are a critical part of any mechanic
's tool kit
. When an engineer
designs an assembly sequence
for a complex piece of equipment
, such as an automobile engine
he specifies a torque
setting for important bolts and screws
. The torque setting determines the level of tightness required to secure the equipment without damaging or stripping any threads
. Maintaining constant torque on multiple bolts is required to seal
a large area. For example, when installing a cylinder head
the engineer specifies a precise tightening sequence and a torque setting so the head gasket
will seal properly. A torque wrench allows the assembler to determine when the precise torque setting has been reached.
There are two types of torque wrenches. The arm type has a pointer attached to the head and scale near the handle where the current applied torque can be read. As torque is applied, the wrench handle moves in relation to the pointer according the amount of torque being applied. This type lasts a long time, lifetime warranties being fairly common. They can also be used to note torque settings during disassembly, if a reassembly torque is not known. But they are slow compared to a rachet type. They cost betwen $30 and US$45.
A rachet type, or mechanical torque wrench is more expensive, but very fast if you are torquing multiple bolts. Because they depend on gears, their warranty is much shorter, a year being a long guarantee, 90 days more typical. Still, their sheer speed makes them the wrench of choice for professional and race mechanics, with the proviso that their accuracy must be regularly checked. A rachet type has a sliding yoke that is moved up and down until the correct torque setting is indicated at the top of the yoke. The yoke is tightened and the wrench is ready for use. When the desired torque is reached, the wrench will audibly click letting you know you're there. This type of wrench costs around US$60.
In the US torque settings come in foot and inch pounds, with one foot pound corresponding to one pound of force applied at the end of a one foot lever. Torque wrenches work with your socket set and you should have a different torque wrench for each size of socket drive. (EG 1/4", 3/8" or 1/2" being normal in the US for auto projects). You cannot use a reducer to use a 1/2 drive torque wrench with a 3/8 socket without losing accuracy. Because torque wrenches tend to be longer than rachets they are sometimes used as breaker bars because the length allows greater leverage. This is not good for their accuracy.
Any serious home mechanic needs at least one torque wrench in his or her kit. Without a torque wrench, repairs can produce their own damage.
Noders doing their own mechanical work are advised to purchase and refer to a good service manual. It will list the torque specs and tightening order for important bolts.