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Tap bolts are fully threaded hex bolts. Unlike tapping screws, tap bolts do not form their own mating internal threads. They are instead designed to be inserted through holes in assembled parts and tightened by torquing a nut, or mated with an internally threaded (tapped) hole and tightened by torquing the bolt's head—just like their hex bolt counterparts. Aside from the vehicle repair industry, they are used in a variety of applications like attaching motors to machinery and in combination with pulleys to adjust belt tension. They have hex heads along with an underhead washer bearing surface. Tap bolts are also known as:
hex tap bolts, fully threaded hex bolts, fully threaded hex cap screws.
Threads are standard right-hand and often Unified inch coarse series (UNC, Unified National Coarse). Tap bolts are always fully threaded, which allows them to accommodate a large material thickness range. The point is chamfered or rounded to protect the first thread from damage and to facilitate entry into an internally threaded part.
Typical sizes range from 1/4" to 3/4" in diameter while lengths span from about 3/4" to 6". Measure length from the underhead washer bearing surface to the threaded end of the bolt.
The tap bolt should be long enough to allow at least two full threads to extend beyond the nut face after tightening, which ensures full thread engagement with the nut. Having many threads within the grip (the area between the head and nut), as is the case with tap bolts, results in a "springier" bolt. It is also recommended that threads not be in the shear plane(s).
The size of the head varies with the size of the tap bolt. Since the bolt is externally wrenched, sufficient space around the head must be allowed for a wrench or socket. Table 1 lists head width across flats, which is wrench size, and across corners, head height and minimum threaded length.
Grades 2 and 5, steel, zinc plated, are common grades, material and finish.
When selecting a nut for use with a graded tap bolt, it is very important that the nut be the same grade as the bolt; one grade higher is also acceptable as one source states. Generally, use SAE hardened flat washers with bolt Grades 5 and 8. As for lock washers, which are often split (helical spring) type, use regular for Grades 2 and 5 bolts and high strength alloy steel for Grade 8.
Always follow the manufacturer's recommendations regarding bolt reuse, thread lubricants/locking compounds and torque values.
Refer to American Society of Mechanical Engineers Standard ASME B18.2.1, Square and Hex Bolts and Screws, and Society of Automotive Engineers Standard SAE J429, Mechanical and Material Requirements for Externally Threaded Fasteners, for specifications relating to tap bolts.
Tap Bolt Dimensions
Table 1. Tap Bolt Dimensions. Nominal size in fractional and
decimal inches, head width across flats and corners, head
height and minimum threaded length, in inches. (Note:
Average size is calculated and rounded to nearest 64th.)