From the FORD GARAGE.COM
https://www.fordgarage.com/pages/brumfieldheadinstructions.htm
When re-torquing, it is usually not normal practice to back off nuts or bolts. However, on a Ford flathead, some of the cylinder head nuts can develop an increase or higher static friction as compared to the static friction of some of the other cylinder head nuts or a resistance to movement after they have been previously tightened or torqued. Or in other words, once tightened they sometimes tend to stick and the amount of torque required to get the nut to move is a little greater than the torque value that was intended to be applied.
When the torque is applied by the torque wrench in the standard way, the force must be great enough to break the nut free and move tighter. In the process, some of the torque is wasted overcoming the higher friction and the wrench may not apply the correct torque amount as set or viewed on the wrench. The result is the applied torque can be incorrect. And, if the increase in static friction is high enough, the nut may not break free at all at the torque amount set on the wrench.
In other words, the installer can be fooled into thinking that the nut is at the value set on the wrench whereas if the nut was not sticking it would turn tighter, as it should when the gasket has not completed its compression process. This possible sticking of the nuts can happen to a degree even with lubed threads after a little time goes by and a few hot/cold cycles. Talk about possible uneven clamp from nut to nut, leaky gaskets and premature gasket failure!! …. Anybody ought to be able to understand it.
If it could be guaranteed that the nuts would never stick, then backing them off would be unnecessary. However, to avoid the problem and to assure accurate re-torques and and even clamp from nut to nut, the nuts should be backed off, or cracked back slightly before being re-torqued and then retightened or re-torqued immediately.
Backing each nut off 1/8 turn does not loosen them in the true meaning of the word, loose. The procedure merely moves the nut only 45 degrees of rotation and just enough to overcome the stick that may or may not have occurred between the nut and stud threads and including the stud boss and nut face.
Since the torque readings are dependent upon the coefficients of friction present under the nut face and in the threads, there must be a smooth, uninterrupted rotation to have accuracy when the re-torque is applied. And last but not least, nuts have been observed that appeared to be tight at the final torque amount, when re-torqued and NOT backed off, only to turn as much as 2/3 to nearly a full turn farther when backed off and re-torqued at the same torque amount!
Apart from gasket surfaces being too far out of flat, insufficient final torque amounts, worn or low strength studs and nuts, etc.; many a leaking or seeping gasket and many a premature gasket failure on Model A’s are caused by the uneven clamp that is possible from nut to nut due to improper re-torques.
Larry Brumfield
July 2016
