Important!! This part of site is an attempt to organize and add to the web resources for the Rover SD1 to form a cohesive and easily usable guide for those of us without easy access to expert repair and/or advice. It is not offered in any way as a definitive source and we take no responsibility for any errors that may exist. the webmaster Rene Winters.

The 77 Mill. Gearbox Introduction The need for an overdrive gear ratio is catered for in our 77 mill. gearbox which features a .833:1 5th gear. Incidentally, the name 77 mill. is derived from the distance between the mainshaft and the layshaft. The 77 mill. gearbox is found on the Rover SD1 series, 5 speed TR7s TR8 and the manual versions of the Jaguar and Daimler 3.4 and 4.2 models. This gearbox has 5 forward and 1 reverse gear. It is similar to the single rail gearbox in many ways. For instance, it utilises a single selector rail for selection of all gears. Reverse gear uses an idler which is brought into mesh with the layshaft reverse gear and a gear cut onto the 1 st and 2nd gear coupling sleeve. It also uses spool type interlock mechanisms. This time two spools are used. The rear one acts as an interlock for 5th gear whilst the front one acts as an interlock for the other 4 forward and one reverse gears. A lip on the rear interlock, which is in constant engagement with the gearbox housing, locks the 5th gear selector fork when any other gear but 5th is selected. When 5th gear is chosen as you see in the bottom left diagram, a gap in the lip allows the pin to move the selector backwards. A lip on the front spool is also in constant engagement with the gearbox housing. When either 3rd or 4th gear is selected, this lip secures both the reverse and the 1st-2nd forks, leaving the 3rd-4th fork free to be moved by the selector pin. If 1st or 2nd gear is selected the spool is turned anticlockwise by the selector pin, this time locking the reverse and 3rd-4th selector forks. The 1st 2nd selector fork can then be moved by the pin in the selector rod. Selection of reverse, causes further anticlock-wise rotation of the spool. Both the 1st-2nd and the 3rd-4th forks are now locked while the reverse fork can be moved. One other feature of this gearbox is its pump fed lubrication system. A gear type pump driven by the layshaft is situated in the rear casing. Oil is fed through passages in the mainshaft to various bearing surfaces. This form of lubrication assists the splash lubrication upon which many gearboxes rely.

Dismantling the Gearbox We start with the box drained of oil and removed from the car, in this case a Rover. First unscrew the clutch release pivot bolt, using ST1 136, and withdraw the release lever and slipper pads and the release bearing and support sleeve then remove the bell housing. At the rear of the box, remove the 4 bolts retaining the remote control housing to the gearcase rear cover. The remote control shaft is connected to the selector shaft by a pin fitted with a nyloc nut. The two shafts can be separated by removing the pin or alternatively, by manoeuvring the remote control housing and sliding the remote shaft from the pin. The pin is then easily removed.

To remove the rear cover, first remove the drive flange and the speedometer driven gear and housing the locating boss for the rear selector spool and the ten retaining bolts and spring washers. The cover can now be lifted clear. Retained inside it will be the oil pump and its supply pipe, the ferrobestos oil feed ring, the rear oil seal, the tail bearing, a collar and the speedometer drive gear. We will be looking at these later. Remove the gasket; then clamp the centre plate to the main casing, with two dummy bolts. Remove the oil pump drive shaft from the layshaft and the circlip from the selector shaft. Remove the two bolts retaining the 5th gear selector fork bracket and lift off the bracket, together with the selector fork and slipper pads and the selector spool.

Remove the mainshaft circlip then the support plate and the synchro-hub and sleeve. If the hub is tight, it can be levered off using two small tyre levers. Remove the synchro-cup and the 5th speed gear, together with its two caged roller bearings, and the spacer. Now the layshaft, first remove the circlip then pull off the collar and the gear using special tool 18G705 and adaptor 18G705/1. The collar and gear must be removed separately, otherwise the collar may bind and damage the shaft. Next, removal of the main casing. Support the box on the centre plate and remove the front cover and gasket. The front oil seal will be retained inside the cover. It can be discarded as a new one will be fitted on reassembly. You may find that the mainshaft and layshaft selective washers are also retained inside the cover. Remove the front selector spool locating boss the plastic cover and breather tube.

Then the two dummy bolts from the centre plate. The casing and gasket can now be lifted clear. Remove the layshaft and input shaft bearing tracks. The tracks are a slide fit in the casing. Lift the input shaft and the 4th gear synchro-cup from the mainshaft. Although the 1st, 3rd, 4th, and 5th synchro-cups are identical, if they are not being replaced, it is important that they are refitted in their original locations. Lift the layshaft from the centre plate then lever the spring clip from the reverse lever pivot pin and remove the pin, lever and slipper pad.

Slide the reverse gear and distance piece from the reverse shaft. The shaft need only be removed if either it or the centre plate are being replaced. In the case of centre plate replacement, the locating dowels and the reverse pivot shaft must also be removed. Incidentally, the centre plate and the main casing are supplied as an assembly and if one is being replaced, so must the other. Release the pressure on the selector shaft detent spring and ball align the 5th gear selector pin with the slot in the centre plate and lift the selector shaft and mainshaft clear. Separate the selector shaft from the mainshaft and slide off the 3rd/4th fork and the front spool. The 1st/2nd fork and the selector pins must not be removed.