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.
Fasteners
Screws are generally smaller than bolts and are sometimes referred to as metal threads, they can have a variety of heads , they're used on smaller components, and often their thread extends right from the tip to the head so they can hold together components of different thickness.
Different screws can be tightened with a range of tools.
An
Allen head screw has a recess for an
Allen key. It’s sometimes called a cap screw. It usually screws into a hole rather than a nut, and it needs tightening with an Allen key.
A
machine screw has a slot for a screwdriver.
Bolts are always threaded into a nut or hole that has an identical thread cut inside.
There are a number of special screws cut their own threads as they go. This is called
tapping a thread and pictured above is a
self-tapping screw. It's made of hard material that cuts a mirror image of itself into the hole as you turn it.
This is also known as a
self-tapping screw but it’s designed for cutting and holding thin sheet metal, so it is often used on car bodies.
Bolts are often bigger than screws, and are used for heavier jobs. They nearly always have hexagonal heads that only fit spanners.
Torx drivers are used with for
torx bolts and are often found in vehicle engines where a particular need for fastening is required.
Often, the thread on a bolt is only as long as it needs to be to tighten onto the nut or into the threaded hole.
A
stud is like two bolts in one, for instance, an exhaust manifold on the cylinder head is normally located and held by studs and nuts.
Studs can have different threads on each end. On one end , there’s a thread that's best for gripping the hole in the cylinder head and to locate the exhaust manifold onto, and on the other end there’s a thread for pulling everything together tightly with a steel nut.
Nuts are often used with bolts. A nut is a piece of metal, usually hexagonal, with a thread cut through it. There are many different ways to keep it done up tightly.
A self-locking or
Nylock nut can have a plastic or nylon insert. Tightening the bolt squeezes it into the insert where it resists any movement. The self-locker is highly resistant to being loosened by the kind of vibration that engines and vehicles experience.
Tightening this style of nut distorts the insert, so it only provides its locking effect the first time you use it. If you remove the nut, it should be replaced with a new one.
A
Castellated nut has slots like towers on a castle. When it's screwed onto a bolt that's been drilled in the right spot, a split pin can be passed through them both and then spread open to lock the nut in place.
A
speed nut isn’t as strong as the others but it can be a fast and convenient way to secure a screw. Once the speed nut is started, it doesn’t need to be held.
Some bolts and nuts need washers.
Flat washers spread the load of a bolt-head or a nut as it’s tightened, and distribute it over a greater area. This protects the surface underneath from being marked by the nut or head as it turns and tightens down. Flat washers should always be used to protect aluminum alloy.
Other washers tackle the problem of nuts working loose through vibration or movement. A
spring washer compresses as the nut tightens, and the nut is spring loaded against this surface, which makes it unlikely to work loose. The ends of the spring washer also bite into the metal. Spring washers are used more for bolts and nuts.
Screws mostly rely on smaller
shake-proof washers. The external ones have teeth on the outside, the internal ones - on the inside, and one has both.
Tab washers get their name from these extensions. After the nut or bolt has been tightened they remain exposed and are folded up to grip the flats and prevent movement.
Chemical compounds help prevent fasteners loosening. They’re applied to one thread, then the other is screwed onto it. This creates a strong bond between them, but one that stays plastic, so in future they can be separated with a spanner if necessary. Other compounds can be applied after assembly.
Some metals react with each other and bind together.
Spark plugs can do it when they're in aluminum cylinder heads. An
anti-seize compound neutralizes the chemical reaction that can make this happen and it prevents threads and fasteners from sticking together.
Larger bolts and nuts must sometimes be tightened to a specified level - tight enough to hold components together but not so tight that the component or the fastener could fail. This level of tightness is called a torque specification. Bolts and nuts are often marked to tell you how strong they are, how much torque can be safely applied to them.
This is a
grade 5 bolt - as these markings show.
This is a
grade 8 bolt, so it can be done up more tightly without danger of it failing. The dots on this nut give similar information - this is a
grade 8.
These are
imperial system markings, the metric system uses numbers stamped on the heads of metric bolts, and on the face of metric nuts. Even studs have a marking system to make sure they're not over-stressed when you tighten them.
Pullers
The most common pullers have 2 or 3 legs which grip the part to be removed.
A center bolt is then screwed in, producing a 'jacking' or pulling action which extracts the part.
