Bursting valves, please, please help

I have a 2000 WR400 all free mods have been done. I have been using mobile 1 oil and changing it about every 250-300 miles. Last weekend two of my intake valves blew up destroying my head assembly. The piston has scaring the cams are banged up pretty bad, pieces of the valves were everywhere including protruding into the spark plug cavity. My jetting was right on (james dean recommendations ekn needle) there wasn’t any popping or backfiring. I am a full time collage student and am going to have a hard time paying $1500 for new parts. Any suggestions as to where I can get a used head assembly? Also what could have caused this, and how concerned should I be about my rod being defective now? I have read other posts of similar types of problems involving the intake valves going out. If I fix my bike what can I do to prevent this from happening again? Please help; any info, suggestions or comments would be greatly appreciated.


See the other post about cyl.



Mike can fix wasted heads.

And he can tell you what happened to it.

[ November 08, 2001: Message edited by: SFO ]

Man that sucks...Sorry to hear it. When you finally get a good diagnosis, let us know.

Good Luck...

Bonzai :)

Could this be part of the reason Yamaha went to titanium valves? I'll check out Mike's web site. Thanks for the info

Ok guys I need your help on this. I need a new head. All five valves were destroyed. The first intake valve broke at the base and fell into the cylinder. As the engine continued to run that valve was pounding the other four valves cylinder and piston to crap. At the same time the valve stem punched through the piston and the valve caps shattered destroying the cams and blowing holes in the engine case. Everyone of the valves were bent, bashed, and twisted into a very awkward formation. Wow, this sucks! My question is, are the heads the same for all WR’s Yz’s 400’s and 426’s? Can I stick a 426 cylinder, head and piston on and essentially have a 2002 WR? Or will I run into problems somewhere? Can I use a 426 head with a 400 cylinder and piston? Any info would be a great help. Thanks in advance.

Hi Speed,

Can you give us a little more info on motor?

How many miles on it? How many times had your valves been adjusted and how many miles ago was your last adjustment?

What type of riding do you do and what kind rpm's do you put the motor through?



It is possible to mix and match the 400/426 heads and barrels.

You could get a 400 head for a 426 and visa-versa.

If you are going to throw your old head into the garbage I would like to see it.

Here is a little bit I learned when my '98 yz400 dropped a valve. The head for the 426 does not have the hot start fitting in it, but can be drilled and tapped if needed. The TI valve heads should only be used with the TI valve train (lots more money than the pre TI stuff). A 426 cylinder will fit a 400, but the piston will not. When they switched to the 426 the top end of the rod was re-designed (bigger pin).

When this happened to my 400, the valve that broke turned over and punched a bunch of holes in the piston. The result was a wasted rod as the aluminum went through the oil holes in the top of the rod, and impregnated between the rod and piston pin. Luckily 400/426 rods are pretty cheap, but tearing down the bottom end will suck.

After much searching for a used head, all I found was a complete engine for $2000.00 brand new in the crate. Thr repairs would be $1000.00 for just parts, $1700.00 if I used 2001 426 parts. I still owed $1000.00 on the bike and was so disgusted I never fixed it. Most of my disgust came when talking to folks at Montclair Yamaha (Ty Davis's bike builders and sponser), Factory Yamaha (the famous BK), and notebles such as stroker. They all said catastrophic failure was very common in 400's. Major changes were made to the 426. Head, rod and carb changes were the biggest. BK told me to scrap all the 400 stuff and go 426 if I ever wanted it to be reliable. It was at that point I bought the coolest motorcycle ever made. The KTM 520 EXC. The blue beast still sits, broken and battered.

Good luck

It’s a 2000 Wr 400 with about 2700 miles. The valves had never been adjusted. I took the bike to my local shop for the adjustment and they said the valves were fine and not to worry about it. That was about 8 months ago. I race desert and do hit the rev limiter on long straight sections. When the bike broke down I was on a long section wide open. Could it be that the gray wire mod changed the rev limiter specs. Which led to me floating a valve? Or maybe it was just the fact that the valves hadn’t been adjusted. As always thanks for your help and input.

Guess what I was doing when my valve broke...racing a grass track grand prix :D with long straits. I was hitting the rev-limiter occasionally.After speaking with BK we came to the same conclusion you did. Probablly floated a valve, disloging the shim under the bucket or a valve keeper, destroying the engine.

BTW as I had expierienced catastrophic engine failure with this bike once before :D (broken con rod),I had a valve adjustment done the week before the race. :)

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