DB18 Gearbox differences

Recently there's been some discussion about the differences between the DB18 and the Special Sports gearboxes, list numbers 27509 and 28509 respectively. Although the gearboxes are physically interchangeable the DB18 and the Special Sports rear axle worm and wheel ratios are different, so swapping the gearboxes alone isn't a good idea.

The DB18 saloon gearbox is comparatively easy to source, but the Special Sports overdrive type certainly isn't, so it seems a good idea to ascertain whether it would be possible to convert the DB18 gearbox to the overdrive type as fitted to the Special Sports. Obviously the best way to do this is to compare the part numbers of the individual components to identify which parts share the same number and which parts don't.

To simplify the record of my findings I broke the gearboxes down to 3 sub-assemblies, namely:

  1. The gearcase with studs, dowels, etc.; including the brake gear, but excluding the input drive shaft/oil pump/clutch pack.

  2. The input drive shaft/oil pump/clutch pack.

  3. The running gear (excluding B).

In the cases of the brake gear and gearcase with studs, nuts, and the input drive shaft oil pump/clutch pack the lists are confined to only the different parts, so those that are not listed are common to the 2 gearbox types.

In the case of the running gear the list is confined to only those parts that are common to both gearboxes because there are so few of them, so it may be assumed the parts that are not listed are different for the two gearboxes.

This article can be summarised as follows: although there are many common parts to the two gearbox types, nearly all of them are confined to the gearcase and brake gear assemblies. It's the author's opinion that to convert the DB18 saloon gearbox to the Special Sports gearbox is certainly possible, but it would not be a viable proposition unless you own a complete Special Sports running gear assembly in a serviceable condition.


* The internal cross shaft, P/N 401728, is common to both gearboxes so the external levers are probably interchangeable.

** The adjuster tables, adjuster rings and adjusting nuts are common to both gearboxes so the adjusting springs are probably interchangeable.

The P/N 503397 gearcase, complete with studs, nuts, dowels, etc. is common to both gearboxes.


*The following is no more than an educated guess:

Overdrive is effected by locking the overdrive sun gear and using the input drive shaft to drive the planet gear carrier round it. In this case the planet gears rotate on their individual axles and drive the annulus at the overdrive ratio of 0.73:1. When the car is being driven in overdrive the engine is rotating slower than when it's in 3rd gear, (road speeds being equal), so it follows that the volume of the oil pump's output is reduced. As the overdrive epicyclic gear is operating, reduced oil flow to it would result in increased wear, so to offset this the oil pump's output capacity is increased, which is easily achieved by merely increasing the throw of the pump's eccentric.

When top gear in the DB18 saloon (or 3rd gear in the Special Sports) is selected, the entire running gear is locked up solid so there is no relative movement between the epicyclic gears; in this instance no more than minor oil flow to the running gear is required.

**Without a breakdown of the individual components I can't ascertain whether the clutch assemblies are identical. However, the driving and driven plates of the 3.5 litre Regency and Empress cars are the same part numbers as the DB18 saloon, so it's reasonable to infer the Special Sports and the DB18 clutch plates are identical.


NB: Although all 4 ball bearings are common to both gearboxes, the input and output drive shafts and all forward gears are different.

© S. J. Moore, January 2008


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