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All 1800cc heads have the number 18 embossed on the top of the head next to the rear stud (on the spark plug side of it).
On the manifold side of the engine next to the rear stud there may or may not be one of several letters:
If no letter is present, this indicates a Mk1 head, originally fitted up till 1971 (approx) .
All heads with a letter featured a redesigned combustion chamber with a smaller beak between the valves. This beak was to intended improve idle but it also provided a hot spot at the peak of it which
contributed to such combustion irregularities pinking & running on, particularly on lower grades of petrol (which includes 96 octane both the current unleaded and the previous leaded 96)
The letter A indicates the head fitted to 18V engines from 1972-74 (part no 12H2709), both MGB and Marina. It is the "big valve" head, having 1-5/8" inlet valves. Although only 1/16" bigger than the Mk1 inlet valves, the throats under the valves were 1/8" bigger.
The letter O indicates a very similar head to the A head, I haven't yet spotted the difference but I suppose there must be one! As far as I can make out, it was fitted to the Austin/Morris 1800 Mk2; certainly the O head on my car came off such an engine having removed it from one myself in the late 70's.
Both the A head and the O head are the best heads to start with if you want to modify one, as there seems to be more wall thickness around the inlet port to
accommodate the all-important port re-contouring..
After 1974, the smaller inlet valve was reverted to, but the revised chamber shape retained. There were a series of letters here,
I've seen E, G, N, X at least. I haven't ever worked out precisely what the original applications were, probably lost interest because of the smaller valves! The main thing to watch with these heads is that some of them have the oil feed to the rocker gear in a different place - not good for rocker shaft life if undetected! The original heads had the feed hole directly in line between the head & rocker studs in the rear
pedestal whereas some of these head had the hole offset forward by about 1/2". I suppose Leyland had a good reason for doing it, but
I've not yet worked it out!
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