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View Full Version : A torquing ? & an observation



Techno
05-10-2001, 03:01 PM
A machanic at work told me today that all aluminum heads should be untorqued in stages to eliminate warping the heads. I just loosened the bolts, then spun them out. Do you have to take the bolts out in stages or can you loosen them all at once?
Haven't removed my heads since installing them but I don't want to warp them in the future.

Two guys were rebuilding a sight glass that contains 2,300 PSI. Each glass disc has a square cover clamping the disc in place-4 bolts each. These covers were torqued to 10 foot pounds! Even considering the expansion the parts undergo at 1,000 degrees I thought it was interesting that only 10 FT.lbs. were required. 1,300 PSI is from a pump so it still has to hold pressure when cold.
I suppose the bolts would expand anyway so they might still be only torqed at the 130 IN.lB. level.
I bring this up because it totaly amazed me and to point out that torquing is actually an attempt to measure bolt stretch.

Rickracer
05-13-2001, 07:15 AM
Hi Techno, whenever I take a set of aluminum heads off, I follow the torque sequence in reverse. Most factory manuals will tell you to do this. I think doing it in stages might be a little overboard, but it surely can't hurt.

jimmyb
05-13-2001, 09:45 PM
techno:

you are correct that bolt torqueing is done as a way to determine bolt stretch. Torqueing is a reletively inconsistent way of determining bolt stretch, though. The reason for this is because of the friction on the threads and the "calibration" (or lack of) of a torque wrench. In order to minimize friction, important fasteners that are to be torqued should be lubricated. automotive and engine manufacturors have very expensive machines that are used to torque important fasteners, these machines actually measure the amount of bolt stretch.

Dave@DunnWright
05-14-2001, 08:44 PM
Interesting subject. Most charts that specify bolt grade and appropriate torque give a dry and wet specification. i use sealant on all bolts that I torque on a Merc motor and use a "wet" number. Mercury specs are for wet torque. Most of the Nascar engine builders like Yunick, Johnson, Moore, Yates and Wilson that I read about while car racing center drilled all rod bolts for a reference point and used a bolt stretch micrometer to measure stretch and discarded any bolt that stretched too much and discarded any bolt torqued more than once. Of course they were not buying Mercury rod bolts!!!!