screw drive systems

slotted head 

cross drive


Frearson (Reed & Prince)

ISO metric












Line head




Every threaded fastener needs a way of turning it. It may have a head with a shape that a driver can engage, as 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”).

Slotted head screws

drawing of a slotted head

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. The advantages of the slotted head are that:

Otherwise the slotted head is the worst screw drive system, and is generally obsolete. 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.

Tip width Tip thickness
332 inch  
18 inch 0.012, 0.020
316 inch 0.031, 0.037
¼ inch 0.030, 0.039, 0.042
516 inch 0.039 
38 inch 0.055

A table showing tip widths and slot widths for American wood screws is here.

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.

Cross Drive Systems

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 head screws

drawing of a Phillips head

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

Size Fits these wood screws Fits these machine & tapping screws
#0 #0, #1 #0 and #1
#1 #2 – #4 #2, #3, #4
#2 #5, #6, #7, #8, #9, some #10 #5 – #10
#3 some #10, #11 – #16 #12, ¼ inch, 5⁄16 inch if roundhead
#4 #18, #20, #24 3⁄8 inch, 9⁄16 inch, plus 5⁄16 inch flathead
#5 5⁄8 inch, 3⁄4 inch

Frearson head screws

drawing comparing Phillips and Frearson screwdriver points

A later cross drive system referred to in ANSI standards as type II recess. Originated by Reed & Prince, it is sometimes referred to by that name. Now found mostly on marine hardware. Note the difference in points: Frearson has sharper V (75°). Any Frearson driver fits all Frearson screws.

ISO metric cross drive screw head


drawing of Pozidriv head

Pozidrive® screw heads

Identified in ANSI standards as type IA. As it doesn't cam out, great torque can be applied. Pozidriv screws can be turned by Phillips screwdrivers, but Pozidriv drivers won't turn Phillips screws.

Size Wood screws Machine and tapping screws
#0 #0, #1 #0, #1
#1 #2, #3, #4 #2, #3, #4
#2 #5–#9 #5 – #10
#3 #10–#16 #12 and ¼″,  516″ in some head styles
#4 #18–#24 516″ to ½″

Supadrive screw heads

drawing of Supadrive head

Supadrive drivers will turn Pozidrive heads.


Triangular recesses and heads

fire hydrant

photo of screww with triangular recess photo of screwdriver bit with triangular tip

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 at right is from Austria.

Square recesses and heads

Square nuts and four-sided heads are now mainly found in farm equipment and on lag screws.

Robertson head screws

drawing of head with square recess

A square recess design was invented by P. L. 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 to its use. 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.

Scrulox fits square recesses, made in four sizes:

Five-sided screw heads

drawing of 5-sided head

photo of 5-sided head on fire hydrant

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

drawing of 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 fifteenth 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. Wrench sizes for some common machine screws and bolts are given in the table.

Allen (hex recess)

Sizes are the flat-to-flat dimension.

Inch sizes are: .050″, 116″, 564″, 332″, 764″, 18″, 964″, 532″, 316″, 732″, 14″, 516″, 38″.

Metric sizes are 1.27 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 4 mm, 4.5 mm, 5 mm, 5.5 mm, and 6 mm.

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″

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® head screws

drawing of Torx head

The Torx system was introduced in 1965 by Camcar. This style of head is now very common in trucks and automobiles. The walls of the recess are not tapered. Drivers greatly outlast similar hex head drivers. Torx driver sizes for Torx recesses begin with a T. The sizes of the external drivers, which are less-common, begin with an E. T-6, T-7, T-8, T-9, T-10, T-15, T-20, T-25, T-27, T-30, T-40, T-45, T-50

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

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 source is

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.

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

ANSI B18.6.1

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