Good to see that you can use Google. OTOH I *have* Machinery's Handbook
and other pubs on my bookshelf, and have personally machined variants
of all common thread systems on my lathe - which is why I have a
particular hatred for 1/2-13. Prime numbers are a bitch.

You're wrong about BSW being any sort of derivative of metric except
insofar as *any* thread pitch system can be considered as a derivative
of or conversion of any other one. Whitworth was developed
independently of and IIRC in advance of any std metric system. It
certainly was developed well in advance of SAE.

PDW

In article , Mys Terry
wrote:

On Mon, 15 May 2006 00:59:34 +0100, Peter Wiley
wrote:

In article , Mys Terry
wrote:

On Wed, 10 May 2006 22:15:46 GMT, "Maxprop" wrote:


"Peter Wiley" wrote in message
...
In article , Frank
Boettcher wrote:

On Tue, 09 May 2006 04:17:36 +0100, Peter Wiley
wrote:

In article et,
Maxprop wrote:


You might try Vermont--I hear they love Subarus up there,
especially
those
horrid things with the flat-four engines.

Umm, Max - every Subaru I've ever seen has had either a flat 4 or a
flat 6. Do they sell something else in the USA?

Vested interest - I have a Liberty AWD sedan (Legacy to you guys) and
I
like it. It starts, runs, is comfortable and reasonably quiet. Mileage
is OK and at 230K, I expect to get at least another 100K out of it
yet.
Never seen a Tribeca here but from the pix, looks a bit better than a
WRX. That's damning with faint praise :-)

PDW


Kilometers?

Well, of course. Doesn't everyone use the metric system?

The USA began the switch to the metric system some decades back, but gave
up
the idea due to cost. Too bad. Now we have both systems--British and
metric, and I have to have two sets of wrenches and sockets, not to
mention
speedometers that read in both systems.

Max


We do not use "British" and Metric. We use S.A.E. and metric. The British
came
up with a system that is worse than their teeth or their cusine, known as
"Whitworth".

Actually an engineer named Whitworth came up with Whitworth, known as
British Standard Whitworth later on. SAE - society of American
Engineers - was a Johnny come lately and they *still* managed to create
1/2-13, which has to take the prize for one of the most stupid thread
pitches of all time.

It's sort of the metric system expressed in fractions.

Wrong, but that's expected.


--Wrong?

--"For this reason, it is imperative that a set of Whitworth wrenches are
--purchased before working on any antique British machinery – otherwise,
expect
--rounded-off hexes and busted knuckles, trademarks of the careless
craftsman."


--Read further, smarty-pants


************************************************** ****************************
******
Most, if not all, British cars, motorcycles, airplanes and machinery up the
1970s – and possibly beyond – used Whitworth thread forms. Sir Joseph
Whitworth
was born in 1803 in Stockport, a grimy area of the industrial north west of
England. Coincidently, not too far from where this writer was born. After
leaving school at the age of 14, Whitworth pursued various engineering
opportunities until 1833 when he started his own tool making business. In 1841
he presented a paper before the Institute of Civil Engineers where he
introduced
his revolutionary thread system. Up to this time no conventions existed for
screw threads. During this presentation, Whitworth also introduced a standard
system of gages which was widely accepted. Size for size, a Whitworth thread
is
stronger than its SAE counterpart. This is partly due to the radiused corners
designed into the Whitworth thread which reduces the possibility of a stress
riser.

Whitworth’s talents did not go unnoticed by the British military. In the mid
1850s, rifling of gun barrels was in its infancy, at least for the British.
Rifling of gun barrels was known since the 1520 to 1525 time frame.
Interestingly, rifling became accepted in small arms long before it was for
larger guns, and the result, in the Whitworth period, was that rifle-equipped
sharpshooters could pick off artillery crews from beyond the effective range
of
that artillery! Consequently, there was something of a panic to shrink the
artillery’s circular error. In the States, rifling was introduced during the
civil war. Not only was it a very tricky machining operation, but much
experimentation was required to arrive at the ideal helix angle and depth of
rifling in order to optimize these new design features. After optimization of
these design features, Whitworth developed the tooling required to mass
produce
rifled gun barrels. Of course, screw threads follow a similar convention to
gun
barrel rifling. Being the consummate engineer, Whitworth made many other
engineering advances including the art of casting and forging. After starting
from very humble beginnings, he died in Monte Carlo in 1887 a very wealthy
man.
His legacy lived on for the best part of a century in the form of the screw
thread that bore his name and various scholarships.

Like U.S. threads, three families of threads were developed; a miniature
series
known as BA or British Association, a coarse thread series known as BSW or
British Standard Whitworth and a fine thread series known as BSF or British
Standard Fine. The U.S. equivalents would be the Unified Thread System,
formally
known as the American Standard but renamed in 1949.



i) The SAE number series would be equivalent to the BA series

ii) SAE UNC or Unified National Coarse is the counterpart to BSW

iii) and SAE UNF Unified National Fine is equivalent to BSF.



Just to confuse things even more is the fact the miniature thread sizes are in
reverse. In other words, with the SAE convention, the larger the number the
larger the bolt size. Naturally, the British go in reverse, the lower the
number, the larger the bolt size so “0 BA" is the largest and “16 BA” is the
smallest. (Yeah, I know, go figure..!!) To put things in perspective, a 10-32
is roughly equivalent to a 2BA.



There is, in fact, method to this madness. Peter Bready points out that "The
BA
size numbers are actually functional in generating the pitch of the fasteners;
and the diameters are derived from the pitches." According to British
Standard
BS 93 - 1951, the basic sizes in millimeters for pitch and major diameter are
rounded off in each case to the second significant figure after calculation
from
the formulae given below:



The pitches, in millimeters, are calculated from the formula
p = (0.9)n

************************************************** ***************************
Did they just say MILLIMETERS? Isn't that the METRIC SYSTEM?
************************************************** **************************
where "n" is the number designating the size of the thread. Thus, for BA #0,
(0.9)0 = 1, and the pitch is 1.0mm


The basic major diameters are calculated from the formula
D = 6 p6/5
And the diameter is 6 x (1.0)6/5 = 6mm.


Whitworth wrench sizes are another source of considerable confusion. With the
more familiar UNF, UNC etc., the number stamped on the wrench or socket is the
dimension of the hexagonal bolt head measured across the flats. Whitworth
wrench
sizes indicate the size of the bolt, i.e., a wrench required for a 1/4 BSW
bolt
will have “1/4W” stamped on it. The wrench required for the equivalent ¼ UNC
has
“7/16" stamped on it, this being the dimension across the flats of the
hexagonal
bolt-head. To further confuse things, Whitworth hexs’ are NOT the same size as
the equivalent UNC/UNF -- they are larger. For this reason, it is imperative
that a set of Whitworth wrenches are purchased before working on any antique
British machinery – otherwise, expect rounded-off hexes and busted knuckles,
trademarks of the careless craftsman.



The derivation of Whitworth wrench sizes is another obscure mystery. From a
Jaguar enthusiasts' site

http://www.jag-lovers.org/xk-lovers/...th_system.html "...the hex
sizes were originally governed by the commercially available steel hex bar
stock
sizes, in the days before automated screw machines, when nuts and bolts were
cut
from hex bar stock."



Bill Allan adds "It should also be stated that some of the early nuts would
have
been machined from round stock, with an integral washer, so round bar stock
sizes would have been involved in calculations. One other problem is that the
original Whitworth heads (AF) were too large relative to the actual bolt
shank,
(that's why spanners/wrenches are the length/size they a so you can't apply
to much torque. In the first part of the 20th Century, the head sizes were
reduced to the size one below. (British Standards specify that the AF
measurement of any bolt not be greater than 1.75 that of the shank: 1/4 inch
Whitworth is almost bang on the button) This causes even more problems when
you
need replacement Whitworth fasteners, for machinery over a 100 years old. It's
also the reason why some old spanners/wrenches have two Whitworth numbers on
them."



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. Most of the coarse threads share the same threads
per inch which means BSW nuts can be screwed onto UNC bolts and vice-versa.
The
exception to this being ½ inch -- ½ inch BSW is 12 threads per inch and ½ inch
UNC is 13 threads per inch. Again, it must be emphasized that this practice
should not be a consideration. Whitworth and UNC/UNF thread forms differ
greatly, the primarily one being the thread angle (see illustrations). BSW and
BSF feature a 55 degree thread angle (47 ½ degrees for BA) and UNC/UNF threads
feature a 60 degree thread angle. Consequently, if these fasteners are
interchanged, considerable loss of holding force, fatigue resistance and
strength will result. Unfortunately, this author has seen examples of
mechanics
-- unaware the these subtleties -- mixing fasteners at will -- gee, if it
screws on it must be okay..!! (Scary thought).



The standard tapered pipe thread in the US is the NPT or National Pipe Thread.
The British use the BSP or British Standard Pipe thread. Although similar,
again, they should NOT, under any circumstances, be interchanged. The sad part
of this thread confusion is the fact so many classic British cars,
motorcycles,
and yes, even airplanes have been butchered over the years by those not
conversant with these thread systems.

The foregoing is a very brief introduction to the complex world of screw
threads
and by no stretch of the imagination does it include all the families of
threads
a restorer may run into. For more detailed information, consult Machinery’s
Handbook.



Whitworth wrenches and sockets are available through most clubs that cater to
Brit cars. It’s also a good idea to get a set of taps and dies to fix the
screw-ups of those who have gone before you in the vain attempt to convert,
intentionally or otherwise, to SAE format. Another point worth keeping in mind
are the fasteners themselves. Several outfits sell so-called Whitworth hardware.
Turns out that often times this hardware has SAE sized hexes with Whitworth
thread forms. Sounds as though people are simply making these fasteners from
blanks intended for SAE threads. Again, the correct fasteners are available. I
know that the Rolls-Royce Owner’s Club has a good selection – but be prepared
for sticker shock.




Terry & Skipper, Clearlake Texas