Gees leon,
If you don't know, where does that leave the rest of us meer mortals ?
AFAIK BSA never changed a part number , apart from adding suffex for finishes.
This went for the entire production range and some of the funny numbers were parts originally made for cars, guns, stationary engines, lathes, mills etc etc etc.
As you have noticed this mainly happens with nuts & bolts.
My first thought were they could be the same then I noticed the upper & lower scrolls started off being the same then in the latter years they got different numbers.
Being that it is an eleptically wound coil spring I would hazard a guess that the G's & M's were different physical diameters but the NOS tagged part puts that to bed.
A club member down here broke his spring and I sent him to Bill Green to see if an M20 spring was the same but what Bill had in stock & what Eric had on his bike were different so he ended up getting some made using the broken spring as a pattern.
Again, this was a multi owner bike so there is no guarantee that the spring fitted was the same as the one the factory fitted
I to have trolled through everything I can get my hands on hoping some one would list the maximum allowable set in the girder springs, but no such luck to date.
I checked the parts books from 1929 up to and including 1940. This is what I found. Up until 1936 they used a different spring (save for one model in 1930). As of 1936 it remained the same up until 1940 when they changed it again. The owners of the bikes that checked the springs were indeed owners of bikes dating from 1936 (like mine) up to 1940, which is in line with the 17.5 coils as against the earlier 17 coils.
1929:
9.86 HP de Luxe 17 coils (S20a)
9.86 HP World Tour 17 coils (S20a)
Milk Carrier 19 coils (26-5282)
1930
9.86 HP G30-15 (24-5211 ) 17.5 coils according to 1929 parts book
9.86 HP G30-16 World Tour (S20a)
1931
G31-12 World Tour (S20a)
1932
9.86 HP G32-10 World Tour (S20a)
Milk carrier (26-5282) 19 coils according to 1929 parts book
1933
9.86 HP G33-13 World Tour (S20a)
Milk carrier (26-5282) 19 coils according to 1929 parts book
1934
9.86 HP G34-14 (S20a)
Milk carrier (26-5282) 19 coils according to 1929 parts book
1935
9.86 HP Twin G35-14 (S20a)
Milk carrier (26-5282) 19 coils according to 1929 parts book
1936
9.86 HP G14 (24-5211) 17.5 coils according to 1929 parts book
Y13 ditto
1937
9.86 HP G14 (24-5211) 17.5 coils according to 1929 parts book
Y13 ditto
1938
9.86 HP G14 (24-5211) 17.5 coils according to 1929 parts book
Y13 ditto
1939
9.86 HP G14 (24-5211) 17.5 coils according to 1929 parts book
1940
9.86 HP G14 (33-5082) no information available
I've never had any NOS springs Rob. Just secondhand ones to copy, which was fine, given that we know the overall length. I don't know why a different spring is called for, for the longer forks as the bit where the spring fits is unaltered. The extra 1/2" is in the lower legs only
Both boxes are marked 66-5153. One is still sealed so I didn't open that. The other one contains a green spring. The box is stamped 40 AUG, which I presume is a date stamp.
The part number for a start, I don't know how you measure the springiness of a spring? but perhaps the colours to indicate the period of manufacture?
But a sample of both part numbered springs would be needed before any comparison could be achieved.
Rob
Coil springs are measured by weight per unit of deflection.
Usually something like 100lb /1"
Thus the different grades of rear damper springs ( not seen on many WM20's ) you see them listed as 80 lb , 100 lb , 120 lb.
In all those cases it is the weight required to compress the spring 1"
Tension springs is the same except it is stretch.
This is used regardless of weather the spring has a full 1" of travel or not.
Thus from the users point of view the number of turns and diameter of the wire become irrelevant .
I would hazard a guess that BSA went to a higher grade of carbon steel to save metal with fewer turns but still the same length.
One turn on an M20 spring is about 2" so x 100,000 that is a lot of inches of steel
Trevor the 1954 BSA M20 inspection standards book agrees.
It only gives the specs of the later 66-5153
No. of coils (L.H. wind) 17. Material W2 No.1 S.W.G., 0.3 inch dia. Min distance between coils when assembled 1/2 inch.
Free length 10 condemn at 9
Length 8 inches compressed under load of - High 330lb - low 320lb - condemn at 300lb.
If the date on Leon's spring is correct as August 1940?? it doesn't match the BSA Ledgers which show the last machine of contract C7287 WM20 42000 as being delivered on 22/8/41 and the first of contract C9310 WM20 44213 as 2/8/41, so this looks like another bit of evidence that the spare parts lists can't be relied upon to date changes in specification.
We now need to find the specs of the earlier spring 66-5150 either in writing or by examining an original NOS spring, but if the difference was just a material change even that may not help.
I think this change was to change the fitting of the nut as means of damping adjustment to the side damper knob.
I didn't look at the other box to see whether or not this was dated.
By the way: the standards book refers to 17 coils instead of 17.5, which is a saving of one inch based on Trevor's estimate of the length of one full coil. So may be the difference is only half a coil. If you look above you will see that the G14 spring originally started with 17 coils.