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

email (option): ronpier@talk21.com

I suspect the spring change is more about specification than length of forks.

Pre war the M20 was a sidecar machine, the fork spring needed to be tough enough to take a heavy sidecar.

When the Army first adopted the M20 it was as a single seat solo machine, a heavy spring wasn't ideal but it did the job for a while.

I have often thought that the lengthening of the forks was a token effort to counter the adoption of a softer spring for solo use.

At some point the Army changed the spec of all heavier solo machines for pillion use but it was only meant to be occasional.

Rob

email (option): robmiller11(a)yahoo.co.uk

Rob,

I still have one or two boxed NOS M20 springs (not to be opened until required for use though). What do you want to know?

Kind regards,
Leon

email (option): leonhop3_at_planet_dot_nl

Hi Leon

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

email (option): robmiller11(a)yahoo.co.uk

Hi Rob,

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.

Kind regards,
Leon

email (option): leonhop3_at_planet_dot_nl

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

email (option): bsansw1@tpg.com.au

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.

Rob

email (option): robmiller11(a)yahoo.co.uk

There was a DME Circular which mentions fork springs, sadly we don't have the actual document to read though.

Immediate action

B.195 10/9/41

Fork springs - Shock absorber adjustment hand control. B.S.A. M/C.

Its interesting that the date is very near to the change in contract mentioned before.

Rob

email (option): robmiller11(a)yahoo.co.uk

Rob,

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.

Kind regards,
Leon

email (option): leonhop3_at_planet_dot_nl