‘ If you cut my veins, Jaguar comes out of it, but it comes in lumps of XK engine’
A variety of carburettors and injection systems were used on the XK, but three SUs are the most familiar
‘ If you cut my veins, Jaguar comes out of it, but it comes in lumps of XK engine’
The biggest update to the road-going XK engines came shortly after the arrival of the E-Type. A 4235cc version was introduced with the same 106mm stroke, but wider 92.08mm bores. It wasn’ t simple to achieve the new bigger cylinders, and the bore spacing and sleeve design had to be altered to accommodate the bigger pistons. The two trios of cylinders in the first XKs were replaced with even bore spacing across all six cylinders, and liners were used because the block casting didn’ t allow enough material between the cylinder walls and the coolant passages without them. Although the new 4.2 block’ s bore spacing was changed, the outer dimensions were retained and the heads remained identical, resulting in offset combustion chambers. Such quirks didn’ t hamper the engine, which only improved the E-Type’ s reputation and its speed.
There were visual changes to the XK, too. In 1967, the polished cam covers were replaced by black painted items with machined ribs. It was still a fine-looking engine, but surely a downgrade after the smooth and shiny covers.
The 4.2 wasn’ t limited to the E-Type’ s engine bay; it found its way into the entire Sixties and early-Seventies Jaguar saloons, including the MkX and 420, XJ6 and the Daimler Sovereign. All 4.2s use the straight port head, but as with the smaller versions, different fuel systems were employed for different applications, from two to three SUs, Zenith- Strombergs and Bosch-Lucas L-Jetronic fuel injection. The latter was for US-spec cars and was combined with a catalytic converter.
In the middle of the 4.2’ s life, the head stud design was changed. The updated XK became known as the‘ long stud’ engine.‘ So on 3.4s, 3.8s and early 4.2 XK engines, the head studs got threaded into the top inch-and-a-half of the block,’ explains William.‘ If you really scrutinise it, you’ re putting an awful lot of stress on the top half of your engine block. And there were issues with the actual decks lifting, which would lead to cylinder head gasket failure. More so on 4.2s, which were a great engine, but blighted by cylinder head gasket issues.
‘ Later, the factory went to a long-stud 4.2, where the head studs are basically a foot long. They thread straight into the centre of the block. The core energy and the strength of the block is centralised. The downside to that is they went straight through the main water gallery. In any late XK or E-Type, if you’ ve got original head studs in there, they’ re basically half rotten out. If they snap, then you end up drilling a foot down into the engine block.
‘ Eagle [ a company that makes restomod E-Types ], when they’ re doing a 4.7-litre engine, they only run later 4.2 long stud engines. Because when you’ re raising compression ratios to 10:1 or 11:1, you
24 The Engine Rebuilder Issue 06