Below is just one of many technical articles featured regularly in the XK Gazette. Many of the past articles are now available online in the Mebers section of this website. In the panel to the right you can see a full listing of the articles available to members.

GUY BROAD UNRAVELS CYLINDER HEAD MYSTERIES

The question of interchangeability arises when a cylinder head can't be overhauled for any reason: corrosion in the water passageways too bad, frost cracks, warping or skimming too far (and many other good or bad reasons I can't think of!). So off we go looking for a replacement, concentrating only on the area we've just been told is the reason our so-called original head can't be rebuilt. But later we can find out that we've overlooked a small but significant problem, the Rev-Counter Drive and Adaptor.

Basically, we can interchange all cylinder heads from any 3.4 or 3.8 litre engine. Inlet Manifolds will interchange, as will exhaust manifolds and front breather housings. As for cam covers and rev-counter adaptors, we need to look a little closer.

CAM COVERS

To start with cam covers, the early XK120 without the three front mounting studs (known as 'studless') can be fitted to the later heads even though the gaskets have three holes. It's still continuous on the outer edge, thereby creating a seal. The main problem with the early cam covers leaking is from over-tightening, warping the covers slightly so not sealing completely. (The same goes for any cam cover). After the "studless" covers, the later 120 through to the earlier 3.4 150 will interchange, the Type "C" Badge being only riveted through the standard cam covers on the XK140 SE cars.

The real problems occur on the late XK150 heads when an electronic generator was fitted to the inlet camshaft. This created a whole new design at the rear of the cylinder heads. XK120/140 Rev. Counters are driven by direct cable from the exhaust camshaft. This causes little problem to change to any later 3.4/3.8 head, and a special blanking plate for the inlet cam can be obtained from 68/69 saloons, or easily made. When all installed, only a keen eye would be able to detect any change. For XK120/140 owners, an immediate power increase can be obtained by fitting a well rebuilt 3.4/3.8 XK150 or MKII head. For XK150 owners, swapping cylinder heads needs far more in-depth knowledge if time and money are to be saved as well as achieving a satisfactory answer.

As already said, all 120/140's are direct drive from the exhaust cam. This means at the REAR of the engine both cams are rotating anti-clockwise, which is not a difficult fact to grasp, but as all 150 rev. counters rotate clockwise you can now see where our problems start. To see why this happens we need to look at production.

CABIN ROOM

To create more cabin room, the bulkhead of the XK150 was moved very close to the engine; in fact, it actually surrounds the rear of the engine. With the lack of room for bulkhead wiring, heater and demist ducting, routing a mechanical cable straight from the engine was not viable without sharp bends and no mechanical cable revolving at speed will work with acute angles. (Early breakage normally follows). So to overcome the problem, an angle drive (gearbox) was fitted to the INLET camshaft drive, sending the cable out at 90° to the engine, so allowing a steady curve to the cable through the bulkhead. The angle drive also converts anti-clockwise rotation to clockwise. So now we can see that earlier mechanical type Rev-Counters are matched to their relative components. (i.e. early 150 head to angle drive adaptor, to angle drive gearbox, to cable, to clockwise rotating Rev-Counter).

As the faces of 120, 140, and 150 Rev-Counters differ so much, any swapping of these is an obvious mistake and will not suit the other instruments. Later 3.4/3.8 150s were introduced with electronic Rev. Counters and new head casting designs accommodated the generator (still driven from the INLET cam shaft). The cam cover for this inlet side of the head was also redesigned for this new instrument and earlier cam covers do not interchange.

We hear very often that the head from, say, a MKII has been fitted on an earlier XK150 and the customer is now looking for the matching electronic XK150 Rev. Counter. As this instrument is unique to the late (1960/61) XK150 and 150 'S' models, it is extremely rare and therefore expensive. In fact, it is easier to find the correct cylinder head (type) than to find the instrument. The earlier type angle drive is not pictured in the factory parts book, so, I have tried to find some diagrams to help with identifying parts if not already fitted to the engine. Also, there follows a list of part numbers to be found on the underside of cylinder heads. These are raised letter castings, NOT stamped by hand later.

HEADS

C2242 & C2242/1   Early Studless Type

C6733 & C6733/1   Standard XK120/140 1952 to 1956

C7707 & C7707/1   Type "C" Special Equipment XK140

C12500   Early 150 3.4 mechanical rev-counter

C12600   Early 150 3.4 mechanical rev-counter (straight port)

C14956   3.4 150 electronic rev-counter (not 'S')

C14958   3.8 150 electronic rev-counter (not 'S')

C14957   3.4/3.8 150 'S' with electronic rev-cournter (straight port)