Question: Kevin, I enjoy your writing very much and over the years have learned a great deal. Thank you. My question concerns the tightening of threaded fasteners. I believe supplied torque values are, unless stated otherwise, for dry components. However, if dry threads during assembly don't feel "right" to me, I apply a little light lubricant. Should I reduce the required tightening torques for fasteners that have been lubricated or had locking compound applied?
Duncan Laidlaw
Maine de Boixe, France
Answer: The Loctite people recommend reducing applied torque by 20 percent from dry values when using their liquid thread-locking compounds on threaded fasteners. You can find tables of wet-vs.-dry fastener torques in places like Thomas J. Glover's "The Pocket Ref" (inexpensive, used, on Amazon), but judging from the values given, a fair estimate would be a 25 percent reduction in torque when tightening an oiled or anti-seize-treated fastener.
Bear in mind that critical fasteners such as connecting-rod cap bolts may have special tightening procedures. Use them. Triumph used to specify a specific “stretch” for their cap bolts, measured end-to-end with a micrometer, which eliminates the problem of tightening friction and measures the very thing you are trying to achieve—the degree to which you are stretching the bolt.
Many engines are built today using the concept of “torque-to-yield” on fasteners such as head and main bearing cap bolts. This is done to get maximum performance from fasteners, and is often done by tightening angle. This may be programmed into torque guns as “contact, plus X degrees, then Y degrees more.”
I have worked on engines whose previous owners tightened Grade-12 6 x 1 case cover screws to tremendous torques, causing the soft case material to flow up around the heads of the fasteners. A similar effect can be produced with a hammer-driver, and in each case produces serious overkill. All you need/want on case covers is enough clamp force to make the gasket seal, and the original design gets this from modest torque on run-of-the-mill fasteners. As an example, 50 inch-pounds are plenty for 6 x 1 cover screws.
I must caution you against "torque-wrench worship." Lots of engines get built using either the torque specs given in the service book or the experience of the assembler. When I was first trying to build racing engines, I went through the super-strength fastener game and probably enjoyed it. But, in fact, the only place where such things are of benefit is when you have a special problem such as head gasket blowing. For normal assembly of engines, service book torque on the specified fasteners gives satisfactory results.
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