Transmission, Clutch, Gearbox and Differential
Clutch, gearbox and final drive
The exam covers power flow from clutch to gearbox, propeller shaft, differential and drive shafts. A clutch must have correct free play and release smoothly; a hydraulic clutch needs clean fluid and proper bleeding. Manual gearboxes use synchromesh rings, so use the specified gear oil, not engine oil. Check linkage adjustment, oil level and leaks. Differential backlash, pinion preload and tooth contact pattern are set with a dial indicator and marking compound. CV joints need intact boots and the correct grease. Automatic transmissions require the specified ATF and level-check procedure; never tow a vehicle far without following the maker’s rule.
Ratios and speed
Overall ratio = gearbox ratio × final drive ratio. Lower numerical final drive gives higher speed for the same engine rpm but less torque multiplication. Wheel speed depends on tyre circumference, so a size change alters the speedometer reading.
Worked example
A ring gear has 41 teeth and the pinion has 11 teeth: final drive = 41/11 = 3.727. In a 1:1 gear at 3000 rpm, wheel rpm = 3000/3.727 = 805. With a 2.0 m tyre circumference, speed = 805 × 2.0 × 60/1000 = 96.6 km/h. Show the ratio first, then convert wheel rpm to road speed.
Traps
Common errors are setting clutch free play to zero, mixing ATF and gear oil, marking a propeller shaft after removal, and measuring differential backlash on a dirty gear. A noisy CV joint is not cured by a new boot alone if water has entered.