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MG Midget and Sprite Technical - Cooper S head - should I ?

Hi Guys
I just by chance fell over this Cooper S head, 12G940, priced at what would equal 596 £, reconditioned.
I know of the seller, he is racing a Spridget and is actually a friend of one of my colleagues. So if I gought it I would feel quite safe regarding the informations about were done with the head.

https://www.dba.dk/topstykke-bmc-12g940-coop/id-1038606121/
Jan Kruber

i had my 12g940 head modded for about the same money, and that included porting and new seats, guides and a skim. I supplied the valves, so it seems an ok price to me. Although i have read the Modify your Mini, and Tuning BL's A series books, i can never remember what head is the best starting point, but others will be along who know more.
So far as my upgrades went, i took the view that just one mod would not do on its own, so the head, exhaust, ignition, carb, cam, rockers and as importantly, brakes all came in for improvement, and then a go on a rolling road to get it all to be friends.
plenty of people would have spent more than i did, and got greater BHP results, so i guess like so many things it revolves around what you are after initially.

P Bentley

According to the valve sizes given, it isn't an original Cooper S head - at least, not according to the specs that I have.

The main differences between Cooper S and midget 12G940 are the extra stud holes and the valve sizes. This is probably just a standard 12G940 with extra holes drilled

When buying a head, it helps to know what the chamber capacities are, whether the ports have been enlarged (and by how much) and how thick the deck is - i.e. how much it has been skimmed in the past.

If it is from a race car, the ports might be too big and the compression too high for a road car.
Dave O'Neill 2

For that kind of money, and not much of a savings from what I can see

I would ask to have it shipped to Peter Burgess in kent for an inspection and recommendation at the least at your cost

I cant decipher much ....im not sure about the valve sizes you want to look that up you don't want anything bigger then 1.15 ex / 1.57 ??? Intake or it runs a high risk of cracking between the valves vary common for the 1969 Cooper S head

The only real difference in casting from the 12g940 and the rev run of the mill head casting is the 940 does not have catoletic ports so the head is thinner

To be honest for that kind of money I'd drop a dime to Peter Burgess and see what he can create for you, then you know you got your money's worth and no nasty surprises

On the porting for street racer ... you dont want the ports enlarged, you want the sand casting junk smoothed down but not slick, and a large bulb of metal that surrounds the valve guild removed... but you want it to be a genital curved slope ... NOT a 90 degree corner angle .... this is a mistake a lot of people make

Next unless your block has the 10th & 11th hole at the front and back of the head, I wouldn't put much focus on that modification ....its a great mod if your going 12CR and rebore of 1380 or a stroker 1460....otherwise it's a wasted expense


One thing I would recommend if your doing a head mod, cosmetic head gasket... there steel with Vinton coating ... expensive but nice you can get them in any thickness and are good for 7 replacements

But there is a procedure for installing that is unknown

Prop
1 Paper

Prop said "...but you want it to be a genital curved slope..."

which is a somewhat graphic but very true description.

Rob
MG Moneypit

hi,
12G940 large valve metro head in classifieds.
thanks bob.
bob taylor

I think a genuine Cooper S 12G940 has 12G1805 stamped near the thermostat housing.
The other replies make a valid point in that the head was probably made to go with other engine mods so you won't be certain whether it will fit in with the state of tune of your engine. You may be in for more major expenditure if you buy it.

Rob
MG Moneypit

Bob,
Will the large valve Metro +A head fit to a pre A engine?
Can you please explain "3 groove collets" - I tried Google translate, not to much help, does it mean "3 cracks"?
Thx
Jan
Jan Kruber

hi jan,
the head has 9 bolt holes. not an expert on whether it fits a pre A engine. perhaps someone on here knows for sure. the difference being 36mm inlet and 30mm outlet valves.the head is sculptured around the thermo housing and heater valve casting. the valve stems have 3 grooves around the top where the collets hold them on instead of the normal 2. no bypass hose.


bob taylor

Metro A+ will fit but there is no connection for the bypass hose that goes between the head and water pump. You will need to replace the water pump with a later one.

The collet retains the spring at the top of the valve stem. The collet is in two parts with grooves on the inside radius. These locate onto concentric grooves machined into the top of the valve stem.

Groove in this context is more like the groove in a record. Not a crack, something intentionally machined into a surface.

Bob beat me to it.

Rob
MG Moneypit

Metro A+ will fit but there is no connection for the bypass hose that goes between the head and water pump. You will need to replace the water pump with a later one.

The collet retains the spring at the top of the valve stem. The collet is in two parts with grooves on the inside radius. These locate onto concentric grooves machined into the top of the valve stem.

Groove in this context is more like the groove in a record. Not a crack, something intentionally machined into a surface.

See image for 3 groove.

Rob
MG Moneypit

Image


Dave O'Neill 2

You can use a 1275 head on a pre 1275 block (1098, ect) but you have to notch the block for the valves to clear... but you can not put the smaller head into the 1275 block

Beyond that... I don't know much more but there is info in the archives... but I pity the fool that goes into that crazy land of info

Prop
1 Paper

This thread was discussed between 14/11/2017 and 16/11/2017

MG Midget and Sprite Technical index

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