Mailbag: Cylinder Head Bolts vs. Studs — and How to Determine the Right Torque Values

Q: I recently purchased Milodon main and head stud kits for a 390 in a Ford Fairlane GT. The torque specifications that came with the kits are general and don’t refer to any particular type of engine. The instructions only refer to stud diameter; for example, they say to torque 1/2-inch studs to 85 ft.-lbs., using oil as lubricant on the nuts.

The problem is, those torque specifications don’t even come close to the specs recommended for my engine. The listed torque specifications for the 390 with 1/2-inch bolts is 85-95 ft.-lbs. for the mains and 80-90 ft.-lbs. for the heads. I know that torque is irrelevant when the desired result is maximum stress tension on the bolts or studs just before they lose their elasticity.

What I need to know is how you ascertain how much tension is required for a stud or bolt, and how much torque is required to reach that tension? Also, is there a difference between bolts and studs and their respective thread pitches when torquing them down? Since I rebuilt the engine with a .030-inch overbore and higher compression heads, would the higher combustion pressures require extra torque on the head studs?

A: Studs and bolts both have their advantages. Bolts are often more convenient, especially where space is at a premium. Bolts can damage the threads in the block because torque is transferred through the bolt into the block. With studs, the torque is applied to the nut and tends to remain there rather than going through the stud and into the block.

As for which torque values to use on the studs for your 390, we would stick with the fastener maker’s recommendations. This will ensure you’re using the correct torque for the tensile strength of the studs.

Just as an example of how torque values differ between manufacturers, ARP recommends torquing its 1/2-inch main studs (190,000 psi tensile strength) to 130 ft.-lbs. with oil on the nuts.

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This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.