head bolt torque

I would venture a guess that the cryo treated grade 5 bolts had the surface of the threads hardened enough that it overall reduced the friction between the bolt threads and the threads in the plate enough that the 40 ft-lbs resulted in more actual rotation of the bolt resulting in the increased clamping force. It is easy to vary the tensile loading in a torqued bolt by as much as 30-50% by varying the friction with a lubricant.

I had a similar situation that one day I was walking through the factory where I worked and saw a worker with a breaker bar and pipe trying to tighten bolts on the cover of a 6000psi pressure transmitter. When I asked why he wasn't using the automatic torquing cow he said that they always leaked when pressure tested when using 174-PH bolts (these bolts were used in place of a B8 bolt when the transmitter was to be installed on an off shore drilling rig to resist corrosion). The standard bolts when torqued to 75 ft-lbs with the cow didn't leak when pressure tested.

I was off on a mission to figure out what was going on. I took some the B8 bolts and nuts and made up a fixture so I could torque them in the tensile tester in the lab. A B8 when torqued to 75 ft-lbs would generate 7500 lbs of force. Took the 17-4PH and did the same thing, I only got 3000 lbs of force. Friction in the threads was reducing the actual rotation of the nut at 75 ft-lbs reducing the clamping force. Now what the factory worker unknowingly was doing was increasing the torque to get back to the same clamping force.

My options were to purchase additional equipment and create a separate work station so the 17-4PH could be torqued at 150 ft-lbs. This was not the best choice since we had limited space at that point in time and the gang torquing cows for all the different configurations was quite capital investment.

Back to the lab. I tried a B8 nut on the 17-4PH bolt and bingo we were back to 7000 lbs of force at 75 ft-lbs. Now the B8 nut was not an acceptable solution because of he corrosion issues. So I tried some plating on the 17-4PH nut and found a zinc plating did the trick. So we revised the spec on the 17-4PH nut to include a zinc plate and green chromate wash (green to make them visibly different that the yellow chromate on the B8's). Problem solved an no more works using a breaker bar and potentially hurting themselves.