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SSI Insert revised (no index tag!): 22-Feb-2016
Article on The Internet Mini Encyclopædia |
Cylinder head, 12G940 identification (without measuring the valves!)
If you see a 12G940 head for sale at a show and you are no able to measure the valves, here is a quick way to ID the head...
Standard 1275 mini heads have:
1275 Metro heads:
MG Metro non-turbo:
MG Metro turbo:
The chamber design is the same for all, but MG maybe skimmed for higher CR. Vizard said to avoid the MG for (eg: bodging onto a 1100/998) because there's not enough metal thickness on the face.
Cylinder head, 12G940 variants
Mini 1275's, metro 1275's and MG metro 1275/turbo all use
12g940's but there are several versions.
The MG has 1.4" inlets as opposed to the mini/metro 1 5/16th's<?>.
The turbo has smaller inlets (1 3/16th's?) and sodium cooled exhaust valves.
It's tricky to distinguish them apart from valves alone. The thermostat area is
flat for standard 1275 heads, and sculpted for MG and Turbo heads. The inlet
ports have a locating step on MG/Turbo ones too. The MG non-turbo head has no
bypass hose outlet and the Turbo head has the bypass blocked with a screw.
AFAIK chamber design is the same for all, but MG maybe skimmed for higher CR.
Vizard said to avoid the MG for (eg: bodging onto a 1100/998) because there's
not enough metal thickness on the face.
Cylinder head casting numbers
| Casting number | Inlet valve size (inches) | Exhaust valve size (inches) | Chamber volume (cc) | Engine size |
| 12A1456 | 1 3/32 | 850cc/998cc | ||
| 12G185 | 1 1/16 | 1 | 24.6 | 997 Cooper, fits as big valve head for 850 & 998 (THIS DATA NEEDS CLARIFYING) |
| 12G202 | 1 5/32 | 1 | 26.1 | Austin/Morris 1100, fits as big valve head for 850 & 998, but square formed inlets |
| 12G206 | 1 7/32 | Early MG 1100 | ||
| 12G295 | 1 7/32 | 998cc Cooper, MG 1100 and MkIII Sprite | ||
| AEG165 | 1 13/32 | 1 7/32 | 21.4 | Cooper S, can be used instead of the 11 bolted 12G940 |
| 12G202 | TBA | TBA | TBA |
All 997cc Coopers had a 12G202 head. Never seen a 12G185?? Maybe you mean the 12G295 as fitted to 998cc Coopers? Speaking of 12G202 heads, I have noticed a couple of people selling these on eBay lately, describing them as rare Cooper heads. All stock 1098cc (except some MG, Wolseley, Riley 1100 variants etc) have the same head. So it is hardly a rare and valuable head! Yet another example of adding "Cooper" to an eBay add to drag in the less knowledgeable. (Info from Brett Nicholson Melbourne, Victoria, Australia.) |
| 12G1805 | TBA | TBA | TBA | is the head as fitted to the last of the MK 3 S and to 1300GT austin and morris cars also some MG1300, Riley Kestral and VP1300 might have had them. Head will have stamp near thermostate housing with S K V etc, (Info from Paul) |
| AFG163 | as above | as above | as above | as above |
"From experience, the 12G202 forms a very good basis for 998cc tuning. However, it needs to be skimmed dramatically - >0.060" in order to get the compression ration up to a suitable figure in the smaller displacement engine. There is a danger of cutting into the rocker oil feed gallery above this figure, although I have frequently removed near 0.090" from there heads. They can also be modified and fitted with the larger inlet valve to produce a "poor" man's 12G295. "
Cylinder head colours and details
1275 metro : Red
Metro Turbo : Sculpted
around tappet cover and heater tap.
Cylinder head, fitting a 1275cc head (12G940) to a small bore block
see Block Pocketing
Cylinder head, fitting BMW 8 port
Another cylinder head that will almost 'bolt on' to a mini is a BMW head from the 4-cylinder bikes (the same as the Police ride in South Australia).
The block requires the stud holes moved, and the head requires some drilling of oilways - the head is normally on its side, and being up the wrong way the oil can't flow back down.
These cylinder heads are available from scrappies (in
Australia) for about $400 complete.
Cylinder heads, modifying
"MG Metro is the best standard head, and easy to improve.
My head is an MG Metro 12G940 which I 'Vizardised' with some grinding tools.
Get a 'Dremel' tool (high speed hand held drill thing) and get some tungsten carbide cutters (5-10 quid each). Don't mess with grinding stones as the cast iron is too hard. Tungsten-carbide is extremely tough and I found the cutter was still sharp after hours of grind the hard iron. I used a flexi-drill-drive in a pillar drill, but that took AGES as the RPM was far to slow.
Start by *thoroughly* reading the section on head mods in "Tuning BL's A series engine". You need to read this as it is easy to ruin a head by grinding too deep and breaking through to the water/oil ways.
The chamber edge can be ground out to make the chamber less restrictive. The 'heart shape' and the 'beak' between the valves is to swirl the gas to mix the mixture more, but you can grind down the beak alot to increase the inlet 'valve curtain area'. This is what *really* improves the performance potential. Increasing the flowability of the gas through the valves. The valve seat vicinity is the biggest bottle neck in the whole system. Improve this and all the other things like big exhaust and hot cam start to work aswell.
Make up a card template of the chamber shape you want. It is vital to get all the chambers the same volume, and preferably the same shape!
Don't be tempted to increase the depth of the chamber, just do around the edges.
It's a slow process, but is worth the effort.
As for porting, the MG Metro head is pretty good as standard as the split point on the inlets is quite spacious. One area that will need grinding is the manifold locating step. Cut this back quite a bit to prevent swirl over the step.
IMPORTANT!!! Wear goggles!!! Don't get any iron filings in your eyes..it is very painful. I got a bad eye infection from this and had to go to the doctors and get some medication. Also, rig up a vacuum cleaner to suck up all the dust as you are grinding. Don't breath the dust either. Use a face mask.
Once you have modified the chambers you will need to get the head skimmed to get the CR up to what you want. Again, read the relevent Vizard section. Don't get the head skimmed before modifying as you are bound to slip with the cutter and scratch it.
I added about 2cc to each chamber and then got a 0.010" skim. This wasn't enough to get my desired 11:1CR so I got a further 0.020" skim a while back."
Maestro/Montego 1275 models - Head information (16/12/02)
Maestro/Montego 1275 models used the MG Metro big valve 12G940 cylinder head and cam to produce 69bhp.
See Maestro to Mini Cylinder head info for more information.
Cylinder head, stage numbering
| Stage number | Modifications |
| 1 | Simple gas flowing, ports, skimmed |
| 2 | chamber mods, serious porting |
| 3 | big inlet OR exhaust valves |
| 4 | big inlet AND exhaust valves |
| 5 | all above + specials (slanted guides, etc) |
What exactly is a downdraft/semi downdraft head?
A standard head is a side draft; the inlet and exhaust ports are located at the side of the head.
A semi downdraft head has had new inlets drilled thought the top so the inlet ports are at about 30 degrees. Such a head can flow about 120CFM (cubic feet per minute) of air, whereas modified sidedraft heads flow around 90CFM.
The airflow increases due to the loss of the corners in the inlets, and the old ports are welded up.
Semi-downdraft heads on Minis typically include such things as Weber DCOE 45s - there's no room for SU's!
Info from Toby Vernon-Smith
Head studs
If you need to remove all the head studs, you will find eight will be easy and the one at the front in the middle will be stuck fast, the hole in the head for this stud is drilled smaller to locate the head, but sometimes rusts up.
Head Gaskets - general choices
Tim Draper says: "my preference is as little metal as possible, as it's not as flexible or 'absorbing' to the irregularities of the head/block surface. The point of a gasket is to take up the indifferences between 2 surfaces, thus creating NO gaps between the surfaces. The 'paper' gaskets will take the dirt, uneven-ness etc and embed it into the gasket, but the all metal gaskets will be pushed around by the dirt, and not give the best gasket effect.
"You won't find (to my knowledge) a head gasket without metal inserts for around the piston bores. but I'd try to get the rest of the gasket as the sticky papery stuff (anyone know what it's called?)"
Ade Law adds: "The Payen AF470 gasket is the black squidgy one which is excellent. The AF460 (think that is right) is the all copper coloured one. It does compress by a fair margin when you clamp the head down."
NB: check the part number for your engine size before buying! (20/12/02)
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