Fasteners and torque: clamp load, sequence and safety wire

A bolt works like a spring. When you tighten it, it stretches a tiny amount and pulls the parts together. That pulling force is called clamp load, or preload. Clamp load is what keeps a head gasket sealed and a rod cap tight.

A torque wrench does not measure clamp load. It measures how hard you twist. Much of that twist goes into fighting friction in the threads and under the head. Only what is left over stretches the bolt.

Measuring stretch is the most accurate way to know clamp load. You can do it with a stretch gauge on a bolt where you can reach both ends, such as a rod bolt. Most bolts go into blind holes, so you use torque, or torque plus angle.

Friction changes with the lubricant. A slick lube lets more of your twist turn into stretch. The same torque reading with a slicker lube gives more clamp load. A dry or dirty thread gives less clamp load at the same reading.

Fastener makers report that a slick assembly lube can cut the torque needed for the same clamp load by 20 to 30 percent. A slick lube used with a torque spec written for oil can stretch a bolt past its limit.

A torque spec is only good with the lube it was written for. Follow the maker's spec and use the lube it calls for, on the surfaces it calls for. Clean the threads first. Run a thread chaser through tapped holes and blow them out. Burrs and debris add friction and steal clamp load.

A beam wrench has a pointer that bends along a scale. A click wrench breaks over and clicks at the setting. A dial wrench shows the torque on a gauge. An electronic wrench shows a number and signals when you reach the target.

Hold the wrench by its handle grip and pull smoothly and steadily. Stop at the click. Do not jerk the wrench, and do not keep pulling after it clicks. Never slide a cheater bar over the handle.

Never use a torque wrench to break bolts loose. That is a job for a breaker bar. Back a click wrench down to its lowest setting before you put it away. Have the wrench checked for accuracy on a schedule, and right away if it gets dropped. Field checks at races have found wrenches reading off by 30 percent or more.

Parts with many bolts, like cylinder heads, intake manifolds and rear end covers, need a tightening pattern. Tightening one side first can warp the part or crush the gasket unevenly. Use the pattern in the maker's instructions. Most head patterns start in the middle and work outward.

Reach final torque in stages, not in one pull. A common method uses three equal steps of one third, two thirds and full torque. Go through the whole pattern at each step.

Angle torque adds a turn after a torque step. You tighten to a set torque, then turn the bolt a set number of degrees more. An angle gauge on the wrench shows how far you turned it. Angle is less affected by friction than torque, so it gives more even stretch from bolt to bolt.

Thread engagement is how much of the bolt thread is screwed into the hole. Too little engagement lets the threads strip before the bolt can carry its load. A common rule is at least one bolt diameter of thread in steel and about two diameters in aluminum. A 3/8 inch bolt needs about 3/8 inch of thread in steel and about 3/4 inch in aluminum.

Torque-to-yield bolts are tightened until they stretch past the point where they spring back. They do their job once and stay permanently longer. Never reuse a torque-to-yield bolt. A reused one can give a false torque reading. It can let the gasket fail, or it can snap.

You cannot tell a torque-to-yield bolt by looking at it, so check the service information. For bolts you can measure end to end, like rod bolts, record the length when they are new. A bolt that grows 0.001 inch or more has yielded. Replace it.

Safety wire keeps a fastener from backing out under vibration. Torque the fastener first. Wire does not replace proper torque.

Run the wire so it pulls the fastener in the tightening direction. If the bolt tries to loosen, the wire gets tighter and stops it. Wire that pulls the loosening way can let the bolt back out.

Keep the wire snug with light tension and an even twist of about 6 to 8 twists per inch. Bend the cut end, called the pigtail, back so it cannot snag a hand. Use new wire every time, because safety wire is single use.