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Hello- > I disagree. A screw thread is nothing more than an inclined plane wrapped into > a helix. Once you realize this, it is clear that the only thing that makes a > difference is the amount of advancement per turn. I found some simple automotive examples online we can work through together. Go to ARP's 2003 catalog, page 24. Let's define some variables: F = preload (lb) T = torque (ft-lb) k = coefficient D = thread major diameter (in) P = thread pitch (1/in) (i.e. P for 3/8"-24 is 1/24) I say that T = 12kFD. A little algebra says that k = 12T/FD. You say that T = kFP. So, k = 12T/FP. Note that 12 is simply to keep units consistent between in and lb. Let's solve k for some examples. First, let's compare 9/16-18 to 5/8-18. ARP says: 9/16-18 - 174ft-lb gives 24,312lb 5/8-18 - 243ft-lb gives 30,660lb My formula gives k=0.15 for BOTH of them. Yours gives k=1.55 for the first and k=1.71 for the second. Next, let's compare 9/16-12 to 9/16-18. ARP says: 9/16-12 - 159ft-lb gives 21,792lb 9/16-18 - 174ft-lb gives 24,312lb My formula gives k=0.16 for the first and k=0.15 for the second (recall that I said that pitch figures in as part of the coefficient in a _minor_ way). Yours gives k=1.05 for the first and k=1.55 for the second. As you can see, your formula holds no consistency. Mine does. I'm sorry, but you are incorrect. Take care, Shad Laws LN Engineering - Aircooled Precision Performance http://www.lnengineering.com ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org