[count-your-change]

The best fix I’ve found for a Jaguar was to drop a GM V8 into it. Tends to cure those overheating problems too.

[Eric in the Ozarks]

When Jaguar was an independent company, they didn't have the money to chase down better suppliers and most of the time, projects were jobbed out. The DOHC 6 cyl engine was made by Standard (they made Triumph parts) and the transmission was initially a non-synchro Moss box. In 1965, when more power was required, a slightly bigger block was required to get proper bore spacing for 4.2 liters but no money was available to make a new head. So even though it doesn't fit perfectly over the cylinders, the 3.8 head continued on the 4.2 engine. I've seen a dozen or more of these engines and every one shows a crack between the 3rd and 4th cylinder. Fortunately, the engine has chrome liners so this “flex” in the block can be accommodated.