The Midget 1500 (GAN6, 1974 to 1979) was fitted with the Triumph single-rail four-speed gearbox in place of the BMC A-Series ribcase unit used on earlier cars. This is a fundamentally different gearbox, sharing no internal components with the A-Series, so all internal parts are specific to the single-rail design. Identifying the gearbox correctly is essential, as A-Series parts will not fit.
Synchromesh and Gears
The single-rail gearbox has synchromesh on all four forward gears, an improvement over the A-Series gearbox, which has no synchromesh on first. The synchro rings (baulk rings) wear with use and are the usual cause of a crunch on changing gear, particularly into second when cold. Worn synchro rings are renewed during a gearbox rebuild, along with inspection of the gears themselves for chipped or worn teeth.
Bearings, Layshaft and Thrust Washers
The mainshaft and layshaft run on bearings that become noisy as they wear, producing a characteristic whine that changes with gear selection. The layshaft (countershaft) gear cluster and its thrust washers control endfloat; excessive endfloat causes noise and poor shifting. A full rebuild renews the bearings, layshaft, and thrust washers together to restore quiet, precise operation.
Selector Mechanism
The single-rail name derives from its single selector rail, which moves the selector forks to engage the gears, a simpler arrangement than the multiple rails of the A-Series box. Worn selector forks and a worn rail cause jumping out of gear and imprecise selection, and are inspected and renewed as needed during overhaul.
Ordering Considerations
All internal components here are specific to the Triumph single-rail gearbox of the 1500; none interchange with the A-Series gearbox of the 1098cc and 1275cc cars. A gearbox rebuild is a precise job requiring correct setting of endfloat and preload, and is best entrusted to an experienced rebuilder. Renewing synchro rings, bearings, and seals together while the gearbox is dismantled is far more economical than repeated partial repairs.