There can be some confusion with identifying various
types of hex bolts. We will cover three types: HEX CAP SCREWS,
TAP BOLTS and MACHINE BOLTS. All three are very similar
looking and can be easily mistaken for one another unless you are
familiar with these slight differences.
HEX CAP SCREWS
Hex cap screws are recognized by the washer faced bearing
surface under the head. They are partially threaded with an unthreaded
area referred to as the "grip". The minimum thread length
is measured as twice the diameter plus 1/4" for screws 3/4"
in diameter up to 6" long inclusive. These screws are cold
form manufactured with rolled threads. When screw length is too
short for the minimum thread length rule to apply, they shall be
Screws with a length longer than 6" have a minimum
thread length of twice the diameter plus 1/2". The3se larger
screws may be either hot or cold headed and the threads may be cut,
rolled, or ground at the manufacturer's option. Hex cap screws are
available in various grades of steel.
|Does not require a heat treatment but may require
stress relief. There are no head markings on Grade 2 bolts and
therefore are most likely to be confused with machine bolts.
To verify check for a washer faced bearing surface under the
head. Minimum tensile strength = 74Ksi.
|Are heat treated, quenched in oil or water and tempered at
800°F. Look for the three radii markings on top of head.
Minimum tensile = 120 Ksi.
|Are heat treated, quenched in oil and tempered at 800°F.
Look for the 6 radii starburst on top of head. Minimum tensile
strength = 150 Ksi.
Tap bolts do not have a washer faced bearing surface
under the head. They are fully threaded to just under the radius
of the head. Tap bolts are generally available only in Grade 2 steel
but can be special ordered in Grade 5 or Grade 8 steel also.
Machine bolts are the newest of the hex bolt products.
They are partially threaded like the hex cap screws but do not have
a washer faced bearing surface under the head. Machine bolts are
currently only available in Grade 2 steel.