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MG Midget and Sprite Technical - Cylinder head identification

Changing out my cylinder head for a MG Metro turbo version....However, on disassembling, the old head looks pretty good. Well ported and polished no less, with some peculiar cast numbers. Valves were OK, inlets sooty, exhausts white crust on the face - indicative of anything?

Double sprung also on the valves - a swine to remove, my old Sykes Pickavant compressor just bent.... no valve seals anywhere either - which may explain my mystery oil loss......

Can any of you clever chaps tell me what I have here?

Other images to follow

Mark O

another pic

Mark O

and another

Mark O

It's a 12G940.

Not a lot to say, really.

Does it have 9 studs or 11? Any numbers stamped into the flat face near the 'stat?

You probably had trouble removing the valve springs because the caps were jammed on the collets, not because of the (standard on midgets) double springs.
It helps if you put a suitably sized socket on top of the cap and give it a tap with a hammer to unstick it first.
Dave O'Neill 2


Just thought the x above the casting number in the 2nd pic and the odd diamond shaped lozenge in the 3rd were unusual - as is the level of porting......

Yes, did give the valves a wee tap before compressing.....

Yes numbers near the stat - will revert.....
Mark O

AFAIK the diamond shaped W is the logo of the company that did the castings.
David Smith

Ports look nice how is the short side radius (inner corner on the inlet)

A turbo head is not a particulary good head.
Just a normal 12G940 with special exhaust valves to cope with the extra heat.

If the guides are good just fit some inlet seals and put it back on
Onno K

"Yes numbers near the stat - will revert..... "

Is the number something like 12G41805?

What sizes are the valves?

Lawrence Slater

Nice... id lay odds that it may have bigger valves also... it has the brass guilds ...are you sure there are no caps on the intake? Its not uncommon to have no caps on the exhaust so to help keep the exhaust valve cooler

Id be in favor of using the 940 head over your metro head

Prop and the Blackhole Midget


I have a new-in-box MGB head with a large X cast into it. It is purely a guess in my part, but I have thought that maybe the X was used to indicate a replacement part rather than factory installed. Pic attached, X at upper left.


C R Huff

to me it sounds like you have the same as me a standard head that has been previously worked on by someone else before your ownership

because of the lack of a good rolling road tuner within reasonable distance and not being able to drive other Spridgets it *might* be that without you realising it you already had a good engine set up which would take a lot more to have better than it in a reasonable way and cost
Nigel Atkins

The X Could be be the signiture of the engine builder that built the head

I want to say peter burguss has a signiture mark as well

Prop and the Blackhole Midget

Is the head 9 or 11 stud

Prop and the Blackhole Midget

OR, maybe the "X" signifies that it once belonged to a Wolverine. ;).
Lawrence Slater

My HAN9 Sprite MkIV's head has 12G940 cast into it. On the flat area adjacent to the thermostat cap it has stamped 12G938.

A bit of googling told me that a number of heads marked 12G938 were produced with a bit more 'meat' in important areas to allow them to be uprated for sporting purposes. I don't think mine has ever been touched but perhaps you have one of these that has received some attention.

Colin Mee

the 940 is fairly close to the 1318 the main differance is the 1318 has the smog ports and a little extra meat to help agianst cracking, but the 940 is still a sought after head by those that can quote vizard chaper and page without his bible being present.

Prop and the Blackhole Midget

The best for normally aspirated A series is MG Metro - big valves and double valve springs. Other Metro heads have smaller valves (and single valve springs) although as stated above Turbo heads have different exhaust valves to withstand the extra power (hence heat) produced by the Turbo and double valve springs.

Mini SPI seem to have bigger valves then Mini MPI.

Earlier 1300 heads from the MG 1300, Riley 1300 and Austin 1300 GT (the 1100/1300 cars) have big valves, double valve springs and 11 studs.

The holy grail seems to be the Mini 1275 Cooper S but that has a different casting number and a propensity to crack between inlet and exhaust on one of the cylinders (4 I think as its furthest away from the water pump and runs hotter).

MG Metro heads are getting as rare as hens teeth now.

Also, late Maestro 1300 heads (and probably Montego) are the same as MG Metro and, in the Maestro's case, because they are difficult to put into anything, can be picked up cheap. I picked one up for 100 off eBay a year or so ago. Well worth it for the head and HIF44 alone.

Rob aka MG Moneypit


I want to to say it was the 67-68 mini cooper S head that had cracking.issues between the valves as I was the 1.15 and the 1.44 in 69 they took the 1.44 to 1.4, and that solved the cracking issue

Prop and the Blackhole Midget

Prop, I think you are right. Trouble is everyone wants the ultimate, biggest everything, judging by eBay prices.

To my mind the MG Metro was by far the best A series head, but I don't think you saw them in the USA?

Rob aka MG Moneypit


Many thanks for your comments. On balance, I think the old head is a modified standard head. Inlets are 33mm, exhausts 29, double springs, but polished and ported as shown. Shown below and in the next 2 posts is the condition of said head - well gunked up as you may see, particularly around exhaust ports. I am thinking that given the soot in the exhausts it may be running rich, and the gunk may be contributing to a perceived lack of performance - though I concede it may be perfectly adequate. Valves are similarly crudded up on rear of faces...

I was thinking of soaking in a bath of hot caustic/cola/vinegar solution and then jet washing off to remove the gunk....thoughts?

The replacement head has raised faces at the stat and heater offtake, the larger 35mm inlet valves, single springs, no water pump bypass and has undergone some porting - of sorts. Initially I thought it was a MG metro turbo - given the raised faces, but it has no thicker exhaust valaves - so bit of mystery. The thinking behing the swap was simply the adavantage of the larger inlets, but on reflection and given the nicely machined old head, I am having second thoughts...Single springs a bit of a mystery - any advantage to retro-fitting to a double set up?

Howvever, the "new" head is now fitted up, so I will flog it up my local hill and compare and contrast....

Mark O

and another....

Mark O

one more....

Mark O

This thread was discussed between 31/12/2014 and 05/01/2015

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