A different challenge...Not an Xm...but of course, a non-runner.

[Dieselman]

Thanks. I fully understand this was my error, but have noticed a mix of metric and UNC on both engines, (iirc, 1985 and 1989). I didn't expect Jaguar to have gone from British Standard, to American Standard, before switching to Metric.
The Uk went Metric for engineering applications in 1965.

I find it hard to believe Jaguar couldn't afford a few new Metric threading tools. They must have been replacing tooling on a regular basis, just pick a change over date and go for it.
I guess Jaguar were just being stubborn, which unfortunately typified the British car industry and in particular British Leyland.

[xantia_v6]

Now that I think of it, the crankshaft pulley bolt is 7/8 BSF, a hold over from the pre-war design/tooling which was bought from Standard when they ceased production of 6 cylinder engines.

Jaguar always struggled to get any tooling expenditure approved by BL. The 1992 facelift of the XJS was designed in about 1980 iirc. The 5 speed manual transmission for the XJS was designed and 20 prototypes made in about 1973 but the tooling was never approved.