Anyone who has serviced engines (as well as certain driveline and chassis components) that date from roughly the early 1990s to the present is aware that many OE threaded fastener tightening requirements call for a combination of torque-plus-angle specifications as opposed to a torque-only spec. Common applications for this procedure include but are not limited to cylinder head fasteners, main cap fasteners and connecting rod bolts.
Regardless of how silly the extra step of angle tightening may seem to some people, there is a legitimate reason for this approach…it’s better.
Remember that clamping load relies on bolt stretch (taking advantage of the bolt’s elastic properties). Since fastener engineers already know how much stretch, and therefore clamping load, will occur based on how far the bolt head rotates, they use the angle of bolt head movement to determine exactly how much load is being exerted.
A torque spec alone cannot be used to exactly determine bolt stretch because of the friction variables that come into play during tightening. A certain amount of torque loss is caused by the friction of the bolt head underside to the parent material contact surface (for example, where the bolt head contacts a cylinder head), and by the friction of the thread engagement. The type and amount of oil/lubricant on the threads provides yet another variable in terms of friction.
Depending on how smooth and burr-free the bolt head contact area is, and on how smooth and uninterrupted the threads are on both the bolt and the female threaded hole, a torque reading alone really can't provide accurate and consistent clamping load information. If enough friction is created by these variables, a reading of, say, 60 ft. lbs. on a torque wrench may in reality only provide the equivalent of (for example) 47 ft. lbs. that actually works to stretch the bolt. And if the bolt was designed to stretch to its just-short-of-yield point at a true 60 ft. lbs., this inadequate bolt stretch will mean insufficient clamping load. (which, in the case of cylinder head bolts, for example) can lead to a gasket leak or gasket failure down the road.
Torque-plus-angle simply provides a more accurate method of achieving the needed clamping load.
If published tightening specs only call for torque, then tighten accordingly. But for later model (domestic and import) applications that specify a torque-plus-angle method, you MUST follow those procedures in order to achieve design clamping loads.
Previously in order to achieve final angle-tightening, common methods included placing a reference mark (paint or pen) on the bolt head, and observing the angle of rotation (a “close enough” guesstimate of a movement of 90 degrees, or 45 degrees, etc., which is “close” but certainly not repeatable or consistent, or accurate). Another method is to use an inexpensive specialty tool that consists of a plastic or metal incremented dial that is placed onto a wrench drive, between the drive square and the socket wrench (angle gauge or protractor). This works, but it’s time consuming (torque to the required spec, remove the torque wrench, grab another square drive wrench, place the dial gauge onto the drive, attach a socket wrench to the dial gauge, re-attack the fastener and tighten to the required angle). Plus, if you bump the angle gauge or protractor, you lose your reference point, which means that you need to loosen the fastener and start over.
Thanks to advancing technology and the efforts of quality tool manufacturers such as Snap-On, those days are gone, providing you pony-up and take advantage of a new precision tool system.
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