MGB replacement engine mystery

[Andrew Bishop]

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Thanks for the link to Flowspeed, very helpful! So, I took the valve cover off as suggested and found the head number 12H4736 AF2H 08 which the Flowspeed website recognises, hooray! and the Pat No agrees although it should be internal on this head. The date casting M 8 5 - the M would be December, so likely 5th Dec 1978, and it sat around for about a year until the car was completed in Nov 79, yes? The head is therefore probably the original and was transferred to the "short" recon engine. Apparently the reason for the replacement engine was a camshaft fault of some kind so this also fits the theory. And the recon block would have to be late 70's as well to accommodate the valve throw of this late head. Sounds plausible?

Feel like I've been watching too much Hercule Poirot...

[Ian F]

Hi Andrew, I think that just about wraps it up! The 12H4736 head was indeed fitted to Bs, but I'm unsure what year? Perhaps Vic (who has a later rubber bumper B) could let us know the head casting number on his car? My reason for slight uncertainty is that the 4736 casting used the slightly smaller inlet valve size (1.56 inches) rather than the later 1.625 inch inlet valve. However it may be that later Bs such as yours reverted to the smaller inlet valves. The date evidence is very convincing though, and I would agree with your interpretation.

With all of this now, I'm not sure that you would even have a replacement block - you may have the original block with just a replacement number tag - to record who did the recon work after the camshaft problem. I don't believe camshafts generally damage the block, usually just need new cam bearings (if that!). The later (18V) blocks all have cut-outs for the exhaust valve clearance, and in any case these are very easily added to blocks which do not have them.

All in all a good result I think!

IanF

[Ian F]

Another update! The rubber bumper cars did indeed revert to the smaller inlet valve size, so the head casting 12H4736 (which I believe had the smaller inlet valves) could well be correct for your age of car. A confirmation from Vic would be interesting nevertheless.

The interesting thing to remember about these engines/heads etc. is that if you are rebuilding them for any reason, they can all be improved to a similar standard regardless of the original slight differences as they came out of the factory.

I would recommend getting Peter Burgess's book "How to Power Tune MGB 4 Cylinder Engines" to find out about these units, and Clausager's book "Original MGB" to find out about the cars in general.

Ian F

[Andrew Bishop]

Hi Ian, yes, would be interesting to see what head numbers are actually found on later cars.

Many thanks for your help and advice.

Cheers Andrew

[John Watson]

One may be usefull piece of information is that back in the 1970’s a friend had a very good engine built by a engine builder in Persore to do the Scottish rally.
Good luck
John

[Vic Butler]

The original head on my 77 BGT was replaced years ago with what turned out to be a Downton head (identified by Downton's former chief cylinder head man when his company unleaded it. It was a "back door" one that should have been scrapped but was welded. In 2013 it cracked and I was lucky to get a 12G2795 head from a mate which was given the Downton treatment by the same man and fitted with MGC inlet valves. I don't know what the casting number of the rubber bumper B's head is but if it's any help the part number is 12H 4735

[Peter Cresswell]

I thought B series cylinder heads had the prefix 12H (and A series 12G)? My understanding is that the 2709 casting was originally designed for the Austin/Morris 1800S, and had the larger 1.625in inlet valves. The MGB used this head for the two years the 1800S was in production. According to Peter Burgess (in his book mentioned above) this is the best head for the MGB, as it flows about 15% better than the early 1326 casting and about 8% better than the later 4735/6 casting. However all heads can be modified to the same level eg Stage 2 or 3 or race, but the older the head the more likely it is to crack or be cracked. The 2709 casting is highly sought after by people racing in the 'standard' engine class and Peter Burgess can get one of these to 90bhp at the wheels (the standard MGB engine has around 60bhp at the wheels) with careful blueprinting.

[Vic Butler]

Thanks Peter, must have been thinking of the Cooper S ( the proper one.) I'll have to put it down to brain fade.
I've got the 12H 2709 and the reason for the MGC inlet valves ( a Downton thing I was told) was because one of the inlet valve seats was damaged due to trying to digest a foreign body.
According to my official workshop manual, all Mk 2 1800's with the 18H engines had the 1.625" inlet valves as well as the S version. The latter had the number 20 stamped on the valve
. Vic Butler

[Peter Cresswell]

I believe some Marinas and Sherpa vans used the 2709 head as well.
I think most people on here would find this a very useful document:
http://www.mg-cars.org.uk/imgytr/pdf/enginehist.pdf

Better than counting sheep as well!!!!

[Ian F]

Thanks for the link to the document Peter, a good read I think!

Ian F