At best the 4.2 litre Jaguar/Daimler engine block could be described as suspect and its quality further declined throughout its production. It was designed in the early 1960s to give more torque and better low to mid range performance than the 3.8 litre engine, which it genuinely did. Early examples (fitted to the last Mark Xs, the E-types and the 420s until about 1968) are easily distinguishable from the XJ6 blocks by two fewer waterways at the rear (watch your cylinder heads), three large core plugs per side and studs screwed directly into the block face. The later engines are usually prefixed 7L (all 4.2 litre engined Jaguars from 1968 to 1975) or 8L (1975 onwards) and these are the ones covered in this article.

The pre-XJ6 block was said to be improved upon by the 7L block, which had a more efficient internal cooling flow. The problem was that the cylinder head studs screwed directly into the base of the block, through the waterways and, if the correct coolant is not used, then they will rot out very quickly. These studs are also twice as long as the pre-1968 blocks, allowing more stretch and therefore more variation in clamping pressure against what has been set with the torque wrench and this makes it more prone to head gasket problems. This improved block cracks, usually hairline cracks appearing in the block, eventually becoming stepped (when one side becomes slightly higher than the other) and the liners drop. The 8L block, a strengthened 7L block, cracks even more than the 7L, and this may be due to an increase in thermostat temperature or to unweathered blocks being used or, possibly, a reduction in standards under British Leyland.