Understanding Spanner / Nut sizes

[Arthurdaily]

Hi

After using modern sizes for years, I'm now building up a set of imperial / whitworth sizes for my new Morris - but don't really understand... For example, I believe I need a 3/8 socket to go on my brace for the wheel nuts - ie so I can use the socket on my torque wrench, but what does the 3/8 actually mean? Cos the nut head has no measurement coresponding to 3/8?

Just out of interest... thanks!

[bmcecosse]

http://en.wikipedia.org/wiki/British_Standard_Whitworth

[Arthurdaily]

So, it is actually the width of the bolt itself, and does not match anything to do with the head? I see... Thanks!

[IslipMinor]

So, it is actually the width of the bolt itself,

Well yes, and then again sort of!

The threads used on the Minor body are mainly BSF (British Standard Fine), which uses the Whitworth thread form. But, the the bolt head nut size can be confusing because BSF head sizes are the same as BSW 'one size down', i.e. a 1/4" BSW head is the same size as 5/16" BSF, 5/16" BSW = 3/8" BSF etc.

Many of the spanners are marked with both the BSW and BSF bolt size, but some only have the BSW bolt size, which means that you will need a 1/4" BSW spanner for a 5/16" BSF bolt, 5/16" for a 3/8" etc. So using the spanner size to get at the bolt size can be a bit confusing and unreliable. BSF heads very consistent and are the same size as the theoretical BSW size 'one down'. BSW heads many years ago were pretty consistent, but today have the BSF head size as well, so you can find that a 'modern' 5/16" BSW bolt will need a 1/4" BSW spanner to fit it!!

Best to check the actual bolt diameter and thread pitch to find out the size and series, BSF or BSW?

[mike.perry]

Now you have confused me
Additionally the more modern threads on a Minor are unified in two pitches, Unified Course (UNC) and Unified Fine (UNF) The bolt shank is I believe a ratio of the head size. Spanners are labled as A/F, Across the Flats of the bolt head, ie the width of the spanner jaws. 1/2 A/F = 1/2 inch jaw width.

[IslipMinor]

Mike,

Yes, the A-Series mechanicals are UNF/UNC, so by the time a few metric replacements have crept in somewhere, all 3 size series spanners are needed!

There is no fixed relationship between the thread diameter and the A/F head size, for example a 7/16" UNF/UNC bolt has a 5/8" A/F head, and the 7/16" UNF/UNC nut to fit it has an 11/16" A/F head! But at least the A/F sizes are common to both UNF and UNC.

[bmcecosse]

And like Metric nuts/bolts - the size required is easily found by measuring 'across the flats'.

[katy]

What is the proper size socket to fit the wheel nuts on a '61 Morris Minor 1000 car?
IIRC the threads are 3/8"UNF, but I could be wrong on that.
The only socket I found that fits reasonably well is 18MM.

[IslipMinor]

Yes, the thread is 3/8" UNF, but the correct socket size is 7/16" BSF (or 3/8" BSW) - very odd combination! Especially as the Spridget and Mini both have the same thread size, but the socket is 11/16" AF for their nuts.

[bmcecosse]

And yes - 18mm socket is very very close. Unfortunately it is a 'non-preferred' metric size, so they are not the most common socket at autojumbles.......

[mike.perry]

A 3/8 Whitworth socket is more or less standard for Morris Minor wheel nuts and the early Series MM BSF wheel bolts. It also fits the spare wheel clamp bolt. The only exception (there is always an exception) is the Series MM UNF wheel bolt which has a 5/8 A/F head.
See http://seriesmm.mmoc.org.uk Tech Info, Front Suspension Stub Axle and Wheel Bolt Threads