When using head stud bolts is it better to use fine thread or just stick with the coarse thread so its the same as what is threaded into the block? Just wondering on how fine thread torques compared to coarse
Coarse threads on the set end and fine threads on the nut end is probably the most common stud configuration used on studs for automotive applications. Since the threads are not as deep in the fine thread series, the tensile-stress area is larger than in coarse thread fasteners of the same size, so, for the same nominal bolt size, you can (THEORETICALLY!) apply a higher torque and develop a higher clamping force by using a fine thread fastener instead of a comparable coarse thread fastener, but this assumes thread length engagement sufficient to prevent stripping (not a problem on the nut end of a stud, as here) and close control of all critical thread dimensions. In practice, it is risky to assume production dimensional tolerances are sufficiently close to take advantage of the difference in available tensile stress area, so the usual practice is to use the same applied torque value for either fine or coarse threaded fasteners. In the application under consideration, if the supplied studs are coarse thread on the set end and fine thread on the nut end, that's fine, if they're coarse thread on both ends, that will work too. You can also make studs in a hurry by using continuously threaded stock cut to the proper overall length, which would mean that you would be using coarse thread pre-threaded stock to match the set end threads. What you would not want to do, simply to avoid annoying assembly problems, is to have a mixture of studs with some fine and some coarse thread series on the nut end. Pick a nut end thread series and stick with it. Most likely, that will be coarse thread on the set end and fine thread on the nut end.