*thanks all*. I cannot find a reply from a Stan or a Colin above - how can I find them?

big believer in RTFM but in this case it’s ambiguous at best.

 

Stan = MrMerlin, ............Welcome to the show my friend !

 

It should be crystal clear that if the WSM method cannot be safely used there is something drastically wrong and indeed I believe there is.

The problem is that when just about any corrosion mechanism takes place atomic hydrogen is liberated by electrolysis and I suspect this is precisely what happens in many 928 head gasket examples once coolant gets between the gasket and the head surfaces. The ethylene glycol is stable until such time as the buffering agent is exhausted and when that happens the glycol breaks down into organic acids such as glycolic acid at temepratures in excess of 55C and therein lies the devil of ethylene glycol. That is when the fun starts and atomic hydrogen is liberated or so I suspect. The liberated hydrogen has no impact on the alloy of the block and heads that it will pass through easily but as and when it hits the high strength steel of the bolting the hydrogen will embed itself into the 10.9 bolting material when in the stressed condition. The atoms do not want to exist in the atomic state and after getting into the lattice structure they tend to combine to form a hydrogen molecule. Unfortunately the volume of the H2 molecule is greater than the volume of the two atoms forming it and this causes very high stresses leading to cracks. This process is known as hydrogen embrittlement and is a big problem in steels that are vulnerable and only happens when the steel is in the stressed condition. This phenomena does not change the yield point or UTS of the material but it does change the stress/strain curve of the material and explains why the required stress is attained in the embrittled condition with less rotational effort. The bolts in the cylinder head are the same material as those in the rear of the front lower control arm and the correct torque for those is indeed 90 ft lbs so using a torque wrench will indeed save the day but the problem is when the bolting has been compromised.

As I stated earlier if the bolting has not been cracked the situation can be reversed by heating but the chances of that being the case are probably very slim. It is not difficult to visualise that if the correct stress levels have been attained by two 90 degreee turns then adding a third turn would likely increaase the stress by another 50% and in such condition will either fracture the bolt or rip the bolting out of the base it fastens into.

Some time ago in one of our dialogues regarding head corrosion GB astutely observed that on many examples he found that when reusing the original studs the logical torque was achieved after only two 90 degree turns- not muhc else to say really!

This also led me to wonder why it was that later models with set bolts only need two 90 degree steps whereas the examples with studs had three 90 degree steps. It should be obvious that if set bolts are used they will have the same threading as that in the casing they bolt into and logically that will be a 1.5mm thread pitch. Two 90 degree turns will thus stretch the bolts 0.75 mm. This leaves me suspecting that in the case of studs they used a different thread pitch for the nuts that logically would be 1mm pitch to give the same 0.75 mm of stretch. Much appreciate if anyone can confirm such just for purposes of my interest as this is the only reason i can think of as to why the procedures are different. If anyone has any other suggestions- for instance a lower grade of bolting material maybe?- kindly chime in.

 

Greg told us back in the thread new studs are an different material than the old ones.
So much room for failure.