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General | Types (Round head, flat head, ...) | Drives (phillips, torx, ...) | Nuts | Terms

The "Machinery's Handbook" categorizes fasteners as:

  • Nails, spikes, wood screws
  • Rivets
  • Bolts, Screws & Nuts
    • Machine screws
    • Cap screws
    • set screws
    • wing screws & thumb screws.
Screw/Bolt types:
  • Bolts - Goes thru a hole and is held by a nut. *
    Sometimes large screws e.g. lag screws are called bolts.
  • Screw - Screws into a tapped (threaded) hole or self-tapping or wood screws. *
  • Machine Screw - Threaded all the way or a smaller shank allowing the screw to go all the way in.
  • Cap Screw - Typically only partially threaded with the solid part the same diameter as the outside of the threads (bolts),
      or, have machine threads and require an Allen (inhex) or other drive tool that inserts into the head.
* The "Machinery's Handbook"

I could not find any official definition of Machine or Cap screw.
The list in the table below is the way they were listed in
"The Machinery's Handbook", but round and flat head were listed as both, the illustrations were similar to the ones above.

Machine Screws
flat countersunk (82° & 100°)

oval countersunk
Cap Screws
round head
flat countersunk
socket head

Drive Type:
phillips, slotted, combinataion, socket, hex, allen, ph, sl, torx, star, drive
See Pozidriv below.

Head Type:
bolt, screw, round head, hex head, flat, head, pan head, oval head, fillster head, button head, SHCS, Socket head cap screw, torix
SHCS - Socket Head Cap Screw

Most head types come with a variety of drive types.
Different combinations of names may be used for the same type. One common list included: Socket Head Cap, Hex Head Flange Bolts, Socket Counter Sank Head, Socket Button Head Cap Screw, Socket Set Screw, Truss, Pan, CSK (Countersunk) Heads, Round Head, cheese head, mushroom head.

Bolts and Screws (grade, torque, ...)
Screws (wood, sheet metal, ...)
Fastner Type Chart at
Bolts and Screws at

Screw head stripping is common when screwing into hard woods or when using Cordless screwdrivers and impact drivers:
Phillip's head screws are actually designed to "cam out". That is, when the screw stops turning easily, the bit is pushed up and out of the screw head. This is to prevent you from over-torquing the screw and damaging the work, screw, or bit.
When using impact drivers, phillip's heads are more prone to stripping.
Pozidriv looks a lot like Phillip's, but has a subtly different shape that reduces cam-out.

Square, (aka Robertson) is easier to work with than Phillip's, but not as nice as Torx (aka star) & hex heads.
Some comments:
Square heads strip more than star, but the star tips are more prone to breaking when using an impact driver. Overall, I prefer the star, but when using them, I'll use the drill/driver more than the impact.

Soap can help lubricate screws in to wood, making it easier and reducing screw breakage.

See: How to Stop Stripping Screw Heads

TORX PLUS is an improved version of TORX® which extends tool life even further and permits greater torque transfer compared to TORX.

Many screw drives, including Phillips, TORX, and Hexagonal, are also manufactured in tamper-resistant form. These typically have a pin protruding in the center of the screw head, necessitating a special tool for extraction.

Phillips screws come in 82° and 100° taper

The Frearson screw drive, also known as the Reed and Prince screw drive, is similar to a Phillips but has a more pointed 75° V shape. Its advantage over the Phillips drive is that one driver or bit fits all screw sizes. It is found mainly in marine hardware stores. The tool recess is a perfect cross, unlike the Phillips head which has a bevel in the center, which is designed to cam out.
See: wikipedia: List_of_screw_drives#Frearson

Pozidriv® is very common in Europe. It looks a lot like a Phillips screwhead, but it includes 4 small ribs in the groove. Common sizes are Pozidriv® #0, #1, #2 and #3.
pozidriv Phillips screws and drivers have an intentional angle on the flanks so they will cam out of the slot before a power tool will twist off the screw head. This is (was) a creative production approach when the device was first invented, but it leaves a lot to be desired when it comes to field service of the screws.

With improvements in production tools to better control the torque of the tool the cam-out action is not necessary, and has very little purpose (except to aggravate people). The Pozidriv screws and drivers have straight sided flanks which are, not coincidentally, much more friendly for field servicing. pozidriv bit With Pozidriv you can apply much more torque to the screw without having the driver bit jump out of the slot.
A phillips bit will strip out the pozidriv screw head in no time. See: ATOS-Magazin 1997: Pozidriv, Supadriv und Philips

wing, hex, domed cap, acorn, tee
Nuts - Hex Nuts, Cap Nuts, Wing Nuts, T-nuts - Bolt Depot
Terms: (See also standards below)
12N - National 12 Pitch
AN - World War II era hardware standards system. Literally "Air Force/Navy"
  or "Army/Navy." A very logical numbering system now being
  bastardized by government changeover to NAS/MS series.
FT - Fine thread (same as UNF)
FH - Flat Head
HH - Hex Head
HX - Hex Head
MS - Military Standard specification
National Coarse thread pitch (same as UNC)
NF - National Fine thread pitch (same as UNF)
NEF - National Extra Fine thread pitch
NS - National Special thread pitch
OH - Oval Head
PH - Pan Head
PH - Phillips Drive
psi - Pounds per square inch.
tpi - Threads per inch
UNC - Unified National Course thread form
UNF - Unified National Fine thread series (same as FT)

ISO - International Organization for Standardization - ISO 898-1, 1999
SAE - Society of Automotive Engineers - SAE J429
ASME - American Society of Mechanical Engineers
COFS - Committee on Framing Standards of AISI
ee Glossaries at:, Defense Logistics Agency

links: - Nuts and Bolts, Screws and Fasteners online
Head Styles - James Glen
Wood and Sheet metal screws | Machine screws and bolts
Fastner Type Chart and Nuts at
US & Metric Bolt Torque by Grade
Screw Finder at
Screw Information at
Screwtypes at
Fastener Material Chart at
Metric Bolt Properties Grade, Strength
Proof Loads at
Strength at
ASTM, SAE AND ISO grade markings and mechanical properties for steel fasteners at
SAE Standards for Bolts
CD Stud Fasteners Specifications at SunBeltWelding
Tap Drill Sizes
screw drive systems at
Screw - Facts, Information, and Encyclopedia Reference article
Corrosion Resistant Bolts at
Aircraft Hardware
Automotive Racing Products (ARP)- Bolts
Stainles Steel fasteners with specs for Aluminum, Titanium also at
Fastener and Screw / Bolt Design, Formula and Calculation at Engineers Edge
Strength Properties of Metals at
The Tempering of Martensite
Types of bolts and screw heads.
Fastener types.
Product Specifications at Barnhill Bolt
Understanding Threads and Fasteners at (local copy)
Fasteners 101
Home Depot Special Order Fasteners at

"The Nuts and Bolts of Bolting" by Joseph C. Dille
Fastners 101
Metallurgy for the Non-Engineer

Gardner-Westcott is a bolt manufacturer.

Pozidriv™ is a trademark of European Industrial Services Ltd.
Pozidriv® is a registered trademarks of Phillips Screw Co.
Torx® is a reg. trademark of Camcar-Textron Inc. USA

last updated 1 Oct 2008