Stainless Steel Bolt Torque Specs

by DarienC

Tightening bolts to the proper torque is extremely important. Bolts that are not tightened enough can fail to provide the necessary support and stability, and bolts that are tightened too much can actually snap unexpectedly, particularly when you apply additional pressure. There are several factors to consider when calculating bolt torque specs, including the grade, size and purpose of the bolt, and the fastener that is catching it. If a bolt should be tightened to 2,000 inch-pounds of torque, but the fastener can only withstand 1,000 inch-pounds, there is a good chance the fastener will bend or break as the bolt is tightened.

Types of Stainless Steel

The two main types of steel used in stainless steel bolts are 18-8 stainless steel and 316 stainless steel. Stainless steel given an 18-8 designation consists of approximately 18 percent chromium and 8 percent nickel. The remainder of the steel comprises several other elements and compounds, including manganese, phosphorus, sulfur, silicon and chromium. Carbon is held to a maximum of 0.08 percent in both basic types of stainless steel. The term 18-8 applies to several similar grades of steel, including types 302, 303, 304, 305 and 384. These types of stainless steel represent the "basic alloy," and there is little difference between them in terms of strength or resistance to corrosion. Type 316 stainless steel is similar to 18-8 stainless steel except that it is more resistant to corrosion because there are higher levels of molybdenum in it. Type 316 stainless steel is often used in marine situations because it is particularly more resistant to corrosion caused by salt water.

Size, Thread Count and Other Factors

In addition to the grade of stainless steel the bolt is composed of, several other factors, such as the length of the bolt, the bolt's thread count, the job that the bolt is being used for, the fastener the bolt is being applied to, any plating the bolt has been coated in and whether or not the bolt is clean, dry or lubricated can affect the the amount of torque that should be applied to a bolt. For instance, a 5/16 bolt with 24 threads per inch requires more torque than a 1/4 inch bolt with 28 threads per inch. The Rask Cycle website recommends lubricating motorcycle bolts with motor oil to reduce the torque by 15 percent to 25; to reduce torque by 50 percent, use a Teflon dry film or a Cetyl alcohol dry wax.

Bolt Torque Specs

Taking the above factors into consideration, the following specs provide a good ball-park range for the preferred fastening torque of several bolts. The torque is given in inch-pounds and is intended for standard bolts (without plating) that are dry at the time of tightening. You should tighten bolts that are 1/4 of an inch long and made of 18-8 stainless should to 75.2 inch-pounds if they have 20 threads per inch and to 94 inch-pounds if they have 28 threads per inch. Tighten bolts of the same size and thread count that are made of 316 stainless steel to 78.8 inch-pounds and 99.0 inch-pounds, respectively. Tighten bolts that are 3/4 of an inch long and made of 18-8 stainless steel to 1,530 inch-pounds if they have 10 threads per inch and to 1,490 inch-pounds if they have 16 threads per inch. With bolts of the same size and thread count that are made of 316 stainless steel, tighten to 1,582 inch-pounds and 1,588 inch-pounds, respectively. Tighten one inch long bolts that are made of 18-8 stainless steel to 3,440 inch-pounds if they have 8 threads per inch and to 3,110 inch-pounds if they have 14 threads per inch. Bolts of the same size and thread count that are made of 316 stainless steel should be tightened to 3,595 inch-pounds and 3,250 inch-pounds, respectively.

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