MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Cylinder head/block compatability

Which cylinder heads are not compatible with which particular block(s). I've heard there are some differences in how oil and/ or water holes line up.

Ta,

Iwan
Iwan Jones

All cylinder heads will fit on all blocks, but 18G blocks will need the exhaust valve clearance cutouts machined if an 18V head is fitted (12H2709 or 12H4736 heads)

The correct rocker gear rear post needs to be fitted - either centre or offset to match the head.
Chris at Octarine Services

Hi,

Thanks Chris; however, I read somewhere that some heads have a oil or water hole blocked if fitted to the wrong or different block.

Ta

Iwan
Iwan Jones

Iwan,

The only oil hole that is different is the one that feeds the rocker gear as I mentioned before.

Chris at Octarine Services

All oil and coolant passages at the mating surface are compatible with each other.

There is an oil passage at the top of the head that feeds the
rockers that you must be concerned with. On the "early" heads
the oil passage is in-line with the pedestal mounting studs.
On "late" heads this oil passage is offset. You must have
the correct rocker shaft pedestal for you application..

Later model heads have a water port located at the top rear
for the carb choke, but this does not present any issue with
compatibility with the block. This port can be easily blanked off
with a homemade metal plate.

Daniel Wong

Chris,

Do you mean all of the blocks with a G regardless of the next letter? That is, I think the earliest block was designated with only 18G, and then later there was the 18GA, 18GB, 18GK, etc. So, are you including all of these later G blocks as well?

Thanks,
Charley
C R Huff

Hi,

Thanks Daniel,

Yep, this is what I'd read. I knew about the offset rocker oil feed & certain older blocks needing extra valve clearance machined etc.

Which casting id. denotes these later heads.

Ta
Iwan Jones

Yes all 18G to 18GK

Also applies to MGA 1500, 1600 and 1622 blocks!

Offset oil feed heads are 12H4736 in the UK
Chris at Octarine Services

Hi Chris
I'm not that bothered about offset oil feeds etc. its what Daniel mentioned (see below) that I'm interested in knowing the id. number for.

'Later model heads have a water port located at the top rear for the carb choke, but this does not present any issue with compatibility with the block. This port can be easily blanked offwith a homemade metal plate.'

Thanks
Iwan Jones

Chris,

Thanks for the clarification.

Charley
C R Huff

Hi
Im in middle of engine&head restoration,GB engine with later head fitted 12H4736,haven't had the engine running myself, but last owners was using car this way before its layup,but there is not a cut out in the block for valve clearance.So can i trust the last owners word or is there something else going on

Cheers Bill
bill sdgpm

Hi
I'm in middle of engine&head restoration,GB engine with later head fitted 12H4736,haven't had the engine running myself, but last owners was using car this way before its layup,but there is not a cut out in the block for valve clearance.So can i trust the last owners word or is there something else going on

Cheers Bill
bill sdgpm

Sometimes the late heads will not pose an exhaust clearance problem with the early blocks. If the valve lift is not very high, or the cylinder has been overbored sufficiently, there will not be a problem. I should also mention that some of the later style composition gaskets are thicker than the earlier copper faced ones were, which will also add additional clearance.

I wrote an article primarily on cylinder heads seen in the US here: http://www.flowspeed.com/cylinder-ident.htm

Sean
Sean Brown

Iwan,

He is referring to the late US heads - you are unlikely to find one in the UK except on a US reimport.
Chris at Octarine Services

This thread was discussed between 12/08/2010 and 19/08/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now