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Hello- > > 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. No, that is incorrect. The pitch (which is more of a relative thing... i.e. a coarse vs. fine, etc.) figures into the coefficient in a minor way. I'm not pulling this stuff out of my butt. This is from standard mechanical engineering formulae. > I think the 2 pitches are > 1.25 and 1.75. M8x1.25 and M10x1.50. Let's look at the relative pitch: (8/1.25) = 6.4 (10/1.50) = 6.7 More-or-less the same. So, their effect on the coefficient should be minimal/non-existant. > 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 You are getting into the nitty-gritty details. All of that stuff figures into the coefficient in a minor way. BTW, FYI, the coefficient is usually around 0.16-0.24. > > 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 That's irrelevant. > > 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 inserts are 12mm (14mm is the VW oversize and 1/2" is an American oversize). While they can take more load before pulling out of the case, VW actually had them stressed _less_. The preload is the same, and the studs are less rigid (they have a smaller diameter... material strength doesn't figure into rigidity), so they will stretch more easily, putting less of a load on the case when the cyls/heads expand. > 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. But it is incorrect reasoning. Sorry... Trust me - I got my degree doing this kind of stuff :-). > 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. 12mm at the case, 8mm shanks, 10mm at the head. Yes - the T4 has an undercut stud. And, so do all the Porsche engines! And, so does the Corvair! This unstresses the threads, removing them as the weak-point of the stud. It's a much better design. In fact, the only German-boxer-like automotive engine I know of that _doesn't_ have undercut head studs is the VW T1, because it was designed to be as cheap as possible. That's why LN Engineering had manufactured ARP head studs for the T1 with 10mm threads and 8mm shanks! > 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.) But preload isn't when they pull out. You don't torque the studs and watch them snap with the engine on the bench. They snap when the engine is hot and they are more highly-stressed. > I > suppose VW decided that step studs were cheaper than inserts, but this > surprises me. No, the other way around. The step stud is more expensive. > Perhaps I'm wrong about the case end diameter; if they are 12mm there that > makes a big difference. You are focusing in on the wrong place. > Do the type 4 head nuts get the same 18ft lbs of > torque? No, 23. It's because the thread at the head side is 10mm, nothing more. > Or it may just be the stronger case alloy that makes all the > difference. What do you think? The stronger case alloy does help it from ripping out of the case, but it didn't factor into the design decisions of the Germans - it really didn't effect how they wanted the studs to clamp the cyls/heads (nor did it for the magnesium vs. aluminum 911 engines, although the former did have more pulled studs than the latter). 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