>Bob Winslade
> I never re-torque heads. BUT, when I install them, I follow the
> manufacturers recommended pattern, (from the center out), and do so
> sssslllloooowwwwllllyyyyy. If you use a "click" type especially. When
> complete I repeat the pattern. If even one bolt turns, I finish the
> sequence (pattern) and repeat. I do so again until I go through the pattern
> twice without bolts turning. Doesn't take long, compared to pulling the
> cover off to re-torque. I worked in a garage for several years, dong
> reparis on the local phone company fleet, and never had a head gasket fail.
>
> Torquing is actually a measure of bolt stretch, but there is no way to
> measure the increased length of a head bolt, so they use the resistance to
> torsional twist. The newer head bolts in many cases are torque to yield
> bolts, because the stretch actually permanently deforms the shank of the
> bolt. If you took a new torque to yield head bolt, and compared it to a
> used one, you could measure a slight difference in both length, and diameter
> of the bolt. This permanent stretch allows for heat expansion, and
> different heat/cool rates between aluminum heads and cast iron blocks, while
> still firmly holding the head in place.
>
> Enough long winded stuff. IMHO, torque carefully once, take your time, and
> forget about it.
Okay, while that may be true of the newer style, torque to yield, head bolts
aren't we using the OEM bolts from Nissan, with a few folks using stud kits
from suppliers like ARP? Bob, you mention the different heat cycle rates of
the aluminum heads and cast iron blocks. That's why I'd think you'd need to
periodically re-torque the head bolts even using an aftermarket torque once
gasket.
If the bolts work lose over time due to the different heat cycle rates of
the head and block, I don't see how any head gasket's going to maintain its
seal. Only twice, admittedly 30 years ago or so, I had head gaskets fail.
Once each on a U-20 and L-16. I had not re-torqued the bolts on either.
Since then, on engines that I've re-torqued regularly I've not had a gasket
failure.
I loosen all the bolts and then re-tighten them in a spiral pattern from the
center out in three passes. I know some folks do one bolt at a time, but it
seems to me that tightening from the center out distributes the clamping
forces more evenly, like smoothing a sheet on a bed from the middle to the
edges. Since I'm not certain my ratchet torque wrench is accurate I usually
use my beam type wrench. One of these days I need to cobble up a torque
wrench test like Tom Walters outlined a while ago.
FWIW,
Ron
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