Identifying an unusual thread: 26 tpi

[Mr Ron]

Do you have enough meat to drill and retap for a 3/8-24 thread? In the fractional thread size, there is a 5/16-27 thread. Could that be it? An M8-1 could also be it.

[10 Wheeler Rob]

The place I have seen the 5/16"-27 thread is on shot gun cleaning brushes. No idea why for such a non critical purpose they use a not standard thread other than it keeps sporting goods stores in business.

[kapullen]

5/16 26 threads were/are used on British pipe fittings.

http://www.stuartmodels.com/accessories ... k%20Valves

lists the valves in question.

wkipedia says;

"Note that M8 × 1.00 metric threaded parts can interchange with 5/16th × 26 tpi parts with a "Class B fit", meaning you should not go back and forth between ..."

There is a slight pitch error and 8 mm is .3149 making it a bit
on the tight side for a tight 5/16-26 threaded hole.

I would use hardened m8 x 1 bolts but you may have to tap the holes out.

Kap

[Bob Flowers]

You are dealing with an "older" (say PRE-WWI) version of the 5/16-26 BSCY (whitworth)!
"About 1900", the British Standard Cycle thread series was "Updated".
All the threads smaller than 1/4-26 were dropped (can still be ordered as BSCY (special), AND will generally now come in the 60 degree thread form vs the old Whitworth thread form.
The Major diameter thread size you measured is 100% due to the rounded crest of the threads (55 degrees) being established after the nominal shaft diameter was chosen. Remove the sharp vee pitch to a rounded whitworth crest and you have the full answer. Using a modern 5/16-26 BSCY from EBay ( enter search: "BSCY 26 TPI TAP DIE SET 5/16" TAP AND DIE set") and you will have a full set of taps and dies cheaper than you could cut it on a lathe! Get a long shanked modern 5/16 screw and cut off most of the threads so you have something to start on and you can make it in about 17 minutes, if you take 2 minutes to admire them!
Other OLD BSCY threads (still available today as BSCY-specials) fit Revolutionary War Charleville muskets, US 1816 Flintlocks and 1842 Percussion Muskets, Springfield .58 calibers and Trapdoor Springfields, PLUS the famous 1853 Enfield rifle used in India and by both sides in our Civil War! Hope this reaches you, I'm a newbie here.

[earlgo]

If it were a UN screw it would be a #19. Easily determined by measuring OD, subtracting .060 and then dividing the remainder by .013.
So .307-.060 = .247 and .247/.013 = 19 Definitely not a hardware store size. The diameter of a UN screw is equal to (screw number x .013) +.06.
So a #10 screw is 10*.013 = .130 and .130 + .06 = .190, and your uncle is Robert.
--earlgo

[toglhot]

It's called a Brass Thread. Brass threads use 26TPI regardless of bolt size. Not in general use anymore.