Threads are cut on screws, bolts, nuts, studs and inside holes to allow components to be attached and assembled.
There was a time when there were many different thread designs used throughout the world. Modern vehicles still use a range of thread patterns but due to standardization it’s getting much simpler.
Nearly all the nuts, bolts, screws and studs on a vehicle have a V-thread cut into them. For something that isn't a V-thread, look at a screw jack, or a clamp. They have what is called a square thread.
The pitch of a thread is the distance between the crests .
It’s measured by a thread pitch gauge, which come in sets. By laying the blades along the bolt it’s easy to find one with teeth that fit neatly into the thread.
Thread tables show what size hole has to be drilled, and what size tool is needed to cut the right thread for any given size bolt. A 3/8” bolt for instance can have a coarse thread, or a fine thread. This is because some threads are required to grip metals that are brittle or soft and this requires more metal in the thread. Such bolts normally need coarse threads.
A thread in a steel nut can be much finer. Fine threads give more grip for a given torque than coarse threads.
The thread on this engine stud for example, is coarse on the end that screws into the cylinder head, and fine for the steel nut that tightens the exhaust manifold to the cylinder head.
Taps cut threads inside holes or nuts. They normally come as sets of three.
The first in the set is known as a taper tap.
It narrows at the tip to give it a good start in the hole where the thread is to be cut. In a piece of flat steel that is having a thread tapped into it a "Tapping size table" is referred to, to give the right drill size for the hole. The taper tap can tap a thread right through the piece of steel to enable a bolt to be screwed into it.
The second in this set is an intermediate tap, and the third, a bottoming tap. There are used to tap a thread into a hole which doesn't come out the other side of the material. Often called a blind hole, a taper tap is used to start the thread in the hole and then the intermediate tap is used followed by a bottoming tap to take the thread right to the bottom of the blind hole.
This tap wrench comes with the set. It has a right-angled jaw that matches the squared end which all taps have.
To cut a thread in an awkward space, this T-shaped tap wrench is very convenient but harder to turn and to guide accurately.
Screw extractors are used if a screw, stud or bolt snaps off in the threaded hole.
A common type of extractor uses a course left hand thread formed on its hardened body. Normally a hole is drilled in the center of the broken screw and then the extractor is screwed in to the hole. The left hand thread grips the broken part and unscrews it. This extractor is marked with the sizes of the screw it’s designed to remove and the hole which needs to be drilled.
To cut a brand new thread on a blank rod or shaft, a button die held in a die stock is used. The button die is split so that it can be adjusted more tightly onto the work with each pass of the die, as the thread is cut deeper and deeper, until the nut fits snugly.
The die nut is more common in the workshop. It’s hexagonal to fit a spanner and it’s mostly used to clean up threads that have been damaged.