BSW and UNC thread identification

I help to restore old machinery. We need to be able to match the bolts on old equipment, so we have a good supply of BSW, UNC and metric bolts, and some of the finer thread sizes. There are around 400 boxes of bolts, of various lengths. Many of these are new but some are up to

100 years old. Some of the boxes are not labelled or, I suspect, wrongly labelled. Many of the bolts and nuts do not have markings that follow any standards that I have found.

I would like to be able to easily identify the difference between BSW and UNC bolts. BSW has 55 degree threads while UNCs are 60 degrees. I have thought of the following ways to identify the difference:

  1. When I place a UNC thread against a BSW thread and look with a magnifying glass, the top of the BSW threads touch the bottom of the UNC threads, and the difference in angle can be seen. Probably this is the best way of telling the difference, for larger thread sizes. I'll check to see if our thread gauges can be used to see the different angle.

  1. The UNC bolts go easily into BSW nuts but BSW bolts don't go into UNC nuts. If the bolts are plated or old this is not a clear test.

  2. The UNC bolts that I'm looking at seem to have smaller heads than BSW bolts. Is this a standard?

  1. In theory the BSW thread is more rounded than the UNC but with a magnifying glass I cannot see any difference. Probably that depends on the quality of manufacture.

Sometimes we do need to have the maximum strength with bolts, and I do not wish to mix BSW and UNC. With sizes other than 1/2 inch this is all too easy.

Does anyone have any other ideas?

Reply to
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Make up a little card with one nut of each UNC and BSW size on it, labeled. Then instead of peering at a mystery bolt, you can just try it in the nut.


Jack wrote:

Reply to
Grant Erwin


Mark Rand RTFM

Reply to
Mark Rand

An excellent idea. I'll make up a big card with both types of nuts and bolts on it, since there are a few thousand nuts to check as well. I note that the 1/2 inch BSW and UNC nuts that I have here are the same size across the flats. According to the "manual" that is incorrect. I also note that standard marks that are supposed to denote Unified threads are often absent.

Reply to

The head sizes are different. That's the easiest way of telling them apart. Is the difference really that important? Unless you are stressing the bolts to the maximum, and can accommodate the different heads, does it matter if you mix them? The spanner will identify them.


Reply to
John Manders

Except that 7/16" AF (for 1/4" UNC) & 1/4"BSW spanners are to all intents & purposes interchangeable (I think I've quoted the right ones from memory ). The 1/4" UNC may have a *thinner* head than the


Cheers Tim

Tim Leech Dutton Dry-Dock

Traditional & Modern canal craft repairs

Reply to

My self-appointed job is merely to sort out the 400 boxes of assorted nuts and bolts that we have. These have been made by a great variety of manufacturers, some of whom have clearly not followed the Unified identification standards. E.g. the UNC nut sizes are not always different from BSW - I know that because I have 1/2 inch nuts here which are both 3/4 inch across the flats but they have TPIs of 12 and 13.

Sometimes the difference is important as the bolts go on trams restored by perfectionists who line up the heads in the same direction, and I know that enthusiasts would whinge if the heads looked different.

Sometimes the bolts are stressed to the maximum, e.g. on tram brakes and the bits that keep the wheels on etc. After a crash I'd rather not be trying to explain to a judge why we mixed UNC and BSW threads just because we couldn't tell the difference.

I'm just listening to this advice: "Another area that causes confusion is the fact that some BSW or BSF nuts can be screwed on UNF or UNC bolts and vice-versa. Under NO circumstances should this practice even be considered ... if these fasteners are interchanged, considerable loss of holding force, fatigue resistance and strength will result."

I now have a sure method of identifying the bolts. The nuts are a harder problem. I have a BSW nut where the top of the thread is flat instead of rounded and the inside diameter is identical to the UNC nut.

Reply to

Don't bet on anything in hardware. A few years ago, I bought a bunch of hex nuts for 1/4 20. They looked a bit odd so I did a quick check - yep, 10mm. across flats. I still have a few that I pass off on people just for laughs. Gerry :-)} London, Canada

Reply to
Gerald Miller

replying to Jack, Old guy wrote: Get yourself a BS spanner set A 3/8 BSW Bolt head fits onto a BS 3/8 spanner A

1/4 BSW Bolt head fits onto a BS 1/4 spanner.etc This is a standard
Reply to
Old guy

When the 'Unified' system was brought in during the war, nut and bolts were given identifications to solve precisely this problem. The various signs are different depending on the method of manufacture: 1) A circular recess in the head of bolts, 2) an interrupted circle on one top face of nuts, 3) a row of small circles (approx 1/8" dia) on one flat of items that were machined from hex bar. These have become somewhat less common now, and much American stuff does not use it, and of course pre-war NC does not.

Reply to
Stephen Westlake

The head size isn't a reliable means to determine whether it's BSW or not as originally BSW used larger heads but was standardised to use the smaller heads of IIRC BSF/UNC around WW2

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see section on spanner size.

Reply to
David Billington

This thread reminds me to be thankful that over the past sixty years I seem to have misplaced all of my original wrenches purchased to work on my first car - a 1950 Austin with BSW fasteners.

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