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Since what we're talking about is a 122 I checked the manual for that car.
The headbolts are very similar if not identical to those on the B20 and B30.
It did not mention lubricating the bolts or threads one way or the other.
My professional experience is with industrial engines and aircraft engines,
and both of them require lubrication of the bolts. If nothing else it gives
some degree of consistency in loading by providing uniform friction characteristics. IPD for a time sold hardened washers for headbolts to assist
in that same situation, although the cylinder head is fairly hard cast iron
and my view is that grease under the bolt head does the same thing there also.
The original clamping force is probably the same whether the last few degrees
of torquing is by advancing the threads or twisting the shank, because as
you twist the shank, the bolt also shortens somewhat. The difference is that
if that torsion of the shank relaxes, especially if by the head backing off
to the untwisted position, you lose some of the clamping force, but there is
much less likelihood of the threads actually unscrewing unless it was not
torqued to the proper level.
It is interesting to me that in 35 years (off and on) of assembling Volvo
engines I have broken 2 headbolts, both on the same engine and both because
of rust in the threads, while I was torquing the headbolts to the final 70
foot-lb. I have never broken one under any other circumstance and I have
been cleaning and greasing the threads for a long, long time. Neither have
I seen any evidence of ill effects of the possible overtorquing.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma
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