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Cylinder Head Stuff

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.

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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
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
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. "

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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

Be careful if you are fitting a 1275 head to a small bore (eg: 998) block. Not only does the block need 'pocketing' to clear the exhaust valves, but there can sometimes be problems with the water pump. The larger water pump pulley will often foul the head, so the smaller type is required. Also, if using a 'high capacity' water pump, there is a risk that the head will foul on the top edge of the pump and not clamp down properly. Check the clearance, and modify the aluminium pump if necessary. Also, the oil ways in to the head from the block don't always match up perfectly, and so to do this properly you might need to plug the original oilways and redrill to get perfect flow.

Ade Law has an online how-to for this particular mod here. By kind permission, the full contents of the article are included here:

"Tuning a 998 isn't really worth it. If you are building an engine then it costs just as much to re-build a 1275 as it does a 998 so if you are after power then stick to a 1275 derived engine. However if you like 998s or don't have the money then a 12G940 head, cam swap and a stage 1 kit is the way to go.

First off lets understand exactly what a 12G940 head is and how it differs from a small bore head such as a 12G295 or the std 998 head (CAM4180). The 12G295 is considered to be the ultimate small bore head and is good but they are getting rare and thus expensive. Here is a quick comparison between the CAM4180, 12G295 and the 12G940.

Casting No. Inlet Valve Size Exhaust Valve Size Chamber Volume
CAM4180 26.98mm 25.4mm 25.5cc
12G295 30.9mm 25.4mm 28.3cc
12G940 35.6mm/33.3mm 29.3mm 21.4cc

As you can see the valve sizes are a lot bigger on the 12G940. The size of the inlets on the 12G940 changes depending on the engine it came off. If it came from a MG metro or Vanden Plas metro then it will have the bigger valves. The ports are bigger too.

However there is a problem, To get the larger valves in they had to move the exhaust valves away from the inlet valves. This means that when you place a 12G940 on the block of a 998 the exhaust valves overlap the block. If there you get too much valve lift then the valve will hit the top of the block.

There are 2 ways round this problem. The first is to not have too much lift.. When I did this I had to keep the std cam because of cost. I was literally swapping heads. In my case the std cam didn't have too much lift. If you are keeping the std cam then there is a pretty good chance that the 12G940 head will go straight on however I suggest you check to see if you have enough clearance as tolerances of the rockers etc are poor and the amount of actual valve you get varies. I did this by attaching a lump of blutack to each exhaust valves and bolting the head onto the block with no gasket and 0 valve clearance. Then I turned the engine over by hand and pulled the head off and looked to see how much the blutack had been squashed. The other way to do it is again bolt the head on with no gasket and 0 valve clearance then turn the engine over until an exhaust valve us fully open then winding the adjuster until the valve touches the block. If it takes about 1mm of extra valve lift then you will be ok.

The other option (preferable as you can change the cam to something more interesting) is to "pocket" the block. This really needs the block to be stripped as it creates swarf what you are doing is cutting the area of the block under the valve away. How you do this is up to you, I have heard of people doing this with a dremel but beware if you slip and go down the bore below the ring line or across the top of the block you will need to rebore the block or get it skimmed.

Compression ratio, as you can see the size of the combustion chamber is smaller than both the CAM4180 and the 12G295. This means that the Compression ratio goes up. On a 998 with flat top pistons (like my 89) the Compression ratio was around 10.5:1

So, I have convinced you that the 12G940 is a better head then even the best "small bore" head. I will tell you where to find them. The bigger valve heads that you are likely to find around are from MG metros (not turbo) and metro Vanden Plas. The other smaller valve head was fitted to all almost all other 1275 A series engines. If you find a 1275 mini, metro and possibly meastro made after approx june 89 then it will be and U/L head!

Once you have sorted the problems the big valves cause there are a few other things you need. The head is a 1275 head so you need a 1275 head gasket and top radiator bracket. The other difference between a 998 and 1275 is the height of the "deck" relative to the water pump. The head sits lower relative to the pump this means that the std large mini water pump pulley hits the head. This is cured by using a pulley from a metro this pulley is smaller and runs the pump/fan quicker which improves cooling too! However if you are running much over 6.5k I suspect you might have problems with the pump cavitating. Also due to head sitting lower on the block the head will hit the top "web" of the water pump hitting the block. This is easy to modify without making the pump any less strong.

Finally if you got the head from a metro then it won't have a bypass hose, this means you will also need a water pump without a bypass hose."

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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.

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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 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 used the MG Metro big valve 12G940 cylinder head!

Maestro/Montego 1275 models used the MG Metro big valve 12G940 cylinder head and cam to produce 69bhp.

According to Haynes they started off with 1.312" inlets and 1.156" exhaust valves.

But from engine No.12H999104761 and 804102434 they had 1.4" inlets.Which is worth about 5bhp.

Get down that scrapyard!

Added 16 Dec 2002

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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!

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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.

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Head Gaskets - general choices

Tim Draper says: my preferance is as little metal as possible. as it's not as flexible or 'absorbing' to the irregularities of the head/block surface. The point in 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. the all metal gaskets will be pushed around by the dirt, and not giving the best gasket effect.

You wont 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.

(Added 20 Dec 2002 NB: check part no for your CC of engine)

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