Industry Standard wrench Numbers (ISN's)
[This is sorta a long term work in progress...]
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:The original (simple) number system:
The original ISN contained only 2 digit numbers, and the overall system makes more sense if we look at that first. The system is fairly straightforward, provided we understand that the numbers apply not only to wrench sizes, but also wrench types. This was probably a mistake in hindsight, but that is the way it was done...
:Zero to twenty:
ISN 0 through 20 were for single ended engineers wrenches. Since there is only one opening, there are no combinations, and the system for these wrenches at least stayed fairly simple. Size 1 through 20 fit USS nuts from 1/4 to 3" in multiples of 1/16. Later addition of a size 0 and a size 00 covered 3/16 and 1/8.
The 700 series (700-709) were added later to fill in the missing sizes. (Because USS sizes are based on odd 32's, roughly every other standard multiple of 1/16th is not in the original set of sizes)
:Twenty one to fourty nine:
ISN 21 through 49 were for double open ended wrenches.
To make sense of these numbers , you first have to look at only the odd numbers. The odd numbers form a sequence where the end of one wrench is the same size as the other end of the next larger wrench. A set of one each of all the odd numbered wrenches gets you 2 wrenches in each size except the first and last.
Like this:
5/16(#21)13/32(#23)1/2(#25)19/32(#27)11/16(#29)25/32(#31)7/8 ...
That is, wrench #27 is 19/32 on one end, and 11/16 on the other, and #29 is 11/16 on one end....
Now, these sizes look a bit pecular, until you look at the bolt sizes that those wrenches fit:
USS:1/8(#21)3/16(#23)1/4(#25)5/16(#27)3/8(#29)7/16(#31)1/2 ...
Thus, a set of odd numbered ISN wrenches, fits all the bolts in the series, for USS at least...
The even numbers are a bit messier, every other even size makes a set that fits Bolts in 1/8th's
USS:1/8(#22)1/4(#26)3/8(#30)1/2(#34)5/8 ...
And finally, the remaining even numbers form a set that covers all the 16'th bolt sizes
USS: 3/16(#24)5/16(#28)7/16(#32)9/16(#36)3/4...
Thus far, the series makes perfect sense, if you are working with only coarse hardware, a set of 1/8 sized
wrenches may be all you need, if you work with everything, the odd wrench numbers give you a general
coverage set, and if you bought the 1/8 sizes and want a complete set, you can fill in the 1/16th sizes
without having to buy sizes that will duplicate half the wrench sizes you already have.
:The 700 series:
The 700 series reflects new thinking from the wrench makers. Instead of a sequence based on bolt sizes, the
sequence is now based on the actual opening sizes. In addition, the now obsolete 32's are omitted. Also the
sizes now change to 1/8th's for the larger wrenches, probably reflecting the fact that Hex cap screws change
to 1/8th steps in the larger sizes. No wrench is provided for 11/16, in the old bolt tables, it does not get
used for any SAE or Hex cap bolt or nut. Oddly, 15/16 is omitted, tho it is an SAE nut size.(It will be
added later as 33C) and inexplicably,no wrench is provided for 1/2x9/16...
The odd numbers for an almost complete set, sans the sized apparently deemed unneeded:
AF:5/16(721)3/8(723)7/16(725)1/2 | 9/16(727)5/8(729)3/4(731)13/16 | 7/8(733)1(735)1+1/8(737)1-1/4(739)1-3/8
Because the odd sizes now have gaps, and change step size, the even sizes are no longer pretty, instead, the
even numbered wrenches provide 'skip' sizes, that give you one size up+one size down.
3/8(724)9/16 [723-725]
1/2(726)5/8 [725-727]
9/16(728)3/4 [727-729]
5/8(730)13/16 [729-731]
13/16(732)1" [731-733]
7/8(734)1-1/8 [733-735]
1(736)1-1/4 [735-737]
Presumably this odd scheme was designed to make the sizes more likely to be the ones wanted, but it lost
what could have been a very simple way to tell which wrench is the next size bigger than the one you have...
It is amusing to note that the odd selection that results from the odd numbered 700 series seems to have
even baffled the wrench makers. Billings 'Mytee Ten' wrench set lists one included wrench , that according
to the table given, does nothing usefull...
75-85 are the ISN's for S-Wrenches....that's for next week ;P
