Cap screw bolts hold machine parts together and fasten automotive engine parts together. The cap refers to the head of the bolt, which is generally a hexagonal shape. This type of bolt is partially threaded on the shaft to screw into a hole and the bottom of the shaft is flat. Cap screw bolts are also called hex head bolts. The most common sizes are 5/8-inch diameter and diameters of 1/4 to 2 inches, in 1/4-inch increments. To purchase a replacement for a cap screw bolt, you need to know the bolt length, its diameter and its thread length.
Stretch a measuring tape out to a length longer than the bolt. Push the measuring tape lock on. Set the tape on a table or counter with the numbers pointing up.
Place the bolt on the measuring tape with the bottom of the head at the zero mark. The bolt length is the measurement on the tape at the bottom of the bolt.
Set the bolt on the measuring tape with the cap facing up and the bottom of the threads facing down. This measurement gives you the diameter of the cap screw bolt.
Calculate the thread length of the bolt by multiplying the diameter times 2 and adding 1/2 inch. For example, multiplying a bolt's 3/4 diameter by 2 equals 1 1/2 inches. Adding 1/2 inch equals 2 inches of threads on the bolt.
Cap screw bolts may have a hex head alone, or a hex head with a screwdriver slot, either Phillips or flathead. Cap screw bolts install with a wrench. Slotted cap screw bolts may install with a wrench or a screwdriver.