Types of screw head and screwdriver
There are many types of screw head, each with its special type of screwdriver. This page lists all the common (and not so common) ones you'll encounter.
Apart from the common slotted and cross-head screws, there are many others you will come across from time to time, some more often than others, which are described here together with their advantags and disadvantages.
- If a screwdriver slips it can damage adjacent components, or possibly cause injury.
- Make sure you know how to drive screws like a pro otherwise you may damage a screw head and make it almost impossible to remove.
Types of screw head
Each of the many different types of screw head comes in a range of sizes. With some types the wrong size won't work at all, and with the more forgiving types the wrong size will at best give you sub-optimal leverage and at worst will damage (or "cam out") the screw head.
The following are the types of screw you are most likely to come across as well as most of the less common.
The commonest and oldest type of screw head just consists of a slot, and is driven by a flat bladed screwdriver. It has the disadvantage that the head is easily damaged and the screwdriver blade easily slips out of one end or the other of the slot. Choose a screwdriver with a blade which best fits both the width and length of the slot and make sure it's centred within the slot.
In a tamper-resistant version the slot is interrupted in the centre, making it impossible to use a standard flat screwdriver. This requires a special screwdriver with a slot in the centre of the blade. This version is quite often seen in domestic electrical and kitchenware items. If you're stuck, you may be able to use a dremel to cut a slot in the blade of a standard screwdriver.
This is a common form of cross-head screw, not to be confused with the Pozidriv (see below). Phillips screwdrivers come in sizes 000, 00, 0, 1, 2, 3 (in increasing order of size), sometimes prefixed with "PH", to distinguish them from Pozidriv. Always use the right size if possible.
Superficially similar to a Phillips screw, a Pozidriv screw is distinguished by the tick marks at 45 degrees and is designed to allow greater torque to be applied without risk of camming out. Pozidriv screwdrivers come in sizes 0 - 5, prefixed with "PZ".
A Pozidriv screwdriver has a less pointed tip than a Phillips and so it is inadvisable to use a Phillips screwdriver on Pozidriv screws as the screw head is liable to be damaged if more than minimal torque is applied.
Yet another superficially similar cross-head screw is the JIS or Japanese Industry Standard, commonest on the Pacific rim. It is distinguished by a raised or recessed dot on the head.
The correct driver is less pointed than a Phillips and the corners of the cross are square rather than rounded. A Phillips screwdriver is liable to cam out a JIS screw.
Common in electronic equipment, a Torx screw requires exactly the right screwdriver. Correctly used, it gives very good resistance to cam-out without having to press excessively hard. Torx screwdrivers come in a wide range of sizes, from T1 (smallest) up to T100. The smaller sizes from around T6 to T10 are commonest in electronic repair.
A tamper-resistant version has a small pin in the centre of the recess, requiring a screwdriver with a corresponding hole in the centre of the tip. If you have a security screwdriver set with both the Torx and security Torx bits, don't be tempted to use the security variant on a normal Torx screw as it's less strong and can shear off if you use it on a tough screw.
These have a hexagonal recess in the head and require either a hexagonal screwdriver or an Allen key, which is simply a piece of hexagonal hardened steel bent into an L-shape. Self-assembly furniture and other items often uses hex screws and come with an Allen key in the pack of screws.
Several unusual triangular screw heads have been invented but this one is sometimes seen on electronic and domestic electrical equipment where it is used to discourage dismantling by unqualified people, particularly where there might be a serious electric shock hazard.
At first glance resembling a Torx screw, these have 5 lobes instead of 6. They are commonly used by Apple as a tamper-resistant screw, but the screwdrivers are readily available. If you order a replacement iPhone battery or screen it will often come with a cheap pentalobe screwdriver. Due to their small size, these can be fairly easily damaged.
In addition to the above, less often you may come across various other types of screw. Many are listed in the Wikipedia article List of screw drives.
A last resort
If you don't have the right screwdriver and can't procure it in the available time, a trick that may work for you if the screw isn't too tight is as follows:
Find a cheap plastic ballpoint pen and remove the writing tip and ink tube. Now gently heat the tip in a lighter flame until it starts to become soft and mouldable. Be careful not to overdo it or set fire to it. Press the softened tip into the screw head and hold it there for 2 to 3 minutes, as straight as possible, until the plastic has cooled and hardened. Whilst still pressing it into the screw head you may now be able to use it to turn the screw, twisting it gently at first.
- A YouTube video Why Are There so Many Types of Screws?! gives an interesting insight into the history and rationale of some of the screw types we see today.