American screw threads

[RogerS]

Can anyone tell me what a 1/4"-20 hex bolt means, please.

Many thanks, Roger

 

[Travis Byrne]

Hello Roger

The 1/4" is the OD (outside dia.) of the main body of the bolt.
The 20 is the no. of threads per inch. The hex head is a 6 sided head taking 7/16" wrench (Spanner?) If there is a nut, it will probably be the same as the head.

American bolts have two standard threading system. the 1/4-20 is course. the 1/4-28 is the fine threaded one. The threaded portion can be a inch or two or the entire body of the bolt.

Hope that some of this helps with the answer.
There also is the question of grades of Bolts, but thats a differant story.
Travis

 

[jasonB]

The angle of the thread may also vary depending on what it is: BSF, UNF, UNC, ME, BSW,BA, BSC, etc.

The bolt in question will as Travis says have a shank of 1/4" and a thread pitch of 20 teeth per inch whick makes it either Whitworth or UNC. and if it is american then UNC is the likely one.

Jason

 

[Bean]

Rodger a good buy for problem threads is a zeus book they are a few quid and usually invaluble.
If you are trying to source some fasteners try Model engineering suppliers as they use them up to 1/4" and sometimes above. Failing that a Landrover dealer or a vintage motorcycle dealer/repairermay be worth a try as they used them as well.

Bean

 

[RogerS]

Thanks, guys. That's really useful information.

Out of curiosity where do people buy small quantities of American hex bolts/screws from? It's just the type of useful thing that Whistons' used to deal in but I've not heard of them for years and years.

Roger

 

[jasonB]

Used these people quite a bit when I was restoring a 1928 Austin Seven.

http://www.namrick.co.uk/

Jason

 

[RogerS]

Excellent site, Jason..many thanks.

I also came across another good site www.chronos.ltd.uk and have just ordered a few bits and pieces from them.

Roger