HOW TO INSTALL AND TORQUE BOLTS
Lots of questions going on about Head Bolts. Here's how I deal with them (never had a head gasket fail) First, when you torque a bolt, although you are measuring the resistance
to the bolts turning, this is not to determine how tight the bolt is,
but rather how much stretch it has.
Head bolts are actually stretched and that’s what holds the
head on. It's just
impossible to measure otherwise. Race
engine manufactures will measure the length of a bolt on a connecting
rod (because they can), knowing how much stretch they need. Before installing the head, run a tap into the threaded holes in the
block, and make sure you clean out any debris. Inspect the bolts for damaged threads, pitting, rust etc.
If there is any damage, toss them unless you absolutely have to
use them again. They will
stretch differently if rust or pitting is present.
Older head bolts are torqued and when removed basically will
return to the original length. You could measure this length if you can find the original
specs. Newer vehicles tend
to use head bolts that are "Torque to yield"
These bolts have to be replaced, as they permanently deform when
torqued. Our old stuff, we
don't have to worry. When you install the bolts, ALWAYS oil the bolt threads (just coated,
not dripping) if the threaded hole in the block is a blind hole.
If the head goes into the water jacket or joins an oil gallery,
use silicone. USING STUDS. When using
bolts, the bolt actually will twist like a torsion bar. Friction in the hole in the block can also help give a false
torque reading because of the resistance (hence the oil, or uncured
silicone acting as a lubricant). Once
you remove the torque wrench, the bolt will naturally try to
"unwind" itself. Tightening
a nut on a stud creates a straight pull on the stud, and not a torsional
twist (which is what a torque wrench is also measuring).
Straight pull equals more precise torque readings. THE
RIGHT WAY TO PUT ON WASHERS If the head bolt has a washer, look at the washer to see which way it
was stamped. One side will
be minutely convex, the other concave.
The convex side ALWAYS goes towards the head of the bolt.
Looking at the concave side, around the hole, you will notice the
edge is slightly sharper than the convex side, from the stamping
process. This sharper edge
could in theory nick the bolt where the head meets the shank.
Yes, this correct way to install washers should always be
followed, not just with head bolts. When I torque heads, cranks, rods etc, I follow the manufactures pattern
and steps first. Then I
repeat the pattern at the final torque setting twice. If even one bolt
or nut turns, repeat, until you successfully go around twice and nothing
moves. Sounds like a lot of work, but believe me it only takes one or two extra
times. Any more, then
something isn't seated, or the bolts are toast. I worked as a mechanic for the local phone company (fleet of about 900
vehicles), and we could not afford the down time to bring vehicles to
retorque the heads after an overhaul.
Like I said, I never lost a head gasket!
My personal vehicle I can afford the time, but only retorque
bolts that were lubricated with oil, and leave the siliconed ones alone. Sorry I got so long winded. Hope
this helps. Bob Winslade
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