Identifying thread types.

Without using gauges, is there a visual way of distinguishing Metric, Whit and AF Nuts and bolts?

I seem to recall the Nut or Bolt head is chamfered in different styles.

Unlike other groups we cannot eliminate all thread types except one as we use all thread types due to the age range and manufacturing location of our vehicle stock.

 

You can usually distinguish metric and imperial simply by looking at them as they are different sizes. M20 and 3/4" BSW are quite close and an M20 nut will screw onto a BSW thread, albeit loosely. It is harder distinguishing between BSW & UNC. UNC bolts usually have a circle on the head but they may also have three small lines at 120 degrees.

 

There are two give aways. Firstly metric and Unified threads are cut at 60 degrees and th whitworth at 55. As said by Steve above some unc nut will go on whitworth bolts. Also some tpi's are difrent.

The other giveaway on black bolts is the making on the head. If I remember the right way round metric ones have a number ushaily 8.8 and imperial ones R. This is the material spec.

Metric bolts also can have smaller heads relative to imperial.

 

Does the chamfer on the nut mean anything?

 

As a first check I would measure the diameter of the thread. The thread will always be a little below the nominal size, even when new. For example a 3/4" thread will perhaps be 0.740" to 0.745" diameter an a 20mm one 0.775" to 0.785". Then check the size of the hexagon, metric fasteners are always a set number of millimetres across the flats (AF), e'g a 12 mm bolt is 19 mm AF, a 1/2" UNC or UNF is 0.750" AF and a 1/2" BSF bolt is 0.820" AF.

As noted above a numerical marking indicates a metric fastener and a letter is for an imperial one. These letter and mumbers have a meaning but it is too complicated to fully decribe that here.

A digital vernier that can switch easily between imperial and metric is very useful.

There is a huge amount of information on the internet, e.g http://classicmechanic.blogspot.com/2011/06/mm-af-bsw-bsf-spanner-conversion-chart.html

 

The chamfer has no specific meaning.

 

I often use the Lister [engine] Threading Tables or, at least, a number of pages that I have printed off and laminated.

It's quite difficult to link direct to .PDF file but if you enter,
http://www.gerardsgarage.com/supportfiles/Threading_Tables.pdf

it should take you to them.

The pages start,
C:\British Cars\Tech Papers\Thread Sizes\Threading Tables.doc

but that is not a reliable link,

The next line is,
Source: http://www.oldengine.org/members/diesel/front.htm

which I think is a broken link.

But once you find the data, it is certainly worth Saving as a file.

I normally find it the other way round. An M10 head is normally a 17mm spanner whereas 3/8" Whitworth can be driven with a 16mm spanner.

It gets even more confusing when you get into German made cars. Presumably for weight/material saving, M10 fixings may have a 16mm or even 15mm head size.

 

I did a presentation on nuts & bolts some while ago for Middleton but I am holiday and it's on my desktop PC. When I get back, I'll try and remember to put some relevant bits on here.

 

Metric nut sizes are far from standard in the smaller sizes. The Japanese, Germans and UK use different sizes for the same thread. Can't remember which ones, though. As in my earlier post, I'll give detail when I'm back from holiday.

 

The OP asked "Without using gauges, is there a visual way of distinguishing Metric, Whit and AF Nuts and bolts?" I do agree though that my first port of call would be pitch gauges.

 

One thing I have not seen mentioned is the difference between pre war and post war whitworth heads. I think they all went one size smaller during the war

 

A lot of Whitworth spanners are marked to show this, e.g. 7/16" Whit 1/2" BSF. The heads on bolts to the old standards do look very oversized.

 

Back from hols. I don't know if the followibng helps:

 

Thanks Steve.

 

I'm not sure if this is 'cross-posting', 'duplication' or simply combining two hobbies!
But if you're following one thread, try the other one.

https://www.national-preservation.com/threads/identifying-thread-types.1416989/#post-2527221

http://www.oldclassiccar.co.uk/forum/phpbb/phpBB2/viewtopic.php?t=21246&highlight=thread+types

 

As long as it's not cross threaded, it's OK

 

Nothing wrong with cross threading, it just makes sure it's double tight!