FD - Final Drive (diff ratio)
What they don’t tell you in the manual!
If you’ve decided the FD fitted isn’t what you want, changing the FD is possible without separating the engine from the gearbox. Removing the diff case will allow you access to the diff assembly to change the crown-wheel, and access to the gear-change linkage. Use a manual for crown-wheel replacement. While you’re in there it would be just as well to replace the thrust washers and diff-pin as these are the source of many a gearbox problem. Particularly for up-rated engines where fitment for the competition diff-pin is highly recommended. Once again, follow the manual here. Now the bit they don’t tell you how to do...
Rod-change types -
Removal of the speedo drive housing gives access to the pinion retaining nut (1.5”AF socket needed and a long bar). To be able to undo the pinion nut (torqued up to 150lb ft !!) you need to engage two gears - 2nd and 4th - to lock the transmission. Rotate the gear selection barrel anti-clockwise as far as possible then pull it back towards you, i.e. away from the gearbox. Make sure both the inner selector shaft and the outer barrel are as far back as they will go. Using a pry bar, or long screwdriver, push the 1st/2nd and 3rd/4th selector quadrants inwards. They will rotate towards the clutch end, and in so doing engage 2nd and 4th gears. Engaging reverse by sliding the gear away from the centre web and into mesh with the outer gear of the 1st/2nd outer track using a long screwdriver assists further. Through the speedo-housing aperture, use a small chisel to undo the pinion lock tab. Now undo the pinion nut. You are going to need help with this, somebody to hold the engine whilst you heave mightily on the pinion nut! Carefully pry the pinion nut off of the shaft - NOT on the edges of the teeth as they can easily be chipped!
Re-assemble in the exact reverse order, adding the replacement parts, ensuring when you re-engage the selector outer barrel and shaft that all the selector quadrants are in line, especially the reverse gear one as this can not be seen. Test for engagement by pushing and pulling a little on the selector shaft at each quadrant level. You will actually be able to see the reverse lever moving if it is engaged. Then rotate the selector back to its standard position. Re-fit the diff as per the ‘book’. But before refitting the speedo drive housing check the speedo drive compatibility information below.
Remote-change types -
Removing the speedo drive housing gives you access to the gear-change linkage and FD pinion nut. You need to undo the 1/2"AF-headed bolt that holds the gear selector peg in place on the shaft running diagonally down from top right to bottom left of the gearbox as viewed through from the speedo drive housing end. Also remove the neutral-positive location 'claw'. To enable un-going of the pinion nut, two gears have to be selected. Using a suitable rod/heavy screwdriver, push on the ends of the right hand and central gear selector rods. This will select 2nd and top gears, locking the gearbox up. Knock off the pinion lock-tab, and undo the nut. Remove pinion and re-fit replacement, and re-assemble in reverse order making sure all gear selector components are in the correct places/positions, lock-tabs bent over, and all nuts/bolts correctly fitted and tightened. Pinion nut torque setting is 150lb ft. Before replacing the speedo drive housing, check the speedo drive compatibility information below
Speedo drive compatibility
Altering the FD can cause speedo inaccuracies. Whilst being a possible excuse for transgressing speed limits, it probably won’t keep you out of traffic cops notepads. This therefore needs attention.
Prior to 1980, matching FD changes to speedo readings was done by fitting a different speedo, an expensive way of dealing with the problem. After this the mix ‘n’ matching went on in the gearbox. More specifically the speedo drive spindle and pinion. So dealing with the easiest one first, fitment of any A+ gearbox with a 3.44 FD to a pre-A+ powered Mini, you need to fit the spindle and pinion out of the pre-A+ gearbox. Simple. If any other FD is fitted, it is easier and cheaper (in the long run) to get the speedo re-calibrated. Now the multiple choice option...check out the relevant table. This assumes 10” wheels and 12” wheels are retained where fitted as standard, but is reasonably accurate for 13” wheels, as the rolling circumference is very similar.
Speedo drive spindle and pinion data -
|Tooth count -||Tooth count -|
|Final Drive Ratio||Pinion||Colour||Part no.||Spindle||Colour||Part no.|
|All pre- 1980||17||white||22A1881||6||none||2A3720|
Useful part numbers:
|2A7062||Diff output thrust washers (2 needed)Pre-A+ located planet-wheel thrust washers (2 needed)|
|DAM5071||A+ located planet-wheel thrust washers (2 needed)|
|DAM6624||Planet-wheels (2 needed)|
|RPS1418||Diff-pin locating roll pin|
|ATA7385||Crown-wheel bolt lock-tabs (3 needed)|
|ATA7043||Crown-wheel to diff cage bolts (6 needed)|
|GUG705569GM||Speedo drive housing gasket|
|GUG705564GM||Speedo drive spindle housing gasket|
|GUG705565GM||Speedo drive pinion housing gasket|
|2A3505||Upper diff housing gasket, all|
|2A3506||Lower diff housing gasket, remote change|
|22G1836||Lower diff housing gasket, rod change|
1.5"AF deep 1/2"-drive socket for pinion nut (also does swivel pins and Flywheel bolt)
Written By: Keith Calver|
Article Date: Nov 02, 2000
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11/27/2011 @ 12:55 PM
A typical 848cc engine ran a 3.765:1 ratio final drive. With standard 10" wheels, that would mean you are running 15 mph / 1000 rpm in 4th gear; i.e., 60 mph = 4000 rpm. With a 4.13:1 you're really spinning the rpms. Before you do anything, confirm the engine in the car by the engine ID number. Making the necessary changes are covered in your workshop manual and the above article.
11/23/2011 @ 2:32 AM
hi i have an 850 mini that i have just bought and i think someone has put an engine and gear box from a morris 1100 in it and that would have had 12 inch wheels my mini has 10 inch wheels so it seems to be reving a lot and going nowhere even in top gear could someone please let me know how to fix this thanks lyle