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MG MGB Technical - 12H-4736 Head

Having just pulled my engine apart I've found I have a 12H-4736 head with valve cut outs in the top of the block.

Is there a reason why these heads require a cut out where others do not?

Is there anything special about these heads in terms of performance (loss or gain).

How can I check and be sure this head has been converted to run on unleaded?

Doing some research another website said that all cylinder heads except mine have the "PAT NO" on the head, but mine does. I assume this is a mistake on their part?

Also as my block already has the cut outs does this mean I could fit another head with bigger valves?

Thanks
K Goldup

If you have unleaded seat inserts, they will appear to be rings that are press-fitted into the combustion chamber.

The cutouts on your block were done for the larger 1.625" intake valves of the 12H 2923 cylinder head casting, so obviously the cylinder head has been replaced. That cylinder head casting was commonly found on engines with the engine numbers 18V-584-Z-L, 18V-585-Z-L, 18V-672-Z-L, and 18V-673-Z-L, all of which it was Original Equipment for.

The 12H 4736 cylinder head casting (BMC Part # 12H 4735) was first introduced on the Austin/Morris Marina and also used on the 18V-846-F-H and 18V-847-F-H UK/European market MGB engines, all of which it was Original Equipment for. It reverted to the original smaller 89.2 gram 1.5625” diameter intake valve size that was used on the first two cylinder head castings, but in redesigned form (BMC Part # 12H 4211), and had a somewhat improved intake port design that produced a 4% increase in flow at maximum valve lift when compared to previous cylinder heads that also used the smaller 1.565" intake valve, as well as offset oil feed in the rear rocker shaft pedestal (BMC Part # 12H 4737) in order to accommodate coolant passages that were redesigned in order to assist in preventing overheating of the exhaust valve of the rear cylinder. This redesign necessitated the relocation of the oil passage in the cylinder head as well as in rear rocker shaft pedestal, which means that if you should choose to install it onto an earlier engine block you are going to need the later rear rocker shaft pedestal (BMC Part # 12H 4737) that accommodates the offset oil port. It should be noted that for the North American Market this particular cylinder head was used solely on the 18V-836-Z-L and 18V-837-Z-L engines that were produced from September of 1974 through December of 1974 for the so-called “1974 ½” models.

Stephen Strange

Its an Ivor Searle rebuilt engine so it is quite possible it has the 'wrong' head. Although I thought it strange that the combustion chamber and head gasket aren't round. Is this because the head has possibly been reworked? I'll attempt to upload a picture to show what I mean.

I measured the inlet valves with a vernier gauge and they are approximately 1.55" so they don't appear to be the bigger type (unfortunately!)



K Goldup

K-
The combustion chambers aren't supposed to be round. Those are the standard Weslake combustion chambers. It appears that unleaded seats have been installed.
Stephen Strange

Hiya

The block cutouts are for the ex valves to prevent them hitting the block. The cutouts were on all 18v blocks. The 18v blocks were fitted with heads having same combustion chamber shape as yours. The chambers were around 10mm deep as opposed to the 11mm deep chambers on the pre18v heads. It is good practice to machine ex cutouts on pre18v blocks as most heads have been skimmed/milled a time ot two.

Peter
P Burgess

This thread was discussed between 17/04/2011 and 18/04/2011

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