screw drive systems

Every threaded fastener needs a way of turning it. It may have a head with a shape that a driver can engage, the way a wrench fits a hex-head bolt or a nut. Or it may have a shaped hole, into which a driver can be inserted (fastener engineers call the hole the “recess”).

screw and screwdriver drawing

Slotted head screws

Using a slot in a screw’s head to turn it is an old idea: drawings from the 16ᵗʰ century show screws with slotted heads. Some advantages of the slotted head are that:

Otherwise the slotted head is the worst screw drive system, and is generally obsolescent in industry, largely because it is utterly unsuited to automated driving. Some of its deficiencies:

To add to the shortcomings of the slotted head, screwdrivers for slotted screws are usually described by the length of the shaft and the width of the tip; the crucial measurement, the tip's thickness, is not given. Any given tip width is sold in a range of thicknesses; the longer shafts usually have the thicker blades. If a driver with the right tip thickness is not available, the tip of a driver can be ground to match the screw's slot width, which is not possible with more complicated recesses.

Slotted heads are almost universally used in the very small screws in clocks and watches. Of course, watchmakers' tools are made and maintained meticulously. In contrast, the usual household screwdriver is also used as a prybar and even as a chisel.

tip thicknesses available for various tip widths of slotted head screwdrivers
Tip width Tip thickness
³⁄₃₂ inch  
⅛ inch 0.012, 0.020
³⁄₁₆ inch 0.031, 0.037
¼ inch 0.030, 0.039,
⁵⁄₁₆ inch 0.039
³⁄₈ inch 0.055
slot width,
min max
0000 0.005 0.008
000 0.008 0.012
00 0.010 0.017
0 0.016 0.023
1 0.019 0.023
2 0.023 0.031
3 0.027 0.035
4 0.031 0.039
5 0.035 0.043
6 0.039 0.048
8 0.045 0.054
10 0.050 0.060
12 0.056 0.067
¼ 0.064 0.075
5/16 0.072 0.084
3/8 0.081 0.094
7/16 0.081 0.094
½ 0.091 0.106
9/16 0.102 0.118
5/8 0.116 0.133
¾ 0.131 0.149
Size of fastener slot width,
cheese head
machine screw
max nominal min
M1   0.25    
M1.2   0.3    
M1.4   0.3    
M1.6   0.6 0.4 0.46
(M1.7)   0.4    
(M1.8)   0.4    
M2 0.5 0.7 0.5 0.56
(M2.3)   0.6    
M2.5 0.6 0.8 0.6 0.66
(M2.6)   0.6    
M3 0.8 1 0.8 0.86
(M3.5) 0.8 1.2 1 1.06
M4 1 1.51 1.2 1.26
M4.5 1      
M5 1.2 1.51 1.2 1.26
M5.5 1.2      
M6 1.6 1.91 1.6 1.66
M7 2      
M8 2 2.31 2 2.06
M10 2.5 2.81 2.5 2.56
M12 2.5      

We provide a table showing tip widths and slot widths for American wood screws.

A tamper-proof slotted head design is available. It is used in low-tech areas where vandalism and theft are feared, such as window fixtures and public toilet stalls. Opposite quarters of the head are cut away so that a flat blade driver rotating counterclockwise has nothing to push against. Special bits are sold that are capable of removing these screws.

Watches sometimes contain small screws whose heads have three parallel slots. The center slot is for driving the screw; the slots to the side are shallower and are a sign that the screw has a left-hand thread.

screw and screwdriver drawing

Cross Drive Systems

drawing of frearson point

From Frearson's 1873 U.S. patent.

John Frearson, an engineer in Birmingham, England, invented a cross drive screw head, being granted a U.S. patent in 1873. His patent describes the major advantages:

It is well known to persons who use screws that if the nicks are narrow and shallow it is difficult to drive the screw without the screw driver slipping out of the nicks, and if the nicks are wide and deep to afford a good gripe, the head of the screw is weakened, and the screw-driver is liable to slip out sidewise and deface the finished surface of the work, and if the screw-driver is the same width as or wider than the head of the screw, the countersink work is liable to be defaced, and the angles of the screw-driver are often broken.

In all cross drive systems the driver will self-align with the fastener. Both the driver and fastener recess are tapered. Camout is possible and can ream the recess and destroy the bit.

Phillips screw heads

patent drawing

From Phillips 1936 patent.

The licensor is the American Screw Co. The Phillips system was invented for use in assembling aluminum aircraft, with the object of preventing assemblers from tightening screws so tightly that the aluminum threads strip. The driver will cam out before that happens. The driver has a 123° point with a blunt tip, tapered wings.

which gauges of wood screw and machine and tapping screw each Phillips screwdriver size fits
Size Fits these
wood screws
Fits these machine
& tapping screws
#0 #0, #1 #0 and #1 M1.6, M2
#1 #2 – #4 #2, #3, #4 M2.5, M3
#2 #5 – #9,
some #10
#5 – #10 M3.5, M4, M5
#3 some #10,
#11 – #16
#12, ¼ inch,
5⁄16 inch if roundhead
#4 #18, #20,
3⁄8 inch,
9⁄16 inch,
plus 5⁄16 inch flathead
M8, M10
#5 ⅝ inch, ¾ inch  

Consumers are likely to think that any screw head with a cross drive recess is a Phillips, which can lead to problems.

JIS (Japan Industrial Standard) screw heads

This recess design was adopted as a Japanese standard in 1958, and revised in 1964, 1974 and 1985. It is only found on metric screws. While the goal in designing the phillips was that it camout, the goal in designing the JIS recess was that it NOT camout. A JIS screwdriver will turn Phillips screws, but a Phillips screwdriver should not be used on a JIS screw; it will camout and likely ruin the head. JIS-headed screws are used in Japanese motorcycles, and many a home-maintained motorcycle in the U.S. has had the heads of its screws ruined by Philips screwdrivers.

JIS screws are found in model airplanes and their servos, and many other Asian manufactures.

It is hard to tell whether an installed screw is a phillips or a JIS. The standard originally called for a dimple on the screwhead. Apparently, however, some JIS screwheads now have no dimple.

Unfortunately, vendors describe bits as “JIS Phillips” The item so described should actually be a JIS bit, the “Phillips” simply being an indication that the item in question is a cross drive.

Mechanics make emergency JIS bits by grinding a flat tip on a Phillips bit. Obviously, it is better to buy the proper bit.

Bits are made in #000, #00, #0, #1, #2, and #3 sizes.

Bit Screw
1 M2, (M2.2), *M2.3, M2.5, *M2.6
2 M3, (M3.5), M4, (M4.5), M5
3 M6, M8

* screws not in the ISO standard series. Values in () are secondary ones.

This vendor provides a good discussion:

Standard: JIS B 1012:1985

Frearson screw heads

drawing comparing Phillips and Frearson screwdriver points

A cross drive system referred to in ANSI standards as type II recess.

In the United States screws with this head were manufactured by Reed & Prince, and are sometimes referred to by that name.

In the United States, Frearson screws are mainly found as the marine bronze wood screws used in boat building.

Note the difference in points: Frearson has sharper V (75°).

Any Frearson driver fits all Frearson screws.

ISO metric cross drive screw head

H (Phillips type) and Z (Pozidrive type)

DIN goes to ISO 19XX

Pozidrive® screw heads

logo-like view of pozidrive screwhead seen from above

Called type IA in ANSI standards and type Z in ISO standards. It doesn't cam out, so great torque can be applied. Pozidriv screws can be turned by Phillips screwdrivers, but Pozidriv drivers won't turn Phillips screws. In an effort to force a Pozidriv driver into a Phillips recess, one is liable to resort to too small a size, and damage the screw, driver or both.

which gauges of wood screw and machine and tapping screw each Pozidriv screwdriver size fits
Size Wood screws Machine and tapping screws Metric screws
#0 #0, #1 #0, #1 M1.6, M2
#1 #2, #3, #4 #2, #3, #4 M2.5, M3
#2 #5–#9 #5 – #10 M3.5, M4, M5
#3 #10–#16 #12 and ¼″, ⁵⁄₁₆″ in some head styles M6
#4 #18–#24 ⁵⁄₁₆″ to ½″ M8, M10

Supadrive screw heads

Supadrive drivers will turn Pozidrive heads.

screw and screwdriver drawing

Regular polygons

Triangular recesses and heads

screwdriver bit with triangular tip screw with triangular recess

Screws with triangular recesses are found in some consumer appliances. Unlike a screw with a square recess, these cannot be turned with a slotted screwdriver, and so the use of these screws discourages do-it-yourself tampering. The bits are sized by the distance from a corner to the midpoint of the opposite side (the altitude of the equilateral triangle, for those who took geometry). They are available in at least 4 sizes (TA18, 0.079 inch; TA20, 0.091″; TA23, 0.106″; TA27, 0.126″) but are uncommon. Try

Triangular heads are used on fire hydrants and similar devices to prevent ordinary, parallel-jaw wrenches from turning the head. The example below is from Austria.

fire hydrant

A tamperproof screw with a triangular recess was patented, and the patent assigned to Lockheed. It was manufactured, but is rarely encountered.

Square recesses and heads

Square nuts and four-sided heads are now mainly found in farm equipment and on lag screws. Four-point socket wrenches do exist.

Robertson head screws

A square recess design was invented by P. Lymburner Robertson in 1908. Its advantages are great resistance to camout and 4 possible positions for the driver. Henry Ford used such screws in the Model A, but dropped it when Robertson refused to give him exclusive rights. Robertson also refused to license other fastener manufacturers, so the design spread very slowly. Many recreational vehicles built in the 1950s use these screws. In Canada, most wood screws have square recess heads.

Size Color Fits these screws
00 orange #1, #2
0 yellow #3, #4
1 green #5, #6, #7
2 red #8, #10
3 black #12, #14
4 black #16 and larger

Scrulox fits square recesses, made in four sizes: double square Stanley??


Five-sided screw heads

Five-sided heads are used for caps and valves of fire hydrants, and in other situations in which a fastener that cannot be removed by commonly available wrenches (most of which have parallel jaws) is needed.

wrench for 5-sided heads
5-sided head on fire hydrant

Hexagonal heads and recesses

hex head

Probably the most common of all fastener heads, hex heads are also very old. Fasteners with hexagonal heads were used to hold armor together in the 15ᵗʰ century.

drawing showing head dimension to be measured

To find the size of wrench needed to turn a hex head (or hex recess), measure from flat to flat, not from point to point. Socket wrench sizes for some common machine screws and bolts are given in the table.

Allen (hex recess)

Sizes are the flat-to-flat dimension. For lists of inch and metric allen key sizes, and the set screws and cap screws they fit, see this page.

Octagonal heads


The common 8-pt (double-square) socket wrench does not fit an octagonal head. True octagonal socket wrenches are made, but are rare.

Twelve-sided heads

drawing of 12-sided head

Found in aerospace, automobiles, engine head bolts, Ford rear ends, etc.

screw and screwdriver drawing

Clutch head

drawing of clutch head

Originated by United Screw and Bolt. The recess in clutch heads looks like a bowtie. In a pinch, a clutch head screw can be driven by a slotted screwdriver. A worn tip on a driver can easily be restored by grinding off the end. Clutch head screws were popular in mobile home construction and electric motors. The size is the maximum diameter in inches of the bit point: 1⁄8″, 5⁄32″, 3⁄16″, 1⁄4″, 5⁄16″.

screw and screwdriver drawing

Bristol Spline

Originated by the Bristol Co. A recess with 6 flutes (except for 2 sizes that have 4 flutes). Sized in inches: .048, .060, .069 (4 flute), .072, .076 (4 flute), .096, .111, .145, .183

screw and screwdriver drawing


The Torx system was introduced in 1965 by Camcar, and patented in 1971. Its great popularity, however, really began in the 1980's, when it became very common in trucks and automobiles.

Both internal and external versions are made. The walls of the recess are not tapered, so camout is absent. Drivers greatly outlast similar hex head drivers.

The same system of sizes is used for both metric and inch fasteners. To match drivers to fastener sizes, see the table below, under Torx Plus.

Internal Torx logo-like view of torx screwhead seen from above

Driver sizes for Torx recesses begin with a T. The dimensions below are measured on the driver bit, from point to opposite point

Size point to point,
T1 0.81
T2 0.93
T3 1.10
T4 1.28
T5 1.42
T6 1.70
T7 1.99
T8 2.31
T9 2.50
T10 2.74
T15 3.27
T20 3.86
T25 4.43
T27 4.99
T30 5.52
T40 6.65
T45 7.82
T50 8.83
T55 11.22
T60 13.25
T70 15.51
T80 17.54
T90 19.92
T100 22.13

External Torx

The sizes of the external drivers, which are less common, begin with an E.

Point to point internally,
E4 3.8
E5 4.7
E6 5.6
E7 6.1
E8 7.4
E10 9.3
E12 11.1
E14 12.8
E16 14.7
E18 16.6
E20 18.4
E24 22.1

Tamperproof Torx

Tamperproof Torx heads are the same as the internal recess heads, but have a post in the center which prevents ordinary Torx drivers from entering the recess. TT-7, TT-8, TT-9, TT-10, TT-15, TT-20, TT-25, TT-27, TT-30, TT-40, TT-45 and TT-50.

screw and screwdriver drawing

Torx Plus®

patent drawing for Torx Plus

From U.S. patent 5207132.

The Torx Plus system was introduced by Camcar in 1991. Like Torx, it is 6-lobed and has straight walls. The improvement is in the design of the lobes, which were changed to elliptical (from circular in Torx). The result is that the force imparted by turning the driver is perpendicular to a radius of the fastener.

A Torx driver can drive or remove a Torx Plus screw, but the added benefits of the Torx Plus design are lost and wear is increased.

External Torx drivers (i.e., sockets) do not fit external Torx Plus fasteners.

If you are reading this on a smartphone, the next table will be wider than your screen. To avoid having to scroll horizontally, please turn your device horizontally and view the table in landscape mode.

Use the controls below to choose what types of screws you want data for.

Do you want inch or metric measure?
TorxPlus drivers
pan head flat head socket head socket button truss head fillister
inch metric inch metric inch metric inch metric inch metric inch
1IP M0.9 M0.9,
2IP M1 M1.2 M1
3IP M1.2 #0 M1.4 M1.2
5IP #0 M1.4,
#1 M1.6,
6IP #1 M2 #2 M2 #0 M2 #2 #2 #1
7IP #2 #3 #1 #3 #3 #2
8IP #3 M2.5 #4 M2.5 #2,
M2.5 #4 M3 #4 #3
10IP #5 M3 #5,
M3 #4,
M3 #6 M3.5 #5,
M3.5 #4,
15IP #6 M3.5 #8 M4 #6 M3.5 #8 M4 #8 M4 #6
20IP #8 M4 #10 M5 #8 M4 #10 #10 M5 #8
25IP #10 M5 #12 #10 M4.5 M5 #12 #10
27IP #12 M6 M5 M6 ¼ #12
30IP ¼ M6 ¼ M8 ¼ M6 ¼ 5/16 M6 ¼
40IP 5/16 5/16 M7 5/16 M8 3/8 M8 5/16
45IP 3/8 M8 3/8,
5/16 M8 3/8 M10 7/16 3/8
50IP 7/16 M10 ½ M10 3/8 M10 ½ M10 7/16
55IP ½ M12 9/16,
M12 7/16,
M12 ½ M12,
M12 ½,
60IP ¾ 9/16 M14 5/8 M16 ¾ 5/8
70IP 5/8 M16 3/4
80IP ¾ M18
90IP M20
100IP 7/16,

Torx Plus Tamper-resistant logo-like view of torx plus tamper resistent screwhead seen from above

Tamper-resistant Torx Plus recesses differ from other Torx Plus recesses in having 5 lobes instead of 6. It also has a center post.

“Tamperproof” screws are used by manufacturers to prevent consumers from opening things they shouldn't. The usual strategy for creating such screws is to add a post in the center of the recess in the screw head, and a central hole in the driver. The post prevents the ordinary driver from entering the recess. The TT Torx screws are an example of such a system. However, today anyone can buy drivers to fit such security screws, on the web, if not in the local hardware store. They are thus not as tamperproof as originally intended.

Torx Plus tackles the weak point in this situation by restricting the sale of the driver bits to equipment manufacturers and their authorized repair services. Those who can produce the necessary credentials can purchase tamper-resistant Torx Plus bits from The identifier is IPR. They come in sizes IPR-8, IPR-10, IPR-15, IPR-20, IPR-25, IPR-27, IPR-30 and IPR-40 (fasteners from M2.5 to M25; #3 to 1 inch).

Mortorq® Spiral

Philips Screw Co. Identifer (on bits, etc), "MT"

inch mm
MT-0 3/8, #10 4.5
MT-1 1/4 6
MT-2 5/16 8
MT-3 3/8 10
MT-4 ½ 12
MT-8 1  

Mortorq® Super Spiral

Identifier is MTS

MTS-0 3/8, #10 4.5
MTS-1 1/4 6
MTS-2 5/16 8
MTS-3 3/8 10
MTS-4 frac12 12

External Mortorq® Spiral

In an emergency, can be turned with regular hex or 12-pt sockets.

MTS-X1 #8 M4
MTS-X44 2 M50

Mortorq® Tamper-resistant

Line head screws

drawing of ALR heads

A Japanese system found, for example, in the IBM PS/2 computers and Nintendo games.

internal: ALR2, ALR3, ALR4, ALR5, ALR6

external: ALH2, ALH3, ALH4, ALH5, ALH6

internal tamper resistant: ALR3T, ALR4T, ALR5T, ALR6T

Line head bits can be difficult to locate. A substitute are the “Gamebits” made by iFixit: These seem okay, but according to many online reviews, most line head bits offered online are of very poor quality.

Combination heads

Fasteners are often made with heads that combine two systems, usually so that service people in the field will be able to disassemble the product with tools in a different system from the one the factory uses. Examples include hex head cross drive, slotted internal Torx, hex head internal Torx, and so on.


Wooden Boat forums. The boat builders' discussion of the comparative merits of slotted, Robertson and Frearson screws is especially interesting.

Assembly Technology Buyer's Guide
Wheeling, IL.: Hitchcock Publishing, annual.

ANSI B18.6.1