Auto Service World
Feature   December 1, 2007   by Jim Anderton, Technical Editor

Clamp It Down

Torque-to-yield is all the rage in cylinder head bolts...But all automotive studs and bolts have TTY properties

In the days before the torque wrench, the standard gauge used by mechanics to measure fastener clamping was also conveniently located at the end of their arms: fingers. That was fine for the monkey-wrench-and-hammer world of sixty or seventy years ago, and can still play a part in the non-critical assemblies; but under the hood, it’s different now. Light weight materials like aluminium and plastics, as well as tight build tolerances once thought impossible in production engines means that making humble nuts and bolts work is no longer a no-brainer.

Not Back to Basics

In most automotive training programs from the high school level on up, fasteners are rarely taught beyond their grades, sizes and thread pitch. It’s basic, but there’s an important body of information that’s before the basics that few consider. It starts with some elementary properties of metals that apply to bolts and fenders … or coat hangers and paper clips. While the metallurgy can fill textbooks, the important concept to get a huge insight into fastener performance is what engineers call the “stress-strain curve.”

The graph below shows a typical curve for a steel part.

Note the graph shows stress-strain properties of a material like steel, not a specific shape of material, like a steel bolt. As you start applying a force to a metal object, like bending a paper clip or torquing a bolt, you move up and along the curve. The flat, rising part of the curve is called the “elastic” or “Hookean” region (Hooke was an English scientist who also invented the universal joint, in 1604). Remove the load or bending force, and you travel back down the curve to the origin. In other words, the deformation goes away and the part springs back to its original shape.

Add more stress or force by stretching, bending or twisting, and you travel beyond the straight part of the line into the “curvy” part of the graph, at the “yield point.” As the name implies, when you take the load away now the part doesn’t go back down the graph and the deformation stays. The metal is “plastically deformed,” literally failing to spring back, which is great if you’re stamping fenders, but bad if it happens to valve springs. For bolts and studs, it’s the principle behind “torque-to-yield.”

In the case of bolts and studs, “yield” means “stretch,” which is the best way to consistently clamp important assemblies like heads and main/connecting rod bearing caps. If accuracy and precision matter, like in high-RPM rotating assemblies using long-throw crankshafts, bolt stretch is the gold standard in assembly measurement. The issue with using bolt stretch instead of torque in these applications is that the higher the strength of the fastener, the less the bolt will stretch for a given applied torque. We’re talking about thousands of an inch, and for repeatable results, vernier callipers aren’t accurate enough. Micrometers are the standard tool, although specialty jigs using dial indicators are available for direct reading.

Naturally, this technique isn’t possible for studs and head bolts, which ironically are the place where consistent clamping force is needed the most. As a result, inferring the fastener stretch through torque or rotation angle is the only possibility, and both techniques can produce inconsistent clamping if done carelessly. Torque is still the standard method, but how do you know that the twisting force applied by your clicking torque wrench actually represents correct clamping force? The problem would be trivial if the wedging action of the threads was totally frictionless and consistent, but in reality the surface between threads is anything but, resulting in inconsistent and generally low clamping forces. The inter-thread friction can be caused by several factors. The first is corrosion. While red rust is all too common, the thread swelling effect of even minor corrosion can take up the clearance between bolt and threaded hole, reducing clamping force at a given torque value.

The obvious solution is to clean the threads, not only on the bolt, but in the block or threaded hole, too. This requires a bottoming tap that can reach all the way down the hole. Bottoming taps can be purchased, or made by grinding the end of a conventional tap. Naturally, the debris resulting from the cleaning needs to be blown out of the blind hole with compressed air. If there’s significant corrosion at the bottom of a blind hole in the block or in a front cover casting, suspect coolant infiltration. This implies cracks, so a pressure test is a logical next step.

Another clamp force killer is galling of the fastener threads. The forces involved in clamping threaded fasteners can be huge, generating both pressure and heat resulting in microscopic welding and tear out of metal at the threads. Friction can skyrocket, again reducing clamp loads relative to the applied torque. Like corrosion, cleanliness helps, as does a lubricant during assembly. Lubing bolt threads always gives more consistent readings than dry assembly, but the type of lube is important.

Anti-seize compounds, for example, are superior at withstanding the extreme pressure at the thread interface, but if the manufacturer recommends engine oil as a bolt lube, the “better” lube will give an inaccurate reading. Use the recommended lube for every critical application, TTY or not. How much? A drop or two should be more than enough, as clean threads should wick the oil evenly along the thread length. Squirt it in blind holes, however, and it’s possible to hydraulically lock the bolt or stud, giving good torque numbers but minimal clamping. If the bolt or stud threads into a water jacket, you’ll be adding sealants, so don’t add additional lube to these threads unless directed to by the manufacturer. Similarly, if a thread locking compound is needed, check before adding lubricants like engine oil to the same threads. They rarely mix.

Check under the head

Even if the threads are clean, straight and well lubricated, there is still friction in the system. The place to find it is at the underside of the bolt head and between head and washer. Critical automotive bolts have lots of head area relative to bolt diameter, and in the case of head bolts, often use thick, hardened washers as well. That large surface area is a source of torque-killing friction, especially if an unwary tech substitutes a non-standard washer in place of the hardened OEM part. Washers are there to spread the clamp load of a fastener over a wide area and in high-force applications like cylinder heads, they’re thick and may be ground for flatness. Use a hardware store soft steel replacement, and the bolt head can “dig in” greatly increasing rotational friction during torque-up. The cheap washer is also thinner, and will “dish” when torqued, concentrating the clamping force over a tiny area at the edge of the head bolt hole.

This can not only give inaccurate readings, it can form a stress riser at the bolt hole, promoting cracking. The secret for head bolt washers? Like the bolts themselves, cleanliness is king. Washers should obviously be crack-free and should be deburred if necessary. The head surface where the washer seats should be equally clean, and in every case, there should be clearance around the washer so that it can centre itself during torquing. If your engine uses different diameters of washers for some bolts, it’s important not to mix them up because the smaller washer on the big seat area will work fine, while the too big washer in the small pocket may cant the bolt head or skew the bolt sideways, both of which will distort clamp force, heads and head gaskets and maybe all three. And the number one mistake? Don’t reuse torque to yield bolts. Remember the stress strain curve? When they’re stretched, they’re stretched and are now permanently deformed. Reliable clamping a second time is a crap shoot, so why roll the dice over a few bolts? It’s happening in shops across the country and some techs are getting away with it, but like used antifreeze or
reground valves, if the new stuff is cheap, why mess around?

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