YZ426 head

I've got a 02 yz426 and can't figure out why the head self destructed. Took out the piston too. It ran fine up until the time I heard an awful sound. Has anyone had the same experience? Does anyone out there have a head they want to sell?

I've got a 02 yz426 and can't figure out why the head self destructed.

Are you saying the piston didn't have anything to do with it?

Heads do destruct, but typically they have help from the piston. At the very least a camshaft steps up to the plate...

It looks like one of the valves didn't get out of the way in time. The piston is in tack, just hammered by the valves.The noise I heard sounded like the timming chain slipping over the gears. The cam and cam gears look fine, but I haven't had a chance to look at the crank gear.

Is very difficult to understand the reasons of a damage only with the noise. Remove head and check all internal parts. If the engine can start and run, there is the possibility that the damage isn't so terrible... i hope...

Did you check your cam timing before you took it apart? It would be interesting to know if your chain did indeed slip over the gears.

Doug

Been there, a year ago my 01 wr 426 went ka-boom. The center valve keeper let go and sucked in the valve. Wasted head, piston and three valves. Bore was fine. I rebuilt with a 13.5 to 1 piston tit.YZ valves and a new head with all new internals. And one new intake cam. Total bill...$1075.00 and I did all of the work. The bike now makes 54.6 ponies on dyno useing a 14/50 gear on 91 pump gas. Jetting was simple to dial in with that pistion, only one problem, the front tire will not stay on the ground in third. Thinking of using tractor weights to hold it down. :) Good luck with the rebuild dude!!

Been there, a year ago my 01 wr 426 went ka-boom. The center valve keeper let go and sucked in the valve. Wasted head, piston and three valves. Bore was fine. I rebuilt with a 13.5 to 1 piston tit.YZ valves and a new head with all new internals. And one new intake cam. Total bill...$1075.00 and I did all of the work. The bike now makes 54.6 ponies on dyno useing a 14/50 gear on 91 pump gas. Jetting was simple to dial in with that pistion, only one problem, the front tire will not stay on the ground in third. Thinking of using tractor weights to hold it down. :) Good luck with the rebuild dude!!

Whoa! Got to love retail. Just remember most of the OEM parts can be marked up 50% from cost, so shop around. Also it helps to know someone that works at a shop or work at one like myself!

I rebuilt with a 13.5 to 1 piston tit.YZ valves and a new head with all new internals.

OASIS_FLAME

My '02 had an intake valve break. Yep, the titanium valve broke off at the base of the stem and wrecked the head and piston, and bent all the other valves. 1600 miles, valve clearance was properly ajdusted.

Anyway, I am interested in your repair. I didn't know the WR and YZ valves were different, can you tell me what is different and why you chose YZ? Are the springs different also? Who makes the 13.5:1 piston and where do I get it? As long as I need to spend lot's of $$$$, I may as well get more performance too.

Was your rod bent? From what I can see, mine looks OK. Does anyone know how I can check to be sure the rod is not bent? I would hate to split the case unnecessarily, but on the other hand I certainly don't want to put it all back together with a bent rod?

Thanks in advance,

x2

Wiseco makes the 13.5:1 piston

Valve breakage is a symptom of a bigger question.

What touched?

99.99% of the destroyed heads I have examined over the years have been traced to OTHER problems.

Something touched the valve head and it broke.

Valves, specifically those of the one piece variety, have a much higher chance of survival than those of the fused variety in situations of contact but they will fail under constant contact.

Your are looking at an effect not the cause.

Something touched.

Sorry to pontificate, this is just my experience.

So you work at a shop do you? Can you get me a deal? Talk to me!

Were did you get your parts?

Call www.ENGINEDYNAMICS.COM

They can fix your head. Probably for way less than a new one. Seriousely. Call them They are good.

YZMAN400,

Thanks for the link, I'll check with them. If they can fix this head, they are good. The valve that broke got pushed sidways into the seat about half way cutting 2 gashes in the seat about 3/8" long.

x2

x2, upon break down of the motor, I discovered the valves were steel, as were the earlier WR heads. In 01 Yamaha changed to tit. valves. A Yamy tech told me the biggest reason for this was to correct the valve and valve keeper problem, valve weight was not a major concern although it helps. My head may have been a late 00 WR head. So I just upgraded to 01 specs. with YZ springs(little stiffer). Wiseco makes a 13.5 to 1 piston just for the WR motor. The small end on the rod is of different size than the YZ. There was no harm to the rod when the motor went bang. The head was too wasted to repair, almost cracked into two pieces. The best price I found was at powersportspro.com for 601.00 bare. Nothing in it! I still have an 01 YZ piston, rings and c-clips for sale, brand new still in the boxes, 12.5 to 1, $100.00 firm and I'll pay for the shipping.

Hope this info helps. BTW the dyno # are using WR timing and stock needle with air box top off and grey wire pulled. I may switch to YZ timing and needle this spring just to un-leash a rocket for alittle while to [@#$%&!] spank this Husky 610 I've been riding with. :):D :D

Oasis, you must have had a 400.

The wrist pin size changed in '01 to 19mm from 18mm.

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