Gearbox and Transmission

The gear ratio's given in the Norton Maintenance and Instruction Manuals and as often found in various contemporary books are slightly confusing when you are looking for a proper gearbox for your bike. The ratio's given are basically the ratio's of the entire transmission system from engine sprocket to rear wheel drum mounted sprocket, and not of the gearbox itself. Civilian 16H and Big4, and WD16H gearbox shells were all identical (casting number N8001E) and do not have any identifying marking on them to tell you what it is. War Department Big4 gearboxes are however indentified by the gearbox ratio's hammered into the rear of the shell. Picture below shows an example of such marking.
As the majority of gearboxes do not have such markings, its believed that only the boxes deviating from standard are actually identified by these markings.
Its likely that Racing gearboxes will also have the ratio numbers added to them, but I have no personal experience of this. The ratio's will however be lower on them than on the standard gearboxes.
Especially for the Big4 restorers its difficult to find a correct bearbox because there are still loads of 16H gearboxes around, but not many Big4 gearboxes.
It has to be kept in mind however that if you are restoring a prewar civilian Big4, the civil/military 16H gearbox is correct.
Only the military Big4 received a gearbox with a different, lower, gearing to haul along the not so very light sidecar through loads of mud.

Pictures above and table below showing the different gear ratio's of the different gearboxes and of the total transmission of the WD16H, WDBig4 and the civilian 4 speed gearboxes (upto 1940) built  by Norton between "model year" 1935 and 1945.

Gearbox ratio's

Survey of gearbox internals showing identification numbers as given  on the individual gear wheels, number of teeths and Norton spare part number.

Survey of Sprocket teeths on the various positions

According to EM Franks (see ref page), "For civilian purposes a 21 or 22 tooth engine sprocket should be fitted to the sidecar wheel drive outfit (W.D. B4), and a 19 tooth to the W.D.16H machine." 

Total transmission ratio’s between crankshaft and rear wheel drum

I calculated the overall transmission ratio's based on the actual numbers of teeths on all the gear wheels and sprockets.
From this I came to slightly different values than actually given by Norton. I guess the differences are caused by rounding off variations etc.
Being actual hard numbers its impossible to calculate other values, there are no 1/2 teeths possible.
I did not find any overall transmission values for the civilian 16H and Big4's.

Dismantling and re-assembly of the Norton 4 speed gearbox is generally relatively straight forward when following the instructions given in the Maintenance and Instruction Manual. There is no difference between the 16H and Big4 boxes apart from the gear wheels.

There are some points of attention.  The M&I Manual does not give any figures for the total wear allowed on the Bronze Clutch Thrust Washer. Additionally, the gearbox exploded view of the M&I Manual shows a dished washer at the end of the main shaft, which in many cases is missing in actual gearboxes, and the same exploded view does not give the Main Gear Wheel Sleeve Bearing Roller Retaining Washer, which is a crucial part in the gearbox! (I have drawn this washer into the original gearbox exploded view below, encircled by a red ring.)

The Clutch Thrust washer is the washer taking up the reaction forces when the Clutch is operated. Its positioned on the main axle facing the hardened steel ring that keeps the rollers of the Main Gear wheel in place. (Main Gear Wheel Sleeve Bearing Roller Retainer Washer).
The Bronze washer has three grooves in one face, which to be correctly positioned should be facing towards the Main gear wheel (Clutch). When these grooves are worn away, the washer needs to be replaced. The Clutch Thrust washer has a thickness of approximately 7/32 inch (5,55 mm).  Maximum permissible wear of the Clutch Thrust Washer is 1,58 mm or 1/16th inch.
Investigation of the Clutch Thrust washer is indicated when the max end float of the clutch itself exceeds the 1,6 to 2,38 mm. This can be roughly inspected by pushing and pulling on the clutch in axial direction.
End float of the main axle should preferably between 0,254 and 0,508 mm (0,01 - 0,02 inch). If the end float is still outside this range while the Clutch Thrust washer is within the specified thickness, the main axle should be shimmed behind the Clutch Worm nut.

This is where the elusive dished washer may be become handy although flat shims will do the job as well.  The exploded view drawings of the gearbox in the Maintenance and Instrucion Manual as well as various books, show a dished steel spring washer at the end of the main axle. I personally have never found such a washer in a gearbox, and wonder if they were actually used on all boxes, or only the later ones. Its not given in the spare parts list of either old or newer contracts or postwar civilian lists. The dimensions are I.D. 5/8", O.D. 1 7/16", steel thickness 1/64" and dished height 1/16" (as measured from an actual washer). 

 Main Gear Wheel Sleeve Bearing Roller Retainer Washer.
This hardened washer has the tendency to fracture. Two of the 3 gearboxes I have showed this to be the case. The washer is given in the spare parts lists (all) under spare part no 3598 (Main Gear Wheel Sleeve Bearing Roller Retainer Washer).  I have however not been able to find original replacements for them, and had them made from a cold work tool steel (AISI D2 or UNS T30402) and hardened to 59 Rc. Remains of the original were measured to be 60 Rc so I think I am close enough.

Below also some pictures of various gearbox parts, (thanks to Olav Jerzykowski)