What is the difference between a tap bolt and a cap screw?
A tap bolt is (typically) a fully threaded hex bolt, while a cap screw is (typically) a hex head bolt that will be installed by turning the head of the bolt (screw) instead of tightening it with a nut. Usually cap screws are being threaded into something (a piece of machinery for example), rather than being used with a nut. A cap screw is not necessarily fully threaded unless you order it that way. Cap screws will follow a standard thread length formula as follows:
Standard Thread Length
For cap screws 6″ or less in length, thread length will be (2 X diameter) + 1/4″
For cap screws longer than 6″, thread length will be (2 X diameter) + 1/2″
To summarize, a bolt is installed by turning a nut, while a screw it installed by turning the head of the fastener. A tap bolt implies that that fastener is fully threaded.