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30/10/2000 |
Terminology –
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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.
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Speedo drive spindle and pinion data –
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Tooth count |
Tooth count |
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Final Drive Ratio |
Pinion |
Colour |
Part no. |
Spindle |
Colour |
Part no. |
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All pre-1980 |
17 |
white |
22A1881 |
6 |
none |
2A3720 |
|
3.44 |
16 |
green |
DAM2905 |
6 |
none |
2A3720 |
|
3.2 |
18 |
red |
TXD1006 |
5 |
red |
TXF1004 |
|
3.1 |
16 |
green |
DAM2905 |
7 |
blue |
DAM6028 |
|
2.76 |
15 |
black |
TXD10004 |
7 |
yellow |
TXF10001 |
Useful
part numbers:
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C-BTA166 |
Competition diff-pin |
|
BTA101 |
Diff output thrust washers (2 needed) |
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2A7062 |
Pre-A+ located planet-wheel thrust washers (2 needed) |
|
DAM5071 |
A+ located planet-wheel thrust washers (2 needed) |
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DAM6624 |
Planet-wheels (2 needed) |
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RPS1418 |
Diff-pin locating roll pin |
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ATA7385 |
Crown-wheel bolt lock-tabs (3 needed) |
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ATA7043 |
Crown-wheel to diff cage bolts (6 needed) |
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DAM6140 |
Pinion lock-tab |
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GUG705569GM |
Speedo drive housing gasket |
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GUG705564GM |
Speedo drive spindle housing gasket |
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GUG705565GM |
Speedo drive pinion housing gasket |
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2A3505 |
Upper diff housing gasket, all |
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2A3506 |
Lower diff housing gasket, remote change |
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22G1836 |
Lower diff housing gasket, rod change |
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TOOL03 |
1.5"AF deep 1/2"-drive socket for pinion nut (also does swivel pins and Flywheel bolt) |