screw drive systems

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

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	Metric 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	M6
#4	#18, #20, #24	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
000
00
0
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: http://revlimiter.net/store/jis-screwdrivers.php

Standard: JIS B 1012:1985

Frearson screw heads

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

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.

Regular polygons

Triangular recesses and heads

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 www.mcmaster.com.

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.

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??

• #0 fits screw gages 3 and 4
• #1 fits 5, 6 and 7
• #2 fits 8, 9 and 10
• #3 fits 12 and 14.

 

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.

Hexagonal heads and recesses

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.

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

axle

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

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

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″.

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

Torx®

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

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, millimeters
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.

Socket Size	Point to point internally, millimeters
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.

Torx Plus®

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.

TorxPlus drivers

driver size	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, M1	—	M0.9	—	—	—	—	—
2IP	—	M1	—	M1.2	—	M1	—	—	—	—	—
3IP	—	M1.2	#0	M1.4	—	M1.2	—	—	—	—	—
5IP	#0	M1.4, M1.6	#1	M1.6, M1.8	—	M1.4, 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, #3	M2.5	#4	M3	#4	—	#3
10IP	#5	M3	#5, #6	M3	#4, #5	M3	#6	M3.5	#5, #6	M3.5	#4, #5
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, 7/16	—	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, 5/8	M12	7/16, ½	M12	½	M12, M14	9/16, 5/8	M12	½, 9/16
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, 1	M22, M24	—	—	—	—	—