I know, you're absolutely right about sticking torque. Only problem is that
some gaskets don't like to be relaxed. Fortunately no LBC that I know of,
but if you work on German cars (my experience is limited to building engines
for rabbits for racing) you find yourself not only blowing head gaskets if
you relax a bolt to retorque it, but throwing away head bolts every time you
use them--they are necked to stretch and don't work a second time.
The old bevel gear ducatis were pernicious about retorquing. If you relaxed
a head bolt you had to pull the engine and replace the head gasket. If you
didn't you'd see the customer again in about five days.
The guys using shim steel gaskets probably shouldn't retorque. I suspect the
water and oil seal of these things is pretty marginal. Every time I pull one
I see signs of water movement under the gasket. Composite gaskets are
another story--you have to retorque them, and I think you should do it more
than once. Of course that's how I first met Greg Solow--retorquing a head
bolt at the Monterey Historics. It was the third time I had retorqued and it
was probably just fiddling. SNAP. Long stud--broke off way down in the
block. Fixed it in Greg's shop in Santa Cruz.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Randall
Sent: Saturday, April 09, 2005 4:01 PM
To: fot@autox.team.net
Subject: RE: re-torque cyl heads
> That all depends on so many things. Torquing a bolt is not very
> precise no matter what.
No argument there.
> If you have a hard washer, good lube, and clean bits the stretch you
> get from a given torque can be much greater than dirty bolt, soft
> washer, no lube.
Quite true, which is why good torque specs also specify the condition. Most
(not all) OEMs specify clean, dry, original quality hardware ... somewhere,
if not where the torque specs are given. Again, I was taught to assume
clean & dry unless otherwise specified.
> That's both good and bad--stretch is what breaks the bolt (close
> enough anyway). If you really want answers about this kind of stuff
> you need Carroll Smith's book on fasteners.
Got my copy of "Screw to Win" right here. But he's really addressing a
different area, that of engineering fasteners, not of following the OEM's
specification. I was talking about how to follow the OEM's specification.
Try it the next time you torque a head nut down dry. Stop 5 ftlb short of
the goal, and then try to turn it again. Many times it won't turn at all.
But if you back it off and turn all the way to the goal, it will turn
farther.
Randall
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