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MG Midget and Sprite Technical - Piston Query


I am new here so hello to everyone.

I am currently stripping down my 1275 midget engine (1974 Car) and have come across a couple of things that I wasn't expecting.

No.1:- I am pretty sure it has a Cooper S head, is this an upgrade or is it normal. Also looks like it has been gas flowed.

No.2:- The pistons after being cleaned up are marked AE 21250 inside the piston body, but, on the top they are marked 21251 +1.016, am I right in thinking these are +.040" and why the anomaly on the id numbers?

Thats all for a start. Thanks
C Phillips


Has it got the S extra stud and bolt?

A pic of the combustion chambers would be nice, plus it out to have a part on it somewhere.

The early S heads were stamped 12A185, later ones AFG163.

These had 1.4" inlet valves 1.216" exhausts, the valves nearly touched in fact!

The later S heads were numbered 12G940 and had the same 1.4" inlets and 1.156" exhausts, apparently to prevent valve seat cracking which the first heads suffered from.

The A-series tuning bible by David Vizard is your friennd here.

Regards Steve.
SR Smith 1

1. what are the differences that you are seeing between a Midget spec and a Cooper S spec head? - I don't know, I thought they were very similar or even identical? (edit - later ones anyway, as Steve said).
2. Sounds like the piston manufacturers follow the convention of having a number for the casting and a separate number for the finished piston, as more than one spec piston can be made from the same casting.
David Smith

The 1275 spridgets never has the high performance S head, ie with the extra stud and bolt.

The later cars were prob the same as the 1275GT, MG 1300 etc

SR Smith 1

MG 1300 did have the 11-stud (10 stud, 1 bolt) head, as did 1300Gt and Riley Kestrel.

It is not uncommon to drill 9-stud heads with an additional two holes.

Original 11-stud heads (12G940 versions) were also stamped '12G1805' adjacent to the thermostat housing.

Dave O'Neill 2

This thread was discussed on 22/03/2016

MG Midget and Sprite Technical index

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