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 but only tooled up after Ford took over. The 5 speed manual transmission for the XJS was designed and 20 prototypes made in about 1973 but the tooling was never approved for production.

[xantia_v6]

This is my 1975 XJ-S, which is 98% original, but rather tatty. It is still running Lucas OPUS ignition and Bosch/Lucas d-jetronic injection. D-jetronic was technically obsolete before the XJ-S was launched, but no replacement had been developed for 12 cylinder engines.

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[Dieselman]

Thanks for the crankshaft bolt info, it makes me feel slightly better...

I do like the first series Xjs.