OK, let's talk torque here. (one of my favorite topics :-)
Bolts (and studs) hold in tension, that is along their axis, however we have
no direct way to measure how much clamping force is being generated. So we
measure how much twist (torque) is applied to the bolt in question and this
gives an idea of how much clamping force is being generated. Why do I say
idea you ask, read on.
Torque specs are always given for new oiled bolts in clean holes. Lets look
at three different cases. First would be a clean new bolt with oil in a
clean (ran a tap through) hole. B) would be a new dry bolt in a clean hole
and C) a used slightly dirty bolt in a dry non cleaned up hole. A) would
have the most clamping force B) the next highest clamping force and C) the
least. An extreme case might be where a bolt has to screw in 1" in order to
be tight, but 3/4 inch into the hole there is so much debris that the bolt
will go no further and the specified torque is reached even though zero
clamping force is being generated!
So what is torque to yield? While the engineers have no clue as to what the
condition of the hole that a bolt is being screwed into is, they can predict
exactly how much force it will take to stretch a bolt of a given diameter.
This will tell exactly how much clamping force is being generated.
So going back to the three examples give above if each of the bolts were of a
torque to yield design and each were turned an additional 90 degrees each
would stretch to give each of the three bolts to the same clamping force. In
other words torque to yield is less sensitive to cleanliness of the hole,
application of oil etc.
Fasteners used in torque to yield applications come in one of three "flavors"
1) replace every time, 2) Replace every x number of uses, and 3) OK to reuse
if total length does not exceed X.
Torque to yield has been around the auto industry since the 70's and is used
by almost all (if not all) car makers. IMHO (and this is my field) it makes
for a better repair as it simplifies the repair process. Needless to say
torque to yield will not cure a poor design or cover for a mechanic who does
not follow procedure.
Jim is correct that no fasteners on MG's are torque to yield.
In a message dated 03/20/2000 11:25:40 AM Pacific Standard Time,
> "Torque to yield studs" were employed in the '80's by Ford as one of their
> terrible "better ideas". The intention was the stud would stretch as it was
> tightened, but would only retract back very slightly when removed. Their
> "effectiveness" was largely responsible for thousands of cracked heads and
> least one safety recall if the head should crack on the exhaust manifold
> (leaking oil + hot exhaust manifold = fire).
> Ford was so remiss in informing mechanics of this innovation, that some of
> own dealers as nearby as Ypsilanti, Detroit and Ann Arbor, MI were unaware
> studs were never to be reused or re-torqued. The results of reuse was head
> cracking and warpage.
> I have never seen nor heard of any instances where MG made use of them.