loose headbolt

[rwbrick]

What dastardly thing would cause a headbolt to loosen. One has loosened completely where it bangs (or wedged) into the top of the rocker cover. It (or maybe its washer) bangs violently against the rocker cover when the engine runs (usually at a slow rpm only). I retorqued it once, but it just re-loosened after 2-4 minutes of engine running. Two adjacent bolts were slightly under torque; all others are at spec. For the record the bolt in question is upper front corner on the rear head.

[s13sr20chris]

hi and welcome to nico

hmmm, what kind of vehicle and engine?

the only thing i can think of that could cause a head bolt to come loose is a bad bolt, bad block, or major vibration. the last one would still take a very long time and would likely affect them all. are you sure the threads in the block are ok?

[rwbrick]

I'm quite sure the bolt and threads are O.K. It torqued (~2 years ago) and re-torqued normally. My engine is the 3 liter V6

[rwbrick]

Thanks for the asistance. I've concluded that the head gasket blew in a peculiar way that in fact opened a passage from the cylinder to the bolt hole. I removed the rocker cover and pulled off a completely free and clean bolt. Turning the engine over (spark disabled) produced a swoosh out of the bolt hole on the compression stroke. There was no sign of coolant or oil. I don't think the head was planed to much before. The bolt would tighten to proper torque; the earlier planing was just enough to level the head face which was warped about 0.002" I'll know for sure after the head comes off (groan!) Don't know why the gasket blew, especially so oddly. I never heard of such a thing.

[DAEDALUS]

Wow... If you ever figure it out completely post back.

[s13sr20chris]

Wow... If you ever figure it out completely post back.

no doubt. is it the sohc motor?

[rwbrick]

My suspicions were confirmed. There was a short, neat, about 1/4" wide channel blow from the headgasket from clyinder #1 directly to the headbolt hole. Now, of course, the mystery is, why would a head gasket blow in such a fashion?????

There is some funny minor pitting of the head in a 90 degree "grooved" arc right at the #1 cylinder boundries; but the rest of the head face and block face in the proximity are fine.

[s13sr20chris]

i wonder...maybe poor machining or material. does it look like you will be able to deck it and go back together?

[rwbrick]

Turns out my timing belt had also slipped one tooth on the crank gear (I surmise from a partially bad tensioner) which delayed the spark about 18 degrees which means its firing at peak compression which burns much hotter and faster which pitted (burned) the little groove in the head face which weakened the head gasket there (the pitted groove was right under the gasket lip). It got a little start randomly in #1, but that little start made the head face more lilely to burn and pit more.