C15 Gearbox

The gearbox was the most criticised part of the C15 and was renown for failing, often termed as exploding. It is therefore worth taking your time when overhauling the engine to make sure that this area is dealt with properly. There is basically two types of gear cluster arrangements, and three sets of ratios. The gearbox was beefed up in 1960 when the top gear cog's were changed from 30T/20T to 26T/17T. In 1963 the ‘stub tooth’ top gears were brought in to overcome teeth breaking off. Next in 1966, there was a re-designed of the gearbox with needle roller bearings for the layshaft, strengthened stub-tooth design and new kick start mechanism on the side points model.

Pinions

Ratio

Wide

Close

Standard

Layshaft

Mainshaft

Layshaft

Mainshaft

Layshaft

Mainshaft

1st
Early
Modified
Models ‘F’ & ‘G’

2nd
Standard
Model ‘G’

3rd
Standard
Model ‘G’

Top Early
Standard

Top 1960 onwards
Standard
Stub Tooth (1963 on)

Model ‘F’

Model ‘G’

33T
40-3299
40-3174
40-3104

30T
40-3100
40-3097

25T
40-3102
41-3099

19T

40-3109

16T
40-3317

17T
40-3301

40-3108

20T
40-3101

25T
40-3103

31T

40-3106

27T
40-3318

29T
40-3295
40-3176
40-3152

26T
40-3132

22T
40-3134

17T

41-3044

21 T
40-3297

40-3151

24T
40-3130

28T
40-3133

26T

41-3042

32T
40-3014
40-3172
40-3260

27T
40-3010
40-3329

23T
40-3011
41-3098

20T
40-3013

17T
41-3027
41-3044

14T
41-3068

18T
40-3012

40-3256

23T
40-3011

27T
40-3010

30T
40-3017

26T
41-3029
41-3042

22T
41-3061

Shafts
Standard
Model ‘F’
Model ‘G’
40-3009
40-3259
41-3068
40-3004
40-3254
57-2661
40-3009
40-3259
41-3068
40-3004
40-3254
57-2661
40-3009
40-3259
41-3068
40-3004
40-3254
57-2661

Modified layshaft 1st gear with wider ratchet steps for kick-start pawl, which is interchangeable with the original version.

Model 'G' layshaft 3rd gear can be fitted as replacement for earlier gear 40-3011, but not the other way around.

The Top Gear for Model's ‘F’ & ‘G’ have the Layshaft and top gear as a combined unit. This has the effect of removing end-float when assembled in the gearbox.

Shimming of the gears and shafts.

Thrust washers (shims) may be required to remove excessive free play between the gears and also shaft end float. Apart from washer ‘C’ which is a standard size, the others can be bought in varying sizes to take account of wear and tear. These are shown on the exploded diagram above and labeled below.

Thrust washer	Size	Part number	Notes	Effect
A B C D	.093" - .094" .098" - .099" .103" - .104" .119" - .120" .070" -.071’’ .075" -.076" .080" -.081’’ Standard shim Washer .078’’ - .080’’ .082" - .085"	40-3020 40-3126 40-3127 40-3119 40-3019 40-3120 40-3030 40-3258 41-3072 41-3074	Mainshaft Top Gear " " " " " " " " " Mainshaft 3rd Gear " " " " " " Layshaft 1st Gear 1965 - on 1966 - on " "	Mainshaft End-float Gear play 3rd Main & Top Lay Kickstart Pawl movement Layshaft End-float

There is basically two types of gear cluster arrangements, and three sets of ratios. The washers are the same independent of ratio, but vary depending on the cluster arrangement.

Washer ‘C’ is fitted to prevent the kick-start pawl moving sideways, but it does not effect the amount of shaft or gear end float.

Tips:

Layshaft end float on pre-1966 models. There is no shim on these models to take up end float, but this can often be achieved by turning the 'Layshaft Top Gear' around, as it tends to wear at the point where the gear rubs against the spline.
Mainshaft end float. If you can get access to a dial indicator gauge, remove all shims, place the shaft in the case, fit the timing side cover and measure the end float on the end of the shaft, then pick the correct shim. You may be able to achieve this with a metal rule and feeler gauges.
Cam Plate. The engagement of gears can be drastically effected by wear in the grooves. This can be taken out by careful welding and filing, and the tracks can be modified slightly to give maximum gear location. (Not a job for the light-hearted)

Remember no free-play is worse than too much free-play.

The contents of these pages are for your reference and while care has been taken to ensure authenticity no liability can be accepted by the author.

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