If I over torque head bolts...

What uses M12?  Engines of that output are probably 8, 9 or maaybe 10mm.

There are a lot of odd size head bolts like M9 and M11.  Keeps you from using any random hardware.

M9x1.25 in 80mm and 100mm length

super stupid but now that I know that I can order from new sources and get stuff sooner. Trouble is getting head gaskets in a reasonable amount of time 

If you didn't heat cycle the engine, the gasket is probably fine.

If you remove the head and none of the graphite "sticks" to the deck surfaces, you are definitely fine.

If you replace the bolts without removing the head, you're 100% fine.

OTOH, you're also probably perfectly fine to just loosen everything and torque to the proper spec.

Yeah I think you're fine.  50lbft isn't enough on m9 to damage the bolt or gasket.  A friend forgot where he was in the torque sequence once because he got distracted, so he just loosened all the bolts and started over.  Turned out just fine.

In reply to Knurled. :

Volvo, apparently.  My Volvo has m12x1.5 head bolts.  Since that's apparently on the big end, I guess that explains some of the bulletproofness of the head gasket on those engines.

M12x1.5 is probably the most common size for lug studs/nuts and are frequently spec'd around 100 lb ft. They're also commonly overtorqued by a whole lot before anything bad happens. So a good m12 bolt can hold a lot of torque. Of course, every bolt is only as strong as the threads it goes into. 

Gotta love headbolts. They're about the most paranoia-inducing and tiring (doing all 10-20 1/4 turns on TTY bolts in the span of a few minutes) bolts you typically run into on the car.

If the op started to feel them let go, I'd suspect he felt threads in the block starting to yield and not the bolt.....

In reply to jfryjfry :

Yep.  The bolt is high grade steel with rolled threads.  The "nut" is cast iron with cut threads.   Part of the reason my Quantum project stalled was because I had to scrap the original engine block when the deck pulled when I was assembling it.

This is generally why studs are better than bolts.  It's not that the studs themselves are stronger, but because the studs are generally going to engage as many threads in the block as possible, and when you torque them you are torquing a high quality nut on those high quality threads.

If OP said he felt them giving way when torquing, I missed it.

Knurled. said:

If you didn't heat cycle the engine, the gasket is probably fine.

 

If you remove the head and none of the graphite "sticks" to the deck surfaces, you are definitely fine.

 

If you replace the bolts without removing the head, you're 100% fine.

 

OTOH, you're also probably perfectly fine to just loosen everything and torque to the proper spec.

I did say that they began to yield, and stopped immediately.

engine did heat cycle, but not more than a half a dozen times at most.

we pulled the head off again and only one burr was left in the corner of the block, it was at most a single layer. If all you did was a visual inspection of the gasket you more that likely wouldn’t have noticed.

for a general story of what happened, block got pulled yet again. This time we bought a straight edge and checked to make sure it was in spec (.15mm warp or less). It was, checked it on 24 different lines per surface and feeler gauge didn’t even try to sneak thru.

buddy and I assumed that the gasket was still good enough since there was no chunks missing or burn marks and threw it back on. (New one is still on route and I’ll probably replace it anyways.) At that point we threw a compression tester on and all 4 cylinders were within 5lbs of 120. We called that good. Since the radiator/water jackets had been flushed about 4-8x depending on which piece you were talking about, figured they were OK. Tested the water pump next by leaving the thermostat out and revving the motor to see water flow changes. There were.

that really only left the thermostat to be the culprit, so back on it went. It VERY quickly warmed up and started to overheat. Also the flow was BARELY moving in the radiator compared to no tstat. It might as well have been standing still.

that seems like the issue this time!

Took this from my Zetec manual.

Bolts are TTY and must be replaced.

Stage 1:   In proper sequence.   12-18 lb/ft.

 Stage 2: 26-33 Lbs/ft.

Stage 3:  Tighten another 105 degrees.

How much torque is added by step 3 seems to be the question.

In reply to iceracer :

Generally it's not very much, if they are genuine torque to yield.

In the case of the OP, he's talking about an engine designed in the late 50s, so definitely not tty. 

In reply to oldopelguy :

Somewhere I was reading that Datsun Roadsters had TTY bolts in some applications.