So, "what happened to 'B-type'?" I hear you ask.  Well, today this is the term given to all non-straight port heads, whether it be XK120, 140, Mk VII, VIII, IX, Mk 1 or XK150.  All these versions are interchangeable with a two-carb set-up, the inlet manifold and carbs will bolt from an XK150 to an XK120 and vice-versa if necessary, the stud positions and inlet ports all match up to each other, as do the combustion chambers, head mounting studs and gaskets.

So, if you hear the term 'B-type head', it is a quick reference to the interchangeability of the inlet manifolds, not necessarily to all the other small differences.



In 1958 the cylinder head took another re-design for the fitting of three 2 in (50mm) SU carburettors, to increase power to a possible 250bhp.  This first appeared on a production 150 'S' of 3.4 litres in 1959.  The 'S' now denoted this significant change of specification, although the standard two-carb version remained as an option.  This type of cylinder head we call the 'Straight Port'.  This is where, along with other smaller changes, you cannot interchange the inlet manifold to the B-type style: the water passageways and stud mounting positions do not line up.  By definition, the inlet ports are straighter on their approach to the valves.  I hope the pictures clarify what I am trying to say.



Strangely, the straight port head and non-straight port head carried on together in production for many years until the last 3.8 engine used in a 1967 S-type saloon. And, for basic information, ALL 4.2 litre engines from the first to the last, are also a straight port configuration. Possibly the most amazing fact is that, with two small water holes plugged up at the very rear of the head, a 1986 XJ6 4.2 cylinder head will fit on to a 1949 XK120 block and work! I wonder if that is a unique Jaguar fact, or could any other car manufacturer claim the same? The reason this all comes about in the first place is the often­asked question about fitting three carburettors to a car that had only two originally fitted. Jaguar did experimental manifolds to fit to the B­type head for three 1 ¾ in SUs, but this never made it into production. There have been one or two aftermarket manifolds for the three SU conversion and the best-known was done in the early '60s by one of the best semi-privateer racers, Warren Pearce. He developed and cast in alloy his own version for his very successful Mk 1 and Mk 2 race cars. We have re-made a new version of this style of manifold, which is available today.



So, a basic rule to work by is a 3.4 or 3.8-litre car with three 2 in SUs will be a straight port, as are all 4.2-litre (with two or three SUs), while all 3.4 and 3.8-litre cars with twin SUs will be of the B-type, non straight port style. There is always an exception to the rule, that being the last Mk 2 variants in 1967/68, known as the 240 and 340: these Mk2s had straight port heads with two SU carbs not three. Then there is of course the infamous 'wide-angle head', used on works racing engines in the later '50s and early '60s - but that is a whole other story!

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