Triumph TR6 J Type Overdrive Conversion Kit & Electrics
Recommended for (1973 to 1976) CR/CF series vehicles.
Operates in 3rd and 4th gears only.
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 shortages. 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 TR6 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, an A type unit only, 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 was fitted as an option to the TR6 (CP/CC series pre 1973) and operates in 2nd, 3rd and 4th gears. It provides a 22% reduction to the engine speed for a given road speed when engaged. The J type overdrive unit was fitted to TR6 (CR/CF series from 1973), operating on 3rd and 4th gears only. It provides a 25% reduction to the engine speed for a given road speed when engaged.
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, synchro's etc - see Gearbox Reconditioning Kits) prior to fitting the overdrive. This also assumes that your existing non-overdrive gearbox is to original specification (i.e., 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 compete, TR6 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 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). Depending on the vehicle chassis type and type of overdrive gearbox (A or J type) to be fitted, it may be necessary to purchase separately the rear mounting bracketry.
Pleas note that our conversion kits are based on receiving an old gearbox in exchange - either a TR6 non overdrive unit or a saloon (2500), Sprint or Stag type gearbox/overdrive assembly.
To select the type best suited to your car, the A type conversion suits CP/CC series cars (pre 1973) and the J type CR/CF models (1973 onwards). The positions of the chassis mounting brackets for the gearbox/overdrive are different, so selecting the 'correct' overdrive conversion makes life easier, i.e. early (CP/CC) non-overdrive gearbox and chassis mountings are the same as the A type overdrive, and late (CR/CF) the same as J type. For the record, the J type locates about 1 inch lower and 3 inches further back on the chassis than the A type. The propshaft is the same for all applications and may be re-used if serviceable.