Cause of a Cracked Cylinder Head?

I noticed my 2006 WR450 was leaking a slight bit from the head gasket area. I took it to the mecahnic who put a big bore kit in it for me last spring and when he tore it down he said the head was cracked!

My question is, what would cause the head to crack? The obvious answer is overheating, but, even though I did ride several times on slow single track in hot weather this summer, it never steamed or boiled over. I did have to top off coolant here or there, but nothing out of the ordinary.

I'm concerned about the money of replacing the head, of course, but I'm just as concerned about how to avoid this major expense in the future.

Make sure you trust this mechanic... Pretty tough to crack a head... I'd want to see it, and hear his reasoning as to why he thought it might've cracked....

maybe you got a reject head,with all the recycled metals now days you might have gotten a bad batch of aluminum?

Thanks for the input. I have no reason to distrust my mechanic, but I will be sure to examine the cracks myself. Could his work on putting in the big bore kit have caused the problem?

Do you really think Yamaha used "bad aluminum?" Has anyone else had this problem on their WR450? If so, how can I get Yamaha to pay for the replacement?

You put a big bore kit in a 3 year old bike it blows up and you want Yamaha to pay for it?

Point taken, beezer. Here was my logic in even raising the question. 1. If the head "meltdown" was due to use of shoddy aluminium, that would have nothing to do directly with a big bore or the age of the machine. 2. In searching TT re: this issue I read of several cases where KTM and Honda had replaced heads because of manufacturing flaws. Hard to believe, but apparently true. 3. The real question was the one about has this happened to others, rather than the delusional thinking that Yamaha would do anything for me.

Never heard a head cracking. In the old days XR250/600's use to crack between the valves and spark plugs due to heat and lack of material. But a crack that leaks water out no way.

I hear you Yammerman, I believed in Santa Claus until I was 28.

You may have gotten a head with a casting flaw in it but I doubt Yamaha will do anything for you.

Most of the 450's that I have seen with big bore kits end up with a rod sticking out of the cases.

Well, I visually checked out the head. It turns out the crack is sort of a hairline fracture near the head bolt closest to the exhaust manifold. My mechanic says there is evidence of the aluminum shrinking from being too hot, thus causing the minor fracture, but also making the head gasket leak. You can tell the head gasket was overheated in that area because the coating is peeling off. I'm guessing the leaking was due to the head gasket itself being fried rather than the head be so warped or cracked that coolant was oozing out.

Based on the location of the "meltdown" in the head and the area fried on the head gasket, the excess heat was clearly concentrated around the exhaust port. This makes a little more sense about what may have caused the problem, but I'm still perplexed as to why there was no indication of an overheat (e.g., boiling coolant).

How can I keep this from happening again? Temperature gauge (I don't know what's out there, but I guess I'll start looking), specialized coolant (e.g., engine ice), radiator fans?

Do you guys think it's all due to the big bore kit? If so, how could I help a cooling process specifically for this 487cc?

Maybe I should get rid of the kit to avoid this and, as beezer warns, a rod punching through my case.

My buddies and I have played around alot with big bore kits on 4 strokes and most of the time there have been problems with head gaskets blowing and connecting rods breaking. We ride flat track and ice in the winter. I think we could fill a small dumpster up with destroyed motor parts.

The bigger motors put out more heat and the cylinder walls are thinner so there are bound to be problems.

I got sick of it and left my YZ450 stock and haven't had a problem....yet.

Beezer have you had problems on your big bore kit on the wr250?

My WR250 has a 262 kit in it and they seem to be trouble free. It's only a 5% increase. I've done 2 other 262's and both are still running. When you go bigger than 270 cc's on them they tend to blow head gaskets.

So, I got my bike back from the mechanic whom I had do some repair work on the hairline crack in the head. I'm not confident it will do the trick, but it was better than the price of a new head. The leak was coming from the fried head gasket rather than any warp or the like in the head. No coolant in the oil, thank goodness.

Here's my question now. In asking the mechanic how I can avoid the big bore kitted engine from overheating again, one of the things he suggested was to use higher octane fuel (than 91). He said it would burn slower and may therefore help reduce the heat put out by the oversized motor. Is this true? I had been running premium gas with no preignition at all.

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