Re: To retorque ... or not retorque
Peit Visser is correct when he says that lubricants can result in over torqing. Many years ago I manufactured ultra high strength engine bolts ( rod bolts, head bolts, main bolts etc) for race engines. I doubt the things are different today.<br /><br />1) Check the service manual before retorqing any bolt.<br />2) If the manual says nothing about lubrication, do not use any. The SAE standard is solvent cleaned and dry.<br />3) If the manual says to use a lubricant of a specific type, do not vary from this by using something different.<br />4) When it comes to Head bolts, Rod bolts, Main bolts, or any bolt that clamps a moving force the torq specification and bolt design (including the material and method of heat treating) preloads the clamped parts in excess of the expected loads under power. In other words the clamping force exceeds the work load. When fibre material gasketing is used between the surfaces, it is common to retorqe 24 hours after initial instalation. Because this is not practical in production, most gasketing is stamped and embosed steel. The retorqing of fiber material is because the softer material will "set".<br /><br />As for lubricants - What I am about to say is easier to understand if you think of a "Torque Wrench" as a "Friction Wrench". In order to measure torque you need to restrict the movement of the device producing the torque - ie: the torque of a motor is measured by restricting the rotation ( a breaking action). <br /><br />A torque wrench measures the amount of resistance (friction) created when the clamping force increase the load on the mating surfaces and threads. If you lubricate these surfaces the friction is reduced even though the clamping load is increased. This usualy results in permanent damage to the bolt. The size and material of the bolt is specific to the clamping load required.<br />The load on the bolt should never exceed its elasticity point, other wise the bolt will be permanently weakend. Yes the bolt will stretch, and should return to its origional length when the load is removed. If not the grain (like in wood) will have been fractured.<br /><br />Various lubricants have varing load capacity. This is when the load on the mating parts forces the oil/lubricant to migrate (move) from the point of contact (Timpkin Test). Here is an example of how much the torque specification can change and still achieve the SAME clamping force :<br /><br />3/8" Diameter Connecting Rod bolt -<br /><br />Solvent Cleaned & Dry OEM Spec = 42 ft Lb.<br />30 wt engine oil = 37 ft lb.<br />graphite oil (moly lube) = 20 ft lb.<br /><br />Once the grain of the bolt is fractured it is permanetly damaged. If a damaged bolt is torqued near or above it's new (lower) threshold it will relax during use.<br /><br />Check the manual and don't vary from the specifications.
