First published in “Front Wheels February 2007, now enhanced and updated

One useful by-product of preparing the on-line drawings catalogue has been the remarkable insight provided into the many aspects of BSA engineering that, to be frank, have in the past been quite hazy.
One of the highlights has been to be able, at last, to really understand the rather complicated subject of gear boxes and the various different ‘innards’ that BSA put into their boxes over the ten year period of FWD production.
Back in the misty past I seem to recall struggling up the narrow staircase at the old Eagle pub with boxes of gears and a complete gear box to give demonstrations of dismantling and re-assembly of a specially prepared gearbox and diff. Looking back, armed with the information I now have, I realise how little I really knew at that time.
When the first three-wheeler appeared, as many parts as were possible at the time were used both from the previous V-twin light car of the twenties and, believe it or not, from BSA’s other marque, Daimler.
It would seem from the drawings that the earliest worm and bronze used in 1929/30 were of.25 to 1 ratio, both items being of BSA design, or were they? The fitting dimensions of these items were exactly the same as the ones that replaced them after engine number 5227 with a Daimler designed worm and worm wheel with six starts, giving a ratio of 5.16 to 1, This fact begs the question,were other parts of the differential of Daimler origin?. Bear in mind the fact that Daimler used under-slung worm-drive for some (or all) of their cars at that time.The Daimler RWD layout meant that engines turned the opposite way the FWD equivalent, making the direction of rotation correct with an overhead worm drive layout.
Many early cars were fitted with worm drive to the back axle, however overhead worm drive in RWD cars would result in the propeller shaft, engine and gearbox being quite high in the frame,Daimler probably settled on under slung for this reason.In the case of BSA FWD the overhead worm was absolutely ideal, enabling a very short gear box/differential unit.
There was a design study by BSA (see series 8? in Tech articles)) which was clearly based on the Citroen layout, this would have solved all the problems of weight distribution and vertical centre-of-gravity, but in the climate existing at that time in the motor industry it was probably far too complicated (and expensive).
Eventually, when the four-cylinder three-wheeler and T9 appeared in 1932,BSA brought out yet another worm and wheel combination of 5.2 to 1 which became the standard till the end of production.
The only other major change to the differential was when the Series 4 Scout made its appearance. As wheel brakes were specified for the front, the sun wheel output shafts were reduced in length, as were the cage side plates, with minor revisions of bearing housings etc, whilst the half-shaft length was increased proportionately.
The gearbox itself has seen many changes from the original design, which was originally fitted with very rugged but somewhat noisy straight cut spur gears throughout. This was not unusual for that era, but BSA obviously had set their sights on producing cars as well as three-wheelers and revised the gearbox in 1932 by designing a set of gears for the primary drive with a double helical (herring-bone) set which they dubbed ‘silent-constant’, which,as the name suggests, gave a much quieter performance in the two lower gears. However, the V-twin models continued right through to the TW35-1 with the old original spur gear layout.
Three different sets of ratios were offered now, the V-twin with its unaltered ratios; the ‘standard ratio’, for the new four cylinder three-wheeler, the T.9,and other models still in the pipeline; and the ‘low ratio’, mainly for what was called the ‘Q’ chassis; which was specified for the FWD four-wheeler van.
This cluster was basically similar to the V-twin layout but for some reason that is difficult to understand BSA also specified a ‘special’ set of primary drive gears (C and F), a special layshaft (with first and reverse gears of 15 and 16 teeth, and a special pair of gears for second. (B and E)
Of spur gear design the primary drive gears were were 14 and 28 teeth, with second gears of 15 and 27.
Presumably the designer wanted 18.6 to 1 for first gear on the van and there was no other way. The ratios for this new set of gears for the van (as reported by Light Car van publication of the time) were 5.2, 9.4 and 18.6 to 1, compared to the car at 5.2, 8.5 and 17.1.
It’s well worth noting that any changes to gear sets can also mean changes to selector forks, selector shafts, speedo spiral gear sets and interlock bars.Many BSA owners over the years have come to grief (including me!) when trying to mix and match in this area.
The gearboxes remained the same throughout the range of models until the advent of the Series 4 Scout when the final big change happened with the introduction of the ‘silent second’ gear cluster.
The gearbox case was revised too, with a strengthening ridge under the ‘neck’ between gear case and differential.
This new gear cluster had the lay shaft and main shaft double helical 2nd gears inconstant mesh,with a new selector fork engaging both.
The primary drive gears C and F remained from the old cluster, but the lay shaft was revised to allow both B and E gears to slide on a splined portion whilst remaining in mesh. with the first and reverse spur gear being used by the new 2nd gear in the rearward position meshing with a spur gear shaped recess.The reverse gear idler shaft was revised to allow space for these bulkier new gears.This new layout remained unchanged through to the Series 6 Scout (and possibly the Series 7), the resultant quietness was in keeping with other manufacturers products of the time, but is not unknown for the weakness in the main shaft sliding internal dog E to break because the 6 teeth of the internal dog do not divide equally into the 22 teeth of the herring bone, leaving one weak point with very thin metal. A drawing of 1939 shows this to be rectified to a certain extent, but war loomed!
I would like to thank Barry Baker for his invaluable assistance in providing a great deal of information and part numbers for T9, and Q, K, and R chassis.Peter Bowler

Here we can see the early V twin Gearbox layout with all spur gears. G is the layshaft reverse gear, and A is the layshaft first gear(both formed as part of the layshaft) B is the layshaft second gear and C is the constant mesh layshaft gear these last two share a common keyway. D is the sliding main shaft gear which moves right to mesh with A for first gear, and left to mesh with the reverse idler gear which is itself in constant mesh with Gto give direction reversal (see drawing below) E moves right for it’s dog to engage with F for straight through drive (top gear) and left to mesh with B for second gear.
This is the “special” ratio Gearbox designed for the “Q” chassis, ( BSA planned to sell this to companies who bought in running chassis and fitted van bodies etc to them), layout is the same as the V twin, except that the ratios were “tweaked” mention is also made of a special reverse gear idler to match these gears, no details exist in reference to this or the size of reverse layshaft gear. therefore they are marked same as the V twin. D is the sliding mainshaft gear which moves right to mesh with A for first gear, and left to mesh with the reverse idler gear which is itself in constant mesh with Gto give direction reversal (see drawing below) E moves right for it’s dog to engage with F for straight through drive (top gear) and left to mesh with B for second gear.
Here we can see the Four cylinder (three-wheeler and T.9) Gearbox layout with “Silent constant” gears. G is the layshaft reverse gear, and A is the layshaft first gear(both formed as part of the layshaft) B is the layshaft second gear and C the constant mesh layshaft gear these last two share a common keyway. D is the sliding mainshaft gear which moves right to mesh with A for first gear, and left to mesh with the reverse idler gear which is itself in constant mesh with Gto give direction reversal (see drawing below) E moves right for it’s dog to engage with F for straight through drive (top gear) and left to mesh with B for second gear.
This is the final FWD gearbox layout with both “Silent constant” and “Silent second” gears. the layshaft was revised to provide a splined portion for the constant mesh gear, the second/top gearwheel was allowed to revolve freely, If main shaft gear D is moved to the left it engages the reverse gear idler (not shown) which is permanently meshed with G to give rotation reversal. If it is moved right it engages lay shaft gearA to give first gear. When the sliding 2nd gears B and E are moved to the right the dog on F (the primary drive gear) fits into the dog on E and gives straight through drive for top gear. When B and E are moved to the left the recess onB fits over the spur gear A and gives second gear.
This is an end view of the FWD gearbox, this applies to all models and shows the relationship between layshaft reverse gear G, mainshaft (I) sliding gear D and reverse idler gear H
This is based on an early drawing dated 1930 of the layout of the original differential on the right hand side it will be noticed that the bearing housing and attachment plate are of a very early layout, this was re-designed to include the facility for sidewards adjustment of the bronze in relation to the steel overhead worm, (see drawing below for details)
This shows the revised layout of the differential that appeared on the Series 4 Scout, and continued to the end of production