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On 18 Jun 2003 at 1:15, Shad Laws wrote: > What they don't realize is that the emperical approximation to relate > tightening torque (i.e. how hard you are twisting the wrench) to actual > preload (i.e. how hard the stud is holding the things together) is: > Torque = PreloadForce x ThreadDiameter x Coefficient More important than thread diameter is thread pitch. I think the 2 pitches are 1.25 and 1.75. This is the factor that actually changes the mechanical advantage. Of course changing amounts of friction make this virtually impossible to analyze analytically, so the > While determining the exact coefficient is a rather difficult task (it > depends on everything!), it doesn't take too much effort to understand a > general trend here: the bigger the bolt, the more torque needed to get the > same preload. > > And, look at this: > > 23 ft-lb * (8mm / 10mm) = 18.4 ft-lb. Or, 23 * (1.25/1.75) = 16.42 > VW specified *approximately* the SAME preload for both kinds of head studs! Agreed, but I wouldn't be surprised if they allowed for a slightly higher preload with the 8mm studs, given that they put them in 1/2" steel inserts. The increase from 16.4 to 18is about a 10% increase, while the increase from 10mm to 12.4mm is a 24% increase, so this sounds like a good engineering decision. BTW, I was surprised a few years ago to find that type 4 head studs are step studs, with 10mm (is that right?) threads into the case, thin shanks, and 8mm threads at the head. This seems odd, since these would then seem just as likely to pull out of the case as our 10mm studs (assuming the same preload.) I suppose VW decided that step studs were cheaper than inserts, but this surprises me. Perhaps I'm wrong about the case end diameter; if they are 12mm there that makes a big difference. Do the type 4 head nuts get the same 18ft lbs of torque? Or it may just be the stronger case alloy that makes all the difference. What do you think? -- Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org