|
Gearbox
Close Ratio Conversion A+ |
Updated 29 March 2008 |
As outlined in the
'Gearbox - standard production gearbox types' article, it is entirely possible
to change the common 4-syncro gearboxes to the helical close-ratio set-up as
used in the Cooper S, 1275GT, 1300GT, and some early MG Metros providing the
right parts can be sourced. And you don't already un-knowingly have the
close-ratio kit fitted! Part numbers are detailed at the end.
The other
pre-requisite for this conversion is that it can only be applied to the early,
pre-A+ type mainshaft gears. That's those that have the square-topped teeth as
supposed to the distinctively pointed A+ profile. This is because despite being
endowed with A+-type fittings for the mainshaft end and layshaft, the
conversion gears have the pre-A+ tooth profile. This is not to say that the
conversion cannot be put into an A+ gearbox casing - it can and is detailed
below. What is essential to understand is that the pre-A+ second and third
mainshaft gears are going to be needed. First gear is
the same in all 4-syncro gearboxes irrespective of whether they're pre-A+ or
A+.
It should also go
without saying that doing this to an otherwise clapped out gearbox is a waste
of time. Take the opportunity to give the unit a good over-haul and fitment of
a centre-oil pick-up pipe. Utilise a good manual for general gearbox
disassembly/reassembly method. Haynes is possibly the best as it's quite
comprehensively detailed. Of course there is also my 'Mini Expert – Issue 1'
magazine available from this site where I cover the stripping/re-building and
parts wear assessment in far greater detail than you get in the Haynes manual.
When utilising the A+
first motion shaft and laygear, this is very straightforward. Simply exchanging
the existing ones for the close-ratio versions is all that's required.
Everything else remains the same.
If only the pre-A+
variants are available/sourced, then you will also need to use a pre-A+
mainshaft, pinion retaining nut, layshaft and centre-web layshaft thrust washer
and relevant bearings. The mainshaft is required to provide the smaller 14mm
first motion to main shaft support bearing, the pinion nut because the thread
is different to the later types, the layshaft for the reduced carrier bearing
sizes used and it's accompanying centre-web thrust washer for correct layshaft
clearance/bearing retainer hole. Fitting the mainshaft is a simple swap for the
pre-A+ one with no other fitting problems.
Fitting the laygear
and layshaft is straightforward without any fettling or fixing required. It is
imperative that the pre-A+ layshaft centre-web thrust washer be used. The A+
one does not locate the bearing properly, and would allow a certain amount of
'float' in the shaft to occur.
Re-assembly and
shimming is as per the standard method/tolerances.
Part Numbers –
Pre A+ parts:
|
22G1094 |
2nd gear, 26 teeth |
|
|
22G1095 |
3rd gear, 21 teeth |
|
|
22A1371 |
18 tooth first motion shaft |
|
|
22G1040 |
Close ratio laygear, tooth count is same as standard ratio laygear but
has a smaller outside diameter on top gear (first motion) end |
|
|
88G396 |
Laygear needle roller, 1st gear end |
|
|
CHM141 |
Laygear needle roller, top gear end |
2 needed |
|
C22A1738 |
Hi grade competition spec layshaft |
|
|
22G931 |
Standard spec layshaft |
|
|
22G860 |
Laygear thrust washer, top gear end |
fixed size |
|
22G856 |
Laygear thrust washer, 1st gear end |
0.123-0.124" |
|
22G857 |
Laygear thrust washer, 1st gear end |
0.125-0.126 |
|
22G858 |
Laygear thrust washer, 1st gear end |
0.127-0. 128 |
|
22G859 |
Laygear thrust washer, 1st gear end |
0.130-0.131 |
A+ parts:
|
DAM3167 |
18 tooth first motion shaft |
|
|
DAM3168 |
Close ratio laygear, tooth count/size detailing as per 22G1040 above |
|
|
13H9513 |
Laygear needle roller, 1st gear end |
|
|
CHM141 |
Laygear needle roller, top gear end |
1 needed |
|
C22A1739 |
Hi grade competition spec layshaft |
|
|
DAM3187 |
Standard spec layshaft |
|
|
22G860 |
Laygear thrust washer, top gear end |
fixed size |
|
DAM3180 |
Laygear thrust washer, 1st gear end |
0.121-0.123 |
|
DAM3181 |
Laygear thrust washer, 1st gear end |
0.124-0.126 |
|
DAM3182 |
Laygear thrust washer, 1st gear end |
0.127-0.129 |
|
DAM3183 |
Laygear thrust washer, 1st gear end |
0.130-0.132 |
Other parts:
|
C-AHT54 |
Centre oil pick up pipe |
|
|
22A542B |
Now GUG705556GM (in AJM804B set) |
2 needed |
|
AJM804B |
Gearbox gasket set |
|
|
18G8085 |
Gearbox lock tab set |
|
|
DAM6140 |
Pinion lock tab (in set 18G8085) |
|
|
DAM4000 |
First motion/input gear lock tab (in set 18G8085) |
|