18 years ago
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
I'll check to see if our thread gauges can be used to see the
2. 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.
3. The UNC bolts that I'm looking at seem to have smaller heads than
BSW bolts. Is this a standard?
4. 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?