Classic mini A+ a series engine information
The A+ engine came on to the mini scene in 1983. It has a
thicker block to box flange and is generally more robust than the standard A
series. Operationally it's pretty much identical and is the same capacity.
Heads/boxes are swappable.
Here are the A+ distinguishing features:
Strengthening ribs on the back of the block at the clutch end
Thicker block-gearbox flange
Dizzy clamp is a forked plate with a single bolt into the block
Dipstick sits directly in block, not in a tube, and is shorter
Verto clutch, slave on plate slanting downwards, short arm. (NOTE: The very first A+ engines had a pre-verto clutch)
Alternator bracket mounting holes are closer to the rad on the A block
'A+' stickers on rocker cover if a Metro!
NOTE : The 1275 crank fouls the inside of the gearbox casting on earlier 1960s unless the 'box came off an "S" originally. They standardised in '68 or '69.
The change over to A+ didn't occur on all parts at once. Some engines had eg: A+ block and pre-verto clutch, and some had A block/box and A+ bits bolted on. Maybe BL running down stock.
One correspondent has an A+ engine as described above,
but it was the original engine from a 1981 New Zealand model 1275GT. It does not
have a verto clutch, but the gearbox is the A+ type, and so is the block. It
could have just been a test by Austin-Rover to see if their engine was any good
before it was released in the UK in 1983.
It is possible to put an A series engine onto an A+ gearbox and vice versa, but the transfer housing and drop gears *MUST* be the same type as the gearbox, as the idler bearings are different sizes. Apart from that nothing to it.
The block will go straight onto the gearbox but there are 2 different primary gears to clear the stepped tail of the 1275 crank. The first motion and idler gears however are the same.
There are A idlers and first motions (excluding for the moment the longer first motion for an "A" type 3 synchro unit which is pretty much not worth worrying about given the abundance of 4 synchros).
Then there are 4 primary gears:
The 998 and 1275 A primary gears share the same tooth pattern and are internally different to suit the tail of the respective crank. Same goes for A+ however they differ of course in tooth pattern to the two A type primary gears.
So, as long as the gearbox is from an A+ then it can be used.
If the gearbox is an A type (non A+) then you would have to source a 1275 A type primary gear. If the idler and/or first motion shaft gears did not wear prematurely in the case that was originally with the gearbox, you can stick with them even if you need to drill and tap for a transfer case breather.
WARNING: swapping transfer cases sometimes results in a mismatch between the bearing housings locations and can cause premature failure.