Well, I'm on a roll here! This is what I spend even more time doing than
balancing engines!!
Basically, this article is very good, and could give any layman as much as they
need to know about fatigue.
Although our cylinder heads are iron, the most relevant bit, about ensuring
bolt heads are adequately torqued, is just as relevant to us. The bolt loads
need to be more than the maximum force exerted by the combustion gases on the
cylinder head, plus a safety factor. Of course, this is also necessary to
ensure proper cylinder head gasket sealing, too.
Aircraft engines (and automotive engines with Aluminium cylinder heads) have an
advantage over us in one area here - Aluminium expands with heat much more
than iron or steel, so as the engine heats up, the head expands, and the bolts
tighten. With steel bolts in a cast iron head, you don't get this.
However, we have an alternative advantage - cast iron is MUCH stronger in
compression that aluminium, particularly at elevated temperatures. Therefore,
we can happily torque our bolts up all we want, without causing damage to the
head, while if you over-torque the bolts on an aluminium cylinder head,
particularly if the bolt bosses are skinny, the bolt boss will start to yield,
and you start to lose some of your bolt load. Not good.
The only problem with over-torqueing on an iron head is damage to the cylinder
head bolts/studs themselves. Many modern engines are designed to use bolts
that are tightened just to the point of yield, to get the maximum load, but
this can only be done with a special torque wrench, that can detect the slight
drop in the torque required to turn the bolt as the yield point is reached.
Without this, you can end up yielding your bolt or stud, and losing strength.
If you want to increase the bolt loads (for instance on an up-rated engine,
supercharged etc.) then you can, but you will have to use bolts or studs of
superior steel - these steels do exist, but whether suitable studs for our
engines are available I would have no idea. There is also no way I can think
of for testing what grade of steel your studs are, without a destructive test
on a sample - and none of us have the equipment in our workshop to do that!
So you just have to trust the supplier, and hope they know what they are
talking about. I have come across cases (usually with less critical
components) of after-market spares being made of a lower grade steel than the
original part - they look exactly the same, and will work for a while, and by
the time they break you are too far away to remember where you got the dodgy
part, and they are probably (just) out of the usual 1 year guarantee.
There is also the issue of head fatigue cracks elsewhere, which can be caused
by the thermal (heat-up when you start your car, cool-down when you stop)
cycle, or the firing (no-load, bang, no-load, bang etc.) cycle. However, if
an aircraft engine goes bang due to fatigue failure, you die; if a Spitfire
engine goes bang due to fatigue failure, you end up sitting at the side of the
road watching steam pour out of your bonnet. Not great, but not dead either,
so we don't need to worry so much about it. And, as many of us who have
dismantled our engines well know, we can pull a head and find that it has a
fatigue crack, but has been running fine for years. So the general message,
(borne of the experience not of me but of others out there) is not to worry
too much about cylinder head fatigue, unless you are taking the head off
anyway - then check for cracks, and replace if necessary while you have the
chance.
There, that's todays lesson over. Next week - elasto-hydrodynamic bearing
analysis (only kidding!)
Richard and Daffy
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