What is the difference between an A motor and an A+ motor?

The A+ motors came out in 1980.
The only Oz vehicles that got it were the last of the 1275 Mokes.

re the 1275,
Basic differences are-
Block has strengthening webs on it to offset the fact they lightened it.
Many blocks were sleeved at the factory due to porosity in the casting.
Crank has rolled fillet radii in the corners of the journals.
Rods are lighter than the Morris 1100S/ Mini 1275LS type.
Oil pump is slot drive.
Timing chain is single row, with tensioner.
Manifold is different, carb is an HIF44 usually.
Distributor drive is different, the dizzy sits directly in block and is clamped by a forked plate and single bolt. Just like a Holden or Valiant.

MG Metro version had around 72HP, with single carby.


At the end of the day it's still a 1275, if modding it there's not much advantage over the earlier A series ones.
<edit> which are better for doing big bores.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Lots
The A series had been around for 50 or more years and had a number of design restraints. When rover continued the production of the Mini well into the ninties it needed an update on the old motor. This became the A plus motor.

mainly the block was a diiferent casting, the distributor was mounted differently and a few other changes to make it a stronger, less noisy engine and smoother. Many parts including can be interchanged with the old A series.

Specific changes can be found on here by doing a search on the subject or you could visit Brickworkx in Adelaide who would be delighted to give a look at a couple of A plus engines and probably a description on the differences.

in my opinion in standard form the a+ shits all over the a. just seems more smooth and has a nicer box and stuff.. prolly cos its newer. id definately recomend a+

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Potato

Yep they are smoother, partly `coz the conrods don't have great heavy lumps on them whirling around...
We have one in Barney.

A+ gear ratios are crap though, and you can't just drop Mk2 Cooper S 1st motion shaft and 22G1040 cluster gear straight in. Teeth angles and layshaft are different on A+ boxes.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.