Triumph TR3B TCF Series, TR4-5/250 A Type Overdrive Conversion Kit - 4 Synchro
If by chance there are 4 synchro TR3B owners out there wishing to fit overdrive to their cars, this kit will suffice so long as the TR3A style switch, body harness and speedometer cable are substituted. Please contact us for details.
Whilst our conversion kits list everything that you should need to convert your car to overdrive spec, in reality we often struggle to supply a complete kit due to old core gearbox shortage, therefore, use this section to work out what is involved, and ensure you have all the parts required (sometimes a serviceable secondhand gearbox is a good starting point) before you commit to the project.
An overdrive conversion is among the most useful modifications that can be carried out on your TR and it is relatively straightforward to do. Overdrive provides useful additional gear ratios, for use under all driving conditions. Creeping about in town, the use of 2nd overdrive and 2nd gear saves on the continual 2nd to 3rd to 2nd gear changes. It nicely bridges that 2nd to 3rd speed ratio gap.
The 3rd gear gives that little extra help for high speed overtaking when an upward gear change could best be ignored. The 4th gear overdrive provides effortless high speed cruising improving long distance touring fuel economy.
The A type overdrive unit operating in 2nd, 3rd and 4th gears was fitted as an option to later TR2s and all subsequent TRs up to 1972 model TR6s. It provides a 22% reduction to the engine speed for a given road speed when engaged. Overdrive fitted early TR2s used an A type unit which operated in 4th gear only - it may be replaced by a later, more flexible unit if originality is not important. 1973 onwards TR6s, incidentally, used the J type overdrive with operated on 3rd and 4th gears only. The J type provides a 25% reduction to the engine speed for a given road speed when engaged. All of the subject TRs in this catalogue were designed to accept the A type overdrive. It is possible, however, to fit a J type overdrive and gearbox if desired to a TR4A, TR5 or TR250 using a conversion bracket kit for the chassis mounting.
To simply 'Bolt On' an overdrive to your non-overdrive box is not possible since the mainshaft (output shaft) at the rear of the gearbox, is different. Whilst it is possible to adapt your existing gearbox for overdrive fitment, this does mean that a strip down is required to change the mainshaft. This, in effect, results in overhauling the gearbox (at least replacing bearings, synchros etc) prior to fitting the overdrive. This also assumes that your existing non-overdrive gearbox is to original specification (ie it has not been changed or rebuilt to a difficult specification by a previous owner) and that the overdrive mainshaft purchased is compatible with your unit. In real terms it is easier (and usually more economical) to buy a complete reconditioned gearbox and overdrive assembly which when purchased with a conversion kit saves all the trouble.
Our conversion kits not only include the gearbox/overdrive unit assembly, but also wiring, column switch, solenoid, drive flange speedo pinion, angle drive (where appropriate) and speedo cable. Just about everything you will need! you should also consider replacing as a matter of course (purchasing separately), the clutch assembly and engine/gearbox rubber mountings whilst carrying out the conversion (refer to relevant sections). The propshaft is the same for all applications and may be re-used if serviceable. Please note that our conversion kits are based on receiving and old gearbox in exchange - either a TR non-overdrive unit or a saloon (2500), Sprint or Stag type gearbox/overdrive assembly.