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MG MGB Technical - UK Heads

I've recently picked up a spare motor - it's an 18V 581 YH series.
The head is not the original for the block. It's not a US head with the air injection holes, and it does not have an outlet for the water operated choke.

The numbers in the head are: 12H4736 - most likely cast in 1977, judging by the clock markings.

It appears to be a small inlet valve head. Were the UK heads / engines ever produced as Low Compression units like the US?
I can't find any references for UK heads, only US ones. I'm going to rebuild this engine, but if the head is a L/Compression - I may not bother with it.

Anyone help out with information?

Rod
Rod A

Rod,

Someone else will have the full story, but I thought the high and low compression models differed by changing the piston dish from about 6cc to 11cc.

Charley
C R Huff

Rod,
There is some information here you may find useful.
http://www.mgbmga.com/tech/mgb11.htm
http://www.mgbmga.com/tech/mgb25.htm


As Charley said, the piston dish varied the compression, so this head will be fine.
In fact, the lower compression engines had a smaller combustion chamber volume (39cc) than the high compression (43cc).
As the volume of your head may have changed over the years due to head skimming you should measure the combustion chamber volume with a burette. In this way you can accurately calculate the compression ratio.
When rebuilding your head, I suggest you speak to your machine shop about using VN Commodore 3-angle valves.

(I'm using an 18V engine in my Magnette running 9.3:1 compression and an identical spec 18GD engine in my MGB.)

Regards,
David

David Overington

Originally there were HC and LC options for all markets depending on the grade of fuel locally available. From the start of the Mk2 the LC doesn't seem to have been available for North America but continued to be available elsewhere. Then from 1971 North American spec engines were *only* available in low compression, and from 1973 other markets only had high compression available. 18V 581 YH was a non-North American export (i.e. not UK either) high compression with HIF carbs, but of course the head could have come from anything else and the block is only as good as its label! According to the Parts Catalogue only the pistons changed between low and high compression, the heads were common.
Paul Hunt

Thanks for your help. I should have known that heads did not vary between LC and HC engines. The dealer I picked it up from, thought it came form South Africa.

I most likely won't use the head because it's a small inlet valve head. It not worth the cost of altering - Austin 1800 heads are a much cost effective alternative over here. Big valves, much more efficient combustion chamber. I'll simply have the exhaust inserts fitted for the unleaded fuel.

regards.
Rod A

Rod-
The 12H4736 cylinder head casting (BMC Part # 12H 4735) was first introduced on the Austin/Morris Marina. It reverted to the original smaller 1.565” diameter intake valve size in redesigned form (BMC Part # 12H 4211) that was used on the first two cylinder head castings, but had a somewhat improved intake port design and offset oil feed in the rear rocker shaft pedestal (BMC Part # 12H 4737) in order to accommodate redesigned cooling passages to assist in preventing overheating of the exhaust valve of the rear cylinder. This redesign necessitated the relocation of the oil passage in the rear rocker shaft pedestal, which means that if you should choose to install it on an earlier engine block you are going to need the later rear rocker shaft pedestal with the offset oil port (BMC Part # 12H 4737).
Steve S.

This thread was discussed between 17/07/2008 and 19/07/2008

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