When most people torque a bolt, they put little thought into how it works. Few realize a threaded fastener is like a spring that stretches to achieve optimum clamping tension.
The proper torque in any fastener is reached by stretching it within a specified range. A fastener’s “rebounding” property is what actually provides the clamping force, and will vary with different materials. Like a spring, under-stretching will not provide enough clamping tension, and over-stretching will exceed its yield strength. If an unloaded fastener is longer than it was when it was manufactured, by even as little as .001”, a partial failure condition has been reached and it must be replaced. It’s damaged and will not rebound enough for proper clamping – it will fail.
There are three common methods for torqueing fasteners. In order of accuracy, they are:
TORQUE ANGLE – This involves calculating bolt stretch by thread pitch and has many variables that diminish accuracy. This generally works in non-critical preload applications that need to be “snug.” These are your “snug and then tighten another half-turn” instances.
TORQUE WRENCH – This is acceptable when measuring rod stretch isn’t possible. Though more precise than the torque angle method, there are many variables that can alter the friction factor. Lubricants, surface finish, and even inconsistencies within the torque wrenches themselves can alter the amount of actual stretch, despite a proper numeric reading on the wrench. The friction factor is at its highest value during the first tightening and lessens with each “tighten and loosen” cycle, as the surfaces plane against each other and level out. A required 50 ft-lbs of torque may only be a third of the proper stretch with the right combination of variables altering the friction factor.
BOLT STRETCH – Bolt stretch is the most accurate method. Regardless of the other factors, and no matter how much actual torque it takes to stretch the fastener to the correct length, it’s not properly torqued until it’s properly stretched.
In an engine, achieving proper torque on the rod bolts is the most critical element of the rotating assembly. Improper preload, or pre-stretch, will cause a separation of the connecting rod and rod cap with each crankshaft revolution. Centrifugal force will drive the cap outward until the bolts stretch enough to bear the load. They will then “unstretch” as the force recedes in the opposite direction during the upstroke. This repeated stretch and relax cycling will work-fatigue the bolt and cause disastrous failure.
A properly preloaded bolt won’t stretch with the cyclic loads. It needs to be adequately tightened to a load greater than the demand of the engine, and measuring bolt stretch is the most accurate way to make sure it’s reached. A bolt stretch gauge is a “must” even when building an engine, and it’s easy to use.
Follow these steps to make sure you do it right, do it once.