Hylton Reid explains how he did it.
© H. G. Reid 2020
Kim adds: “For many owners of classic cars, including Sprites and Midgets, and especially those examples used often for long distances (in normal times!) the installation of a five speed gearbox (incorporating an ‘overdrive’ ratio top gear) provides major benefits in terms of lower engine rpm at high road speeds, with consequential improved fuel consumption, less engine wear when cruising, and a more relaxing drive. In addition, if a relatively modern gearbox is used, it is likely that it will have synchromesh on all forward gears, by contrast with the original transmission which lacks synchromesh on first gear.
Grateful thanks to Hylton Reid for his detailed account of this job, which should help other owners contemplating this upgrade; all photographs and text/captions by him.
The general principles of Hylton’s conversion could be applied, with suitable modifications, to other BMC rear wheel drive cars powered by ‘A’ Series engines…”
Hylton explains… There was an article some time ago about fitting 5 speed gearboxes into Midgets and Sprites. The original popular choice was the Toyota T50 gearbox, for which Frontline provided a replacement bell housing to match them together. This has now become almost impossible because of the lack of supply of Toyota gearboxes, and Frontline no longer provides the bell housing.
Other choices are the Datsun gearbox or the Ford Type 9 gearbox. I don’t know anything about the Datsun box, but the Ford box is heavy, being cast iron cased, and is a saloon car gearbox.
Another alternative is the Getrag gearbox from a BMW. These are relatively easily available at sensible prices. However the Getrag box has a number of disadvantages. It is also a saloon gearbox, which means that first gear has a very low ratio of 3.764:1, (the Ford is 3.65:1) whereas the standard Sprite is 3.2:1. The Getrag is quite short, however, and light, but it uses an electronic speedometer and not a cable drive. To fit a speedometer would require a new instrument and some sensing system and wiring.
Whilst pondering all of these options it occurred to me that a suitable gearbox might just be from a Mazda MX 5. This is a sports car gearbox, with a decent first gear ratio (3.163:1), with good intermediate ratios and a 0.81:1 fifth gear. It is light, with an aluminium alloy case, but is quite long, being over 7 inches longer than the Sprite box. Hence the gear lever is appropriately 7 inches further back than in the Sprite. It doesn’t have a removable bellhousing, but making an adaptor plate to fit the engine to the gearbox proved to be fairly straightforward.
This article is a description of how the MX 5 gearbox was fitted to my Sprite. Starting from the front of the gearbox and finishing at the propeller shaft, I propose to show how it was done and include pictures at each stage.
The MX 5 gearbox has two large protuberances on the right hand side, used to locate 2 large beams which fix to the differential. They take up space in side the gearbox tunnel and need to be removed.
The Engine Backplate
Since the gearbox does not have a removable bellhousing, you will need to make a new backplate to match the engine to the existing bellhousing. This isn’t as difficult as it sounds.
You need to find a sheet of aluminium about 10mm thick and an original Sprite/Midget backplate with its locating holes. Make it about 2 inches larger in all directions, this gives sufficient extra material to match it to the MX-5 gearbox bellhousing. Drill out the block fixing holes and crankshaft, starter and oil pump holes. Note the oil pump hole is slightly recessed to take the lip of the oil pump cover. Please see Plate 1, below: