Factory Broken Head Bolt?

[Real638]

I've owned my '78 (a Black Pearl) since '81. It hasn't been driven since about '90. Over the past few months I've been working on getting the old girl running, (which it does - sort of). It has 184K on it & the engine has never been apart. Original down to the injectors, timing gear/chain, etc.. While replacing manifold gaskets I noticed a lot of oily goo on the block coming from under the edge of the head at the #4 cyl. Today, while replacing valve stem seals, I found out where it was coming from. The head bolt between #4 intake & #5 exhaust is broken & obviously had been for a long time. I planned on rebuilding the head anyway, & the machine shop can take care of the broken bolt. Cam turns free so there is little or no warp of the head. Again this engine has never been apart so --- my guess is some guy doing engine assembly at the factory broke it. Since the car would run fine, he simply left it rather than pulling an otherwise good engine of the line. Anybody had a similar experience? By the way, while replacing the clutch, (also original), I found one flywheel bolt had backed out almost half way. All the others were torqued to spec.

[siteunseen]

My '77 had one rolling around in it's N47 head. I didn't know what it was at first, the recess for the hex bit was completely full of carbon. I figured it broke over time, heating and cooling down of stressed bolt. Replaced with the ZX turbo bolts.

[conedodger]

It broke over time. Heat cooling cycles do that. Very common in Porsche 911 SC 3.0 liter motors.

[hr369]

Hopefully there is enough sticking up above the block surface that they can get a
tool on it to extract it or weld on another bolt. My 78 also had a broken head bolt too. I bought a set of craftsman bolt extractors to get mine out. They're the kind that you hammer down over the top of the broken bolt and it cuts in. Then you just wrench it out. Fortunately i had about 1/2" sticking out
above the block surface.

I replaced the bolts with a set of turbo bolts for a little bit better clamping force and hopefully better
elasticity.

You're saving yourself alot of grief having the machine shop fix it.

[Real638]

It broke over time. Heat cooling cycles do that. Very common in Porsche 911 SC 3.0 liter motors.

I love these theoretical discussions. Had the same on the subject of the always broken rear manifold stud a few months ago. I've broken lots of bolts & studs off in lots of engines - always with a twisting motion. I absolutely agree that heat & time weaken them. But I have difficulty with the concept that heat stress alone will cause a bolt to simply break. If it has been cracked, even so small as to be invisible, the two factors together seem the much more likely culprit. Thats been my idea on the manifold stud break. The stress put on it when the engine is lifted cracks it. Maybe not visibly, but enough so that heat then takes its toll & eventually it breaks where the crack was - where the stud goes into the head. I'll be keeping the machine shop busy.

[Zed Head]

This web page does a good job, in my opinion, of explaining cyclic loading based fatigue, and how it is different from yield strength. You only get so many cycles. Cracking or other stress-risers play a part also.

Metal Fatigue and The Factors Which Influence Fatigue, by EPI Inc.