The blue gears are all free to spin on the orange output shaft and are constantly driven by the layshaft, which is constantly driven by the engine. The pink selectors are keyed to the orange output shaft. To engage each gear the gear selector fork moves one of the pink dog-clutches into engagement with one of the blue output gears, to lock it to the output shaft and provide drive through to the prop-shaft and wheels.
The additional purple idler gear between the red laygear and the blue reverse output gear reverses the direction of rotation of the output shaft when in reverse.
An oddity with this drawing is that despite each successive forward gear being larger on the laygear and smaller on the output shaft as one would expect, the laygear reverse gear is somewhere between 3rd and 4th and would give. In practice (and as on the MGB further down) reverse gearing is similar to 1st gear - sometimes slightly higher and sometimes lower:
In the MGB gearbox the laygear is not used for fourth, the 4th gear selector engages directly with the drive dog teeth on the 1st-motion shaft to give a 1:1 ratio. For that reason the order of gears is the other way round to the drawing above i.e. the 3rd gear wheel is on the left of the mainshaft with the 4/3 selector between it and the 1st-motion shaft gear, 2nd gear to its right, then the 2nd/1st gear selector, then 1st gear, then reverse.
1st-motion shaft containing the gear 'A' to drive the laygear and dog teeth 'B' that the 4th gear synchroniser assembly engages with to select 4th gear:
Output/mainshaft with the '4/3' synchro assembly moving left for 4th and right for 3rd, ditto the '2/1' assembly, and near-equal 1st and reverse gears. Reverse gears are 'straight cut' with no synchromesh hence the car has to be stationary for a moment or two for silent selection. 'A' is the needle roller that engages in the end of the 1st-motion shaft to support that end of the mainshaft, 'B' is the worm gear for the speedo drive on a non-OD box:
The laygear has 1st and reverse about the same size on the laygear - this is the 4-synch where 1st is 3.44:1 and reverse is 3.095:1 i.e. reverse has a lower ratio/higher gear than 1st. The 3-synch has 1st at 3.64:1 and reverse at 4.76:1 i.e. the other way round with reverse being a higher ratio/lower gear than 1st. This fits in with old-school advice when a very steep hill defeats 1st, which is to try reverse:
The reverse idler gear having similar input and output gears:
In context: The large gear on the laygear is engaged with the gear on the 1st-motion shaft. The reverse idler gear shown in its engaged position, with its driven gear meshed with the laygear and its driving gear meshed with the mainshaft. When disengaged the reverse idler moves to the right with the driven gear in the space between the laygear reverse and 1st gears and the driving gear in the cut-out in the bearing housing on the right:
Gear selection: The selector/synchroniser assembly is in two parts - an inner part that is keyed to the main-shaft with splines and outer part which can slide back and fore on the inner part and is keyed to it with splines at 'C'. The selector forks move the outer part towards gear 'B' which is engaged with the layshaft and spinning freely on the mainshaft with minimal longitudinal movement. The teeth in 'A' engage with the teeth 'D' to lock the gear to the mainshaft and provide drive when the clutch is released. 'A' and 'C' have a spring and ball detent between them so a certain amount of force has to be applied with the gear lever before 'A' will move along 'C':
Synchronisation:
A 'three legged' baulk (brake) or synchroniser ring (Brown & Gammons) sits in the three cutouts of 'C'. This has an inner taper which bears on an outer taper of the gear (to the left of the driving teeth 'D') as the gear lever moves the synchroniser ring towards the 'new' gear. Until the detent releases the two tapers are pressed together which matches the speed of the gear (and hence the layshaft as the clutch is released at this point) with the speed of the synchroniser assembly which is being driven by the road wheels.
Here the 1/2 selector fork (A) has moved the outer part of the 1/2 selector/synchroniser assembly (not visible) to the left to engage 1st gear exposing the 2nd gear baulk ring (B):
Here the 3/4 selector fork (C) has moved the outer part of the 3/4 selector/synchroniser assembly (not visible) to the right to engage 4th gear exposing the 3rd gear baulk ring (D):
| Axle | Ratio | % change | Per 1000 (WSM) | Per 1000 (calculated) | Per 1000 (GPS) | |
| 3-synch | R | 4.76 | 3.76 | |||
| 3.909:1 | 1 | 3.64 | 4.92 | |||
| 1 to 2 | 2.21 | 64.71 | 8.10 | |||
| 2 to 3 | 1.37 | 61.31 | 13.07 | |||
| 3 to 4 | 1 | 37.00 | 17.90 | 17.90 | ||
| 3 to OD3 | 1.09874 | 24.69 | 16.29 | |||
| OD3 to 4 | 9.87 | |||||
| 4 to OD4 | 0.802 | 24.69 | 22.30 | 22.32 | ||
| 4-synch | R | 3.095 | 5.82 | |||
| 3.909:1 | 1 | 3.44 | 5.23 | 5 | ||
| 1 to 2 | 2.167 | 58.74 | 8.31 | 8 | ||
| 2 to 3 | 1.382 | 56.80 | 13.02 | 12.5 | ||
| 3 to 4 | 1 | 38.20 | 18.00 | 18.00 | 17.5 | |
| 3 to OD3 | 1.13324 | 21.95 | 15.88 | |||
| OD3 to 4 | 13.32 | |||||
| 4 to OD4 | 0.82 | 21.95 | 22.00 | 21.95 | 21.5 | |
| V8 | R | 2.819 | 8.16 | |||
| 3.071:1 | 1 | 3.138 | 7.33 | |||
| 1 to 2 | 1.974 | 58.97 | 11.65 | |||
| 2 to 3 | 1.259 | 56.79 | 18.27 | |||
| 3 to 4 | 1 | 25.90 | 23.00 | 23.00 | ||
| 4 to OD4 | 0.82 | 21.95 | 28.00 | 28.05 | ||
| V8 Mazda | R | 3.758 | 6.12 | |||
| 3.071:1 | 1 | 3.136 | 7.33 | 6.5 | ||
| 1 to 2 | 1.888 | 66.10 | 12.18 | 12 | ||
| 2 to 3 | 1.33 | 41.95 | 17.29 | 17.5 | ||
| 3 to 4 | 1 | 33.00 | 23.00 | 24 | ||
| 4 to 5 | 0.814 | 22.85 | 28.26 | 27.5 |
Notes:
- While the 4-synch has reverse at a lower ratio than 1st so 1st will climb a steeper hill, for the 3-sync and the Mazda gearboxes reverse has the higher ratio and conforms to the time-honoured advice that if you can't get up a hill in 1st then try reverse!
- Factory gearbox ratios show a very similar percentage change from 2 to 3 as for 1 to 2. This means that with the 4-cylinder cars there is a relatively large gap between 2 and 3, noticeable when pulling away up a steepish hill when more revs are needed before 3 can be selected or it bogs down (the torque of the V8 means it's not a problem on those cars). The Maxda has a more even spread of ratios.
- The 3-synch has just less than a 10% change in ratio from OD3 to 4 meaning there is not much difference between the two. The 4-synch has just over 13% which means it is closer to half-way and makes a good intermediate gear for long inclines or pirited twisty bits.
- Whilst V8s originally had OD on 3rd gear as well testers were breaking the gearboxes from high torque reversals by switching it out at high rpms so it was removed starting with gearbox 1404. It's a confusing story as to when this was change was applied to production cars.
- North American cars also lost overdrive on 3rd gear - but for a completely different reason - in early 1977.