Vintage Mustang Tech Questions and Answers
Vintage Tech Advice
Torque Specs and You
So this happened. I was torqueing the rod bolts on an engine, and No. 5 didn’t want to tighten up. I knew better, but I kept cranking on them—and then—snap! I broke a rod bolt. These are ARP bolts, the best, and yet they didn’t want to torque. The person who installed these bolts stretched them beyond their ability to spring back and set me up for failure.
You bolt something together to keep it from moving. When you do this, you are applying a preload or “clamp load” to the stack you are trying to hold together. A tightened bolt stretches like a spring. This stretching puts tension on the items being held together and is also enough to keep the bolt itself from backing off due to vibration, heat, etc. If you remove the bolt, the elasticity of the bolt pulls it back into place. So clamp load is what holds your stuff together. The key to a bolt holding things together is the amount that it is stretched. If it is not stretched enough, the bolt can come loose from vibration, and if it is stretched too much, the bolt will not spring back to its original position like this rod bolt and will exceed its yield point and break.
There are three ways to measure the stretch of a bolt: a stretch gauge, a torque wrench, and a torque angle. A stretch gauge only works when you can see both ends of the fastener (like a rod bolt), and most items like head bolts can’t be measured this way. Torque angles can be used, but they need specific calibrations per application. Most of us are going to use a torque wrench to measure the stretch of the bolt.
Torque measures the friction of the bolt as it tightens. As the threads begin to push against each other, the friction between the two surfaces increases and begins to stretch the bolt. When the bolt reaches a certain friction, it is close to the clamp load of the fastener. The torque required to tighten a bolt is determined by the material used, finish of the bolt, strength of the bolt (grade), lubricant used, and condition of the threads. A lubricated bolt requires less torque to stretch, because less friction is involved; remember it is the stretch, not the torque, that determines the clamping load of the fastener.
Most every bolt type has been calculated to a torque specification, which can be found all over the Internet and in many mechanical handbooks. One other thing to remember is something called “friction factor.” According to the ARP website, a bolt has its greatest friction on the first tightening, and as the bolt is loosened and re-tightened, the friction lessens and then levels out. ARP recommends torqueing some of their fasteners at least three times to get close to their recommended torque specification.
Many manuals show a range in which the bolts are torqued. The head bolts on a 390-428 engine torque to 80-90 lb-ft. If there is a specific torque spec for this bolt, then why the range? Ford is not being sloppy; this is telling you a top and bottom limit based on the bolts used. Torque wrenches can vary quite a bit, so this spec says that below this range things can vibrate loose, and above this range you get close to the yield limit of the bolt. Some people torque to the highest recommended number, and with the variability of torque wrenches, you run the risk of falling outside the limits. Torque to the middle of the range.
Torque to Yield
Torque-to-yield (TTY) bolts are used in engines with lots of aluminum parts that heat up and expand more than cast iron. Torque-to-yield bolts are designed to be stretched to a yield range that is at the bolt’s elastic limits, which gives them a more consistent clamping force. Because they are stretched to this yield range, they can’t be reused.
Torque spec is used to measure the stretch of the fastener so the correct clamping force is applied. When tightening bolts, make sure the threads are clean, use the right lubricant on the threads, keep your torque wrench in good condition and have it calibrated every year or so, and torque standard bolts several times to achieve consistent results.
Photography by Dave Stribling