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M20 G14 Girder Fork Springs

Dose anybody know the difference between the M20 and the G14 girder fork spring. The ones I have look exactly the same but according to the parts book they have different part numbers.

Thanks, Ian.

Re: M20 G14 Girder Fork Springs

I suppose a G14 spring must be stronger, as the bike is much heavier then a M20...

Re: M20 G14 Girder Fork Springs

I sent an M20 spring and an original spring taken from a G14 to my spring guy to get some made. They looked identical to me and my spring guy confirmed that there is no difference in the dimensions or wire gauge. The only difference was that the G14 spring was chrome plated. Could that be the different allocated part numbers Ron

email (option): ronpier@talk21.com

Re: M20 G14 Girder Fork Springs

Ron you may well be correct but my only reservation is that the difference in finish was donated by the number at the end of the part number. I was hoping that someone would say the G14 had an extra coil to make it stronger or something.

Re: M20 G14 Girder Fork Springs

Ron Pier
I sent an M20 spring and an original spring taken from a G14 to my spring guy to get some made. They looked identical to me and my spring guy confirmed that there is no difference in the dimensions or wire gauge. The only difference was that the G14 spring was chrome plated. Could that be the different allocated part numbers Ron


The only problem with your logic Ron is there no guarantee that the spring you removed from your G was the same one that BSA fitted.
WM20 parts have until the last few years been the cheapest girder frame parts available and thus in the "if it fits in the hole it is the right part" logic supported by the "whatever part is cheapest is the right part" means that lots and lots of pre-WW II bikes are running around with WM20 parts.
I know 2 G series BSA running around with WM20 rear wheels fitted back & front ( after grinding off the sprocket teeth) .

Now BSA made their own fork springs and all of the medium & heavyweights use the same spring holders top & bottom.
These never changed part number till WWII so one would guess that a M20 spring will mate up into a G series fork .
The parts book I have show the G series with 17.5 turns as std and 19 turns for heavy side cars & milk floats.

Now I am lazy and the 1914 to 1945 parts books I have are David Brailys CD which does not work on macs properly so I did not check each & every year just
1929 which lists the same spring on both twins and the same number for deluxe ( chromed ) and std ( black painted ).

1930 numbers were the same but no lengths
1939 all numbers different and the M20 in both deluxe & std used the same 66-5150 spring which was a new number and having a 66 prefix was a new part designed for the M series ( 20-21-23-24 ) with the G using a different number And now the G & M had a different lower mounting block, but the same nuts & washers which means you could fit the M series stuff into a G series.

If some one has paper copies which makes comparison easier year to year they might like to check to see if lengths were printed in other years than the ones I have checked.
Dealers parts lists ( the ones with prices in them ) usually had more information than owners parts books which do not have lengths for springs, bolts cables or spokes.

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

Re: M20 G14 Girder Fork Springs

You could quite well be right Trevor. I only have paper parts lists for the 1938 and 39 models (apart from WW2 M20's) and both my lists show one illustration of a spring with four different part numbers under it and no dimensions for the different models.

The guy who removed the spring from his G14 for me to get copied is a renowned 1930's BSA expert/collector/forum member. Hopefully he'll see this and chip in with his thoughts. Ron

email (option): ronpier@talk21.com

Re: M20 G14 Girder Fork Springs

Ron is referring to me. I had the G14 springs made based on the spring fitted to my G14, a NOS spring owned by another G14 owner with the original parts label still fitted and measurements from different Y13s and G14s across the world. We measured the number of coils (17.5 like Trevor mentioned) the maximum width, the width at the bottom, the width at the top and the thickness of the wire. This all matched with my used spring save for the overall length which was determined from the NOS spring wich turned out to be 8" like an M20 spring. The number for the finish is indeed behind the slash in the parts book so the part numbers are not changed because of the finish. The only thing we didn't do was test the spring loading which could be the only difference with all the other dimensions being the same as that of an M20. However I have found more parts during my experience with 1930's parts that are dimensionally the same but with different parts numbers for different models. I don't see the logic but that doesn't mean there was none. It just means we don't know. Going through all the parts books I have from 1913 up until 1952 means that you see that BSA did use some form of standardisation. All the bolts on an M20 with a letter and 3 digit date back to the earliest production. Quite a few other bolts with the 2 first digits in the lower regions also date back to the vintage or post vintage period. The parts starting with 65 or 66 date for the late 1930's and were introduced at the introduction of the Val Page designed B and M models.

Since there are hardly any period witnesses left it is deduction and trial and error to determine what could have been the reasoning and what may have been the original specs until written evidence can be found in factory documentation from the correct period. I do by no means claim to be an expert on what is right or wrong. From experience I know there is always a (factory) exception to the rule. And after all BSA was a business and delivered what the customer ordered. Like some of the forum members I search through original factory documentation and period documentation like Pitman's Book fo the BSA, D.W. Munro, the Motor Cycle,MotorCycling and some other sources. It is a strange hobby but it keeps you busy and at least nobody gets hurt.

So to cut a long story short I don't have a clue whether I am correct but so far evidence points in the direction that M20 and G14 springs are dimensionally the same. Here endeth the lesson

email (option): leonhop3_at_planet_dot_nl

Re: M20 G14 Girder Fork Springs

Hi Leon

Is an M20 spring 8 inch or 10 inch?

Regards

Pat

email (option): sacombsashtrees@hotmail.com

Re: M20 G14 Girder Fork Springs

Leon has made a typo! They are 10" long. Ron

email (option): ronpier@talk21.com

Re: M20 G14 Girder Fork Springs

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.

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

Re: M20 G14 Girder Fork Springs

Hi Trevor,

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

And Ron is right the length should have read 10".

Kind regards,
Leon

email (option): leonhop3_at_planet_dot_nl

Re: M20 G14 Girder Fork Springs

There appear to be two wartime M20 spring part numbers.

66-5150 used up until contract C7287 plus RAF, RN combination contracts, 1939 and 1940 civy machines.

66-5153 contract C9310 on

The change in spring part number appears at the same time as the change of forks from the short 66-5014 to the longer 66-5016.

Ron when you compared a NOS M20 spring with a spring from a G14 did it come with part number attached?

Rob

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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

Rob Miller
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


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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

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

Re: M20 G14 Girder Fork Springs

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

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