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Block Pocketing

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."

[Ed JB: or you can use a piece of oil cooler hose held on to the bypass port of the pump with a Jubillee clip and long enough to allow you to put a piece of metal rod down the hose to clamp another hose clip around - needless to say the bit of rod just fits inside the hose. AND the hose is short enough to clear the head.

B.T.W. oil cooler hose makes indestructible bypass hose so long as you don't make them too big to foul on the head! ]

07 June 2003

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