Proper tightening of fasteners is essential in the assembly of parts. Experienced mechanics have the skill to approximate the correct force in tightening fasteners, but even they cannot apply the precise force for critical components such as head bolts and wheels. Properly tightening fasteners requires a torque wrench to apply the proper torque. For most people, using a torque wrench to apply the correct force during assembly is strongly recommended.
It is common to see leaking valve covers and oil pan gaskets with deformed flanges. Too much torque was applied to the thin sheet metal. Not only are the flanges bent, the pressure on the gaskets is no longer uniform and leaks occur.
Automotive engineers determined the correct torque required for the assembly of most all fasteners involved in the assembly of the automobile. These engineers take into account the following:
- The size of the fasteners;
- The strength of the fasteners:common, Grade 5, Grade 8, etc.;
- The type of thread: national course or national fine;
- The type of materials being clamped together;
- The lubrication on the threads;
- Any gaskets between the joined parts.
These correct torque values are provided in a vehicle’s shop manual, but can sometimes be found online (beware of the source, however). If a specific torque is not specified in the manual, it is safe to assume that it is not critical, and the torque value associated with the size of the fastener and its thread is a good guide.
Torque is a rotational force applied around a point or, in this case, a nut. Put a 1-ft.-long wrench on a nut and apply 10 pounds of force to the opposite end. You’re now twisting that nut with 10 lb.-ft. of force (distance times force, or 1 foot times 10 pounds). Use a 2-ft.-long wrench and apply 50 pounds of force, and you’ll have 100 lb.-ft.
When the nut and bolt are tightened, the two pieces are clamped together. The threads are a form of inclined plane or wedge. As the inclined plane is wedged (turned) into the threads, it applies a force along the bolt’s length, effectively making the bolt a tension spring. This tension in the bolt shank clamps two parts together.
The amount of tension created in the bolt is critical. If it is too low, the fasteners can work loose, or the associated gaskets or seals may leak. If it is too high, the fasteners may permanently stretch and no longer apply the required clamping force. The fastener could also break. Other problems can arise from too much torque: the metal of the associated part may deform (such as at valve cover flanges) or the seal or gasket may deform, leading to leaks.
The Necessary Tools
The simplest torque wrench is the bending beam type. This is the wrench for those who don’t regularly need a torque wrench. The large center beam bends as you apply torque, while the unbending pointer beam lets you read the torque directly. If it goes out of calibration, simply bend the pointer back to zero with a pair of pliers. The single biggest disadvantage is that your eye must be directly above the pointer while you read the scale, which is often not easy.
The better torque wrench is the micrometer clicker type. With this wrench, the tool is set to the correct torque and it will audibly click and tactilely when it reaches the pre-set value. It’s highly repeatable and accurate, but should be returned to zero after every use. Don’t use your torque wrench like a ratchet for disassembly — save it for final assembly. Also, it must be calibrated regularly depending on use, handling and storage. This must be done by specialty shops with the necessary equipment; the cost is modest $25 or so.
Typically, the specified torque value assumes clean and dry parts. Clean means no dirt, rust, dried-up gasket sealer or anything except shiny metal. Wire-brushing the threads will help remove rust or sealant. Engine fasteners, such as head bolts or main cap bolts, are often specified to be torqued with 30-weight engine oil wetting the threads and washer. If you’re installing a fastener that has a dry torque spec, and the threads and bolt face are oiled, you’ll need to reduce the torque by 15 to 25 percent, because the slipperier surfaces will decrease friction. Teflon-bearing lubes or moly-sulfide engine assembly lubes can reduce friction enough to require a 50 percent reduction in tightening torque. Even casually substituting a zinc- or cad-plated bolt or washer for an unplated one calls for, respectively, a 15 or 25 percent reduction in applied torque, because the plating acts as a lubricant. Fail to heed this advice and the fastener will be seriously overtightened. You’ll either snap it or crush a gasket to the point where it leaks.
On the other hand, rust or burrs on the threads can increase friction enough that a fastener tightened to the specified value won’t provide enough clamping force. The shop manual may specify whether the fastener is supposed to be dry or lubed. In either case, prep your bolts. Don’t forget that residue from the parts washer or the solvent used to clean parts has oil in it. Even a blast of compressed air to dry off a fastener will leave an oily film behind, ultimately affecting torque. Similarly, an anti-seize compound will also lubricate and affect torque.
Whenever there are more than two fasteners securing a part, it’s important to alternate the tightening among them. Start all the bolts or nuts a few threads and then run them hand-tight. Often, the shop manual will show a specific tightening sequence. Tighten mildly by hand, then to the specified torque
1. Circular patterns of bolts (typically the lug nuts on wheels) should be tightened not in a circle, but in a crisscross or starfish pattern.
2. Larger, oblong parts such as cylinder heads often have a specific sequence for proper tightening, generally starting near the middle and circling around. This method assures that all of the bolts clamp the two parts together with nearly identical force so critical parts such as head gaskets won’t leak.
Proper tightening fasteners is essential in assembly of critical parts and good practice for all others. A torque wrench is needed to apply the proper torque. Torque wrenches are precision tools and need periodic calibration to maintain their accuracy.
“Resto Basics” columnist William C. “Bill” Anderson, P.E., has been involved with the automotive hobby for more than 40 years with experience ranging from hot rods, to sports cars, to sports car racing, and to restoration of vehicles from the ’30s through the ’80s. He is an author, magazine editor, car show judge and professional engineer. A member of several car clubs and a leader in some, through Anderson Automotive Enterprises — www.andersonautomotiveenterprises.com — he restores and appraises cars.
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