On 13 Jun 1994, Rob Reilly wrote:
> Subject: Time:11:38 AM
> OFFICE MEMO MGB nuts Date:6/13/94
> >My '65 MGB tourer (roadster) has a banjo rear axle
> with axle nuts of a size I cannot find among my collection of SAE and
> metric sizes.
>
> I am assuming you mean you can't find wrenches to fit the nuts.
>
> >These nuts hold the oil seals and wheel bearings in
> place. Are these relics of MG's Whitworth sizing in earlier series?
>
> I don't know specifically about MG's, but my guess is they are British
>Standard
> Fine thread nuts and bolts, which are sometimes incorrectly called "Whitworth"
> because they use the same "Whitworth" threadform as the Whitworth coarse
>thread
> series. My '51 Jag has many BSF (Whitworth) nuts and bolts. I used to get by
> with using metric wrenches until I found some British ones.
>
> Caution: do not put an SAE nut onto a BSF bolt, because even though the
> diameter is the same, the thread angle and number of threads per inch is
> different. But you can usually substitute the nut and bolt together - BSF for
> SAE.
>
I was hoping someone who knows more about this than I do would respond to
this, but they haven't and I think more needs to be said. I will chance
adding to it, and if I screw up (heh, heh) at least someone with the
appropriate engineering degree or experience will be motivated to sort it
out. BSW (Whitworth) and BSF (British Standard Fine) fasteners use the same
wrench sizes, but the threads are different.
A case in point is: 5/16 inch nominal. BSW has 22 tpi, whereas BSF has
18 tpi. Both have hex heads 0.518 to 0.525 inches across the flats, hence
both will use the same wrench, but you are not about to put a BSW nut on a
BSF bolt, by accident or by design. Between 1/4 inch and 2 inches, I
don't think there is any case where BSW and BSF have the same tpi. I
believe the proper wrenches for these are marked with the nominal thread
diameter, rather than the flat spacing of the hex heads, for example a
wrench might be marked 5/16 BSW/BSF and have an opening of 0.527 or so.
Probably marking BSF/BSW on the wrench has given people the idea they are
the same thread systems. Nope, just the distance across the hex flats is
the same. What fun.
Life is relatively easy for owners of many lbcs made after Robs. The
Brits, Canadians, and USA decided in 1949 on a standardized thread series
of fine and coarse (UNF and UNC) threads that are interchangeable and
these gradually replaced the Whitworth and BSF threads. I suspect this
happened first in marques exported to the new world. In all probability
one can buy a perfect replacement for virtually any nut or bolt on a 65
MGB from an american supplier.
To make life interesting and to continue Rob's warning, you don't want to
put a UNF or UNC nut on a BSF bolt, but you have to be pretty ham-handed
(no "mechanics feel") to do that since they have different threads per
inch. A more likely mistake is to try to mix UNC nuts with BSW bolts and
vice versa. BSW and UNC have the same tpi, but (I haven't looked it up) I
suspect the profiles of the threads are different. I have never tried
this to see if one could feel anything wrong, but it is not something one
should do, especially not where it matters if the item being attached
stays attached at speed.
To get back to the original poster's problem axle nut. He should say if
the problem is that the nut was of a peculiar size, or if it was simply
not one of the wrench or socket sizes present in most common sets. I
suspect the latter; the nut on my frogeye vintage 1960 is larger than the
largest of my 1/2 drive sockets, and a size intermediate between two of
the sockets of my 3/4 inch drive set. Better yet, the nut is made with
rounded corners, so a socket will slip if it does not fit it perfectly (a
6 point socket is best). If it was easy, it wouldn't be fun.
Nevertheless, it is neither Whitworth nor BSF, simply an uncommon size.
One can go to a good tool store, and for about $30, buy a standard 6 point
american socket to fit it perfectly. And believe it or not, that is the
way to go. Measure across the flats with a vernier caliper, and you will
find it is precisely 2-1/8 or something equally absurd. Go to a good tool
store, and buy a socket to fit. The alternative is that you will have to
destroy the nut taking it off with a punch or chisel, and that you will
never be able to torque the new one properly when you reinstall it.
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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