Clutch, Gearbox  

Clutch 

The clutch, which is fitted in the flywheel, is of the single dry plate type. It has 12 springs exerting a total pressure of 120 kg and is operated through the gear box by a push rod with thrust (ball) bearing.  

Pressure and counter pressure plates are unlined and therefore have no holes for rivets.

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The earliest pressure clutch plates are made of 2 mm steel plate. This thickness was increased to 2.5 mm during 1934. The edge of the counter pressure plate is not bevelled and the central hole is 95 mm in diameter. Starting with number 1550 the clutch had a sprung hub. In order to create space for the springs, the diameter of the hole in the counter pressure plate was increased to 116 mm.

1934 clutch. Please note that the flange of the clutch hub is hidden behind the clutch plate and therefore cannot be seen.

The 1934 clutch plate is made of 2.5 mm steel plate and has 6 straight slits.  The lining is riveted on both sides of the clutch plate.
Very evident for the 1934 clutch plate is the relatively large hub flange which, as opposed to later models, is riveted against the back of the clutch plate. In order to create space for the springs in the 1935 clutch, the flange of the clutch hub was reduced from 68 to 58 mm diameter.
The clutch hub is copper plated. The reason for this is unknown.

The counter pressure plates have, as mentioned, a punched central hole with a diameter of 95 mm. The punched-out circle was used by F&N as a cover plate at the front of the crank case housing.

39.

Gearbox  

The 1934 gearbox at first sight looks like the later models up to No 7500; yet there is a long list of differences, both internally and externally.  

The gearbox does not have the raised section around the second bolt hole from the top which was introduced later.
Viewed from the clutch, the bearing housing has rounded off edges and between these, a casting pattern, resembling a butterfly, can easily be seen.

Gearbox housing

The cover of the gearbox has a 15 mm high collar without a cork gasket for the solid drive shaft and its bearing housing has rounded edges.
Most Nimbuses with solid drive shafts from before No 2560 were exchanged for the new, rubber sprung shaft type. In order to create room for this new shaft, the collar of the gearbox had to be shortened. If this was omitted, the rotating axle would cause the collar to shorten by friction and wear. It is therefore very difficult to find a 1934 gearbox cover having a collar of 15 mm.
The gearbox has three gears, which was very common for motorcycles from the twenties and early thirties.

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The 1st gear cog has 34 teeth and the 3rd gear cog has 24 teeth both having strong rectangular gear claws. The 2^nd gear cog has 29 teeth and has corresponding rectangular slots.
The lay shaft has a guide for the drive shaft and does not have lube oil cups near the drain for the oil ducts.  

The gearwheels or cogs of the main shaft have 14, 19 and 24 teeth for 1st, 2nd and 3rd gear respectively. The bore in the main shaft for the thrust bearing of the declutching rod is shorter than those of the next models, because no cork gasket has been fitted around the declutching rod, as the later models have.
The 1934 main shaft can be recognised by measuring the depth of the bore for the thrust bearing. The easiest way to do that is with an ordinary ruler, because it easily fits into the hole. The depth should be approximately 33 mm for a 1934 housing and about 37 mm for a shaft designed for the use of a cork gasket.  

The gear lever gate has four notches for the 1st, neutral, 2nd and 3rd gear respectively.  

The declutching rod is controlled by an 8 mm bolt with a small 10mm hexagonal head.  

The rectangular gearwheel claws of the 1934 gearbox have proven to wear out rather quickly, resulting in frequent popping out of 3rd gear. The fact that the problem was widespread can be concluded from the large number of 3^rd gearwheels with heavily worn out gear claws or claws that have been abused with a rotating grinding stone or an angle grinder. 
In order to solve the problem, F&N has modified the claws of the gearbox from having a rectangular to a round shape, starting with number 1526. Simultaneously, the gear lever was modified to give 5 notches instead of 4. The extra notch allowed a neutral position between the 2^nd and 3^rd gear, which was an advantage when changing down from 3^rd to 2^nd gear. To create room for the extra notch, the notch for the 3^rd gear has been moved 2 mm outward in order to reduce the height of the new gear lever. Consequently it is impossible to use a lay shaft with 5 notches in an original 1934 gear box, without making modifications.   

The 1934 gearbox is pressure lubricated with oil from the lube oil pump through the lube oil pipe mounted in the right hand side of the crankshaft housing and entering into the bore of the gearbox shaft. The bearing bushes of the 1^st and 3^rd gearwheel are lubricated through the holes in the gearbox shaft, after which the oil runs down, resulting in splash lubrication for the other parts of the gearbox. The remaining oil flows back through the drain ducts to the crank case.

As stated previously, an unrestricted oil flow can be jeopardised when the oil pressure is not high enough. Many 1934 gearboxes show

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Gearwheels of the gearbox, main shaft, lay shaft and shifting shaft.

 modifications to improve lubrication, e.g. increasing the oil sump level by fitting a vertical copper pipe in the oil drain hole and quite often primitive oil cups have been ground around the mouth of the oil duct in the gearbox shaft.
Moreover, F&N introduced oil cups in the gearbox shaft late in1935.

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