Broken Valve!!

[yrider426]

So I was riding down the trail a week ago on my 2000 yz426, and a valve broke and lodged into the piston and wreaked havoc on my head. Does anybody know how or why this happened or what I can do so that this doesn't happen again. Heads are kind of spendy (know where to find a good used one?). I had the valves checked about 2 years ago and the shop said they were fine. I don't ride all that much. Any help?

[Murmy26x]

i believe most commonly a valve spring fails, and the piston hits the valve, breaing it and the start to wrecking stuff.

Keep track of hours on the engine, and take the valve springs out to check them, or even just install a new set, every so many hours, or every year.

unfortunately grenading 4 strokes is a tad expensive.

[mikedabike]

I just did the same thing to my 04. If the head is damaged it is not worth fixing unless you can find a sweetheart deal on a used one. I had a valve go through the shim and bucket and even damaged the cams. I needed an entire head complete with valves and camshafts. The parts alone at cost were going to be over 1500 bucks. I am going to part it out. You can get a used 426 for 2-2.5k.

[mj2412]

This is the life cycle. Titanium valves has the bad habit to break suddenly when you have enough hours on it. Steel valves will wear out more so you are able to recognize it. The only thing you can do is to change the valves and springs after a defined time. But how many hours i can´t say and i think it is really a matter of the maintaince and riding style.

[grayracer513]

The 2000 model YZ426 has stainless valves, for one thing.

Stainless valves are made from a material that has enough native hardness and wear resistance to survive in their assigned role without help. Ti valves, OTOH, are not, and are dependent upon an extremely hard, but also extremely thin, hard coating, usually Titanium Nitride, or a similar compound.

The life cycle of a titanium valve is that it will perhaps settle into the seat at first, requiring one minor adjustment early, perhaps not. Then, it may go literally for years without once needing an adjustment. Then one day, the coating wears through and it suddenly needs a big correction. Riding the bike just a few more hours immediately requires another big correction, and the owner is pretty much aware that the valve needs replacing.

Stainless valves, however, can be a little sneakier. Whereas Ti valves basically don't wear at all for a long period, SS valves begin to wear, very slowly of course, right from the start. They will require periodic, minor adjustment, and even though they may need this, they will usually wear very little over time, even so, perhaps needing no more than 0.1mm total adjustment.

The trouble is that there is no way to know that the wear to the valve face and the seat is uniform across the entire surface of both parts, and what happens is that as the worn valve closes against the worn seat, it may start to scrub to one side as it seats. This flexes the stem, and if it is repeated over a long enough period, it can lead to fatigue and failure of the valve stem.

[King_Air]

The 2000 model YZ426 has stainless valves, for one thing.

Stainless valves are made from a material that has enough native hardness and wear resistance to survive in their assigned role without help. Ti valves, OTOH, are not, and are dependent upon an extremely hard, but also extremely thin, hard coating, usually Titanium Nitride, or a similar compound.

The life cycle of a titanium valve is that it will perhaps settle into the seat at first, requiring one minor adjustment early, perhaps not. Then, it may go literally for years without once needing an adjustment. Then one day, the coating wears through and it suddenly needs a big correction. Riding the bike just a few more hours immediately requires another big correction, and the owner is pretty much aware that the valve needs replacing.

Stainless valves, however, can be a little sneakier. Whereas Ti valves basically don't wear at all for a long period, SS valves begin to wear, very slowly of course, right from the start. They will require periodic, minor adjustment, and even though they may need this, they will usually wear very little over time, even so, perhaps needing no more than 0.1mm total adjustment.

The trouble is that there is no way to know that the wear to the valve face and the seat is uniform across the entire surface of both parts, and what happens is that as the worn valve closes against the worn seat, it may start to scrub to one side as it seats. This flexes the stem, and if it is repeated over a long enough period, it can lead to fatigue and failure of the valve stem.

Well said! Based on this statement how many hours do you recommend between inspecting the valve faces, seats, etc, or do you think that replacing them at a set time as prentivative maintenance is a better option. I am curious as I have a 2000 426. I don't know how many hours were on it since I got it in 2005. I checked the clearance when I got it-in spec. I put 23 hours on it and did the 450 exhaust cam and reshimmed of course for the different cam. I now have 14.8 hours on it since the new cam and plan to check clearances @ 20. As far as I know the valves, springs, seats, etc are all orginal.

Thanks

[grayracer513]

I don't have a recommendation for you, really. I'd go by the total amount of wear to the valves, judged by the amount I had to shim them, and how long it's been run since it moved that much.

One thing about the OEM stainless valves, they're cheap. Real cheap. The exhaust valves are $18 each, and the intakes are $8 apiece. If there's any doubt, just replace them and have the seats refinished.

[King_Air]

Thanks Gray. I am planning on going through the top end this winter.

[yrider426]

Do you know the factory shim size. I looked in my service manual that came w/ the bike and I couldn't find anything about shim sizing. Thanks for your help.

[grayracer513]

The factory shim size is whatever size it takes to correctly adjust the valves at the factory. I find they are usually between 175 and 190.