanks for the input guys. The head checked out great so that is not the issue.

As far as rebuilding, it's looking pretty ridiculous as far as pricing. ~$400 for a cylinder and looking at around $1k for crank, cylinder, piston, and rings, not to mention all the bearings, gaskets, etc. It does look like the 426 parts are a hair cheaper though.

$1200 was my budget for a new bike, and I figure if rebuilding this one is more than half of that, it's getting parted out.

On that note, I'm taking the cylinder to a machine shop on Tuesday and having it checked for roundness, and that should help narrow things down.

Also, this bike starts first or second kick every time, whether it's been sitting or taken apart. Something isn't right. The plug, head, exhaust, etc were black black black. It was not sticky, but powdery, but the mechanic said oil was being burned and I trust his word.

Where do you guys get parts? Aftermarket?


good questions , i was wanting to 426 my 400 when i look on ebay i see vesrah rod kits and hotrods kits175 for a hotrodsrod kit?&%$#@! anyone know whats good worth while stuff to buy?

Looks like Millennium Technologies can do a cylinder core exchange for ~$250, and I can get a piston with rings, and the pin etc for ~$160 and keep it 400cc if the crank checks out ok. I have a feeling I'll need to take it out to check to see if it is straight or damaged and that's not something I really want to do.

Still looking for options here.

I managed to run the cylinder by a machine shop at lunch today. The guy said that the cylinder was round, but that the top and bottom were a little under a thousandth wider than the middle. Is that normal or acceptable wear? He said that the cylinder was ok to continue using.

He also said that cylinder was 95mm? This is indicative of a 426 cylinder correct?

Any input fellas?



EDIT: he also said I could have it bored out and sleeved. Now I know that these are nikasil coated and that the general consensus is that resleeving is bad, but he said he would also use different rings for the piston as well that were designed to wear with a iron/steel sleeve.

Edited by nnamssorxela

Still searching for info on this. In the mean time, here is a picture of some old spark plugs that were fouled. Oil fouled?


Some crank marring from the previous owner blowing the motor up, look ok?


Heat on crank, I'm under the impression this is to assemble the crank and rod?


The rod can wiggle side to side, and the piston can slide side to side on the pin. I'm under the impression that this is normal? Is there a reason I should take out and inspect the crank, or should I throw on a replated cylinder, new piston and rings and call it a day?


Do you still have the decomp. lever on the handlebar? If yes how was compression, could you kick it over TDC easy?

From what i read in the first post, a newer carb would have fixed your trouble I think :-/

Yes, still have the lever on the bars. It felt pretty tight, much better than when I bought it since I had the head redone (I must use decomp now, before I could kick it all the way through). I was thinking that, but at the same time, I've read that tolerance on the cylinder is 1/2 of a thousandth which I think I was at or beyond. Also, why would I be burning oil? I thought it looked like carbon, but I've been told oil, and why do you think the issue is the carb?

My list now is:

426 cylinder replate

426 piston and rings (do I need the pin and circlips too?)

05+ 450 carb

Do I need to check the crank, or can I assume it's good?

Any reason I should keep the 400 carb for breaking in? The shop said it was in good order but running a bit rich. Would this help with break in before I start tinkering with a 450 carb?


If you cant kick it past TDC without the decomp. its how it should be, if you can stand on kick lever with your full weight (80-90 kg) for 3-5 seconds before passing TDC its really good. Oil burning mostly comes from bad piston rings.

I think carb because you said backfire and sputters, many of the old carbs is just worn out. My own wasnt starting right, popping, sputtering, wouldnt idle etc. and an 08 carb made it pretty much a new bike. Be sure to get a 06 + carb, it has a leak jet, it makes the bike a lot more smooth. The leak jet does so that the acc pump is only activate if needed, where the old ones always just follow the slide.

When you order the newer carb you need the clutch lever bracket and all the cables, and remember to seal the old hotstart manifold.

Since you have everything a part i would check everything, get the manual to see all the tolerances, i think there is one for the roundness's too. If everything is ok, I would just go with new rings.

Do ALWAYS change the circlips if you had then out. As far as i know the 426 piston dont fit without a new rod, the diameter of the pin is different. See: http://www.thumpertalk.com/topic/935821-400-to-426-conversion-the-easy-way/

Thanks for all the input. I think I'm going to swing by another machine shop to get some things measured. As far as the 426 stuff, the cylinder bore was 95mm and that leads me to believe the previous owner already switched all that out. I guess I was asking if I could reuse the old pin.

What is the clutch lever bracket?

The pin is proberbly OK but check it anyways, and check the needle bearing too.

I guess its called the clutch perch then :thumbsup: with the newer carb you will have the hotstart up there..

Ah, I see what you mean about the perch, I think I have an ASV lever with a built in lever kicking around somewhere.

Is the needle bearing on the crank? I was trying to avoid splitting the cases, but I suppose it would be silly not to? I'm worried by the time I get it all apart I'll need a new crank after all, as that seems to be my luck. I'm looking at ~$380 for a replated cylinder (if it needs one, see below), piston, rings, gaskets, etc, and ~800 when I throw the crank into the mix with it's 2 bearings.

I just got back from the machine shop and they said it was round, but that I was .003" over 95mm. Is the cylinder suppose to be exactly 95mm? The manual didn't say, only that the tolerance was .002". Is there supposed to be room for the rings, or is the piston a little smaller than 95mm to allow for the rings.


The rod has a needle bearing where its connected to the crank.

Pistons are a bit smaller in diameter, the limit is 0.1 mm. With that in mind you can easy measure if there is too much clearance.. If you are in doubt you can call wiseco for example to hear there min/max clearance..

The manual says that the round limit is 0.05 mm :thumbsup: Read the manual carefully, the two of us will just get inches and mm mix up I think :thumbsup:

Thanks again for your help. To make things simpler, I will convert for the both of us: my cylinder was 95.076mm. If I'm understanding you and the manual correctly, the cylinder should measure 95.000mm brand new, and 95.050mm is the loosest it should be before replacing, and I am past that limit, so I should replace my cylinder.

There is a little slop at the bottom of the rod side to side, and I was told this is normal?


The piston to cylinder clearance should be minimum 0.04mm and maximum 0.11mm - So measure your piston..

The rod needs freeplay from side to side, so yea thats normal. But up and down there shouldnt be much, about 0.03mm I'd say... If it feels like alot measure it, but it can be tricky to measure correct..

I felt no play up and down on the rod, so I'm going to leave the crank and rod alone. I just ordered a new 426 piston, rings, pin, circlips, base and head gaskets, and head bolts (manual says not to reuse them IIRC). Once that comes in I'll ship the cylinder out to get plated.

Fingers crossed.

What is the proper way to break this in, in your opinions?


Great. Dont know what the break in procedure is, i never had a new bike or replated any of mine :thumbsup: but the manuel says something about it :thumbsup: Hope you get it running soon..

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now