I have always understood that these are set once and for all, provided it´s done very carefully.
So can I just forget about this topic on all my 50s and 60s Jaguars? (provided the engines run without any particular problems)
Things wear, and valve clearances can increase or decrease (usually the latter due to wear on the valve and wear (recession) of the valve seat). Decreased (or no) clearance doesn’t allow adequate time for the valve to cool whilst in contact with its seat, which leads to burned valves. Increased clearance (and consequent noise) is not as serious but can still cause problems. IMHO.
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Thanks, and I understand your points.
But on a car that only covers 1000 miles per year, I ought to be safe for the rest of my life…
I would recheck clearances every 2 years.
Why? They‘d shrink with seat wear?
Rechecking is easy though, you‘d have to remove the cam covers when they leak in any case. Probably turns out to be every two years.
Valve/seat recession, in general, yes.
On a rarely driven engine, properly set up, biannually is plenty.
Are the shims typically sold in thise boxes with many small compartments, so that I get a couple of all the various sizes, with a letter on each?
Or do I order the ones that I calculate are needed?
Yes. But, due the vagaries of the whole process, offtimes you’ll calculate you’ll need an 0.080” pad, put it together, spin and measure, and find you really needed a 0.079” one.
Patience.
Often? Always! We had to redo about three and we measured thrice. Don’t the clubs at least in the US have a box you can ‚lend‘? Of course what makes sense is that you get a relatively full box and stock up after removing the ~10 you actually needed, then sell. If you have all winter you can order until they fit, but that’s a little unrewarding!
One time, way back–ONE TIME–I was helping Dad, setting up an XK head, so we tossed in 6 pads, on a side, and thought that should be “about right.”
Turn cam over a few times…and bugger me, if 5 weren’t dead-nuts on, and one needed adjusting!!
Lather, rinse, repeat, on the other side…and all SIX were in spec!
Dad and I bowed towards the east, lit candles, said mantras…and that NEVER happened again.
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did you mean to say twice in a decade? or even a “score”?
Nick
Biennially (thanks, RW!!)… every two years.
Or biennially if you prefer. 
(Biannually is twice a year; but my favourite is fortnightly.)
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Oh gosh… thanks, Robert; Paul’sheimer’s strikes again!!!