Setting theupper timing chain tension

How does one gauge how much tension is correct for the upper timing chain on my 52 XK 120?

You could follow the instructions in the service manual for setting chain tension…in the section for setting valve timing…Steve

Page B41, slight flex, not dead tight.
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Don’t force anything and c h e c k that no length of the chain ends up excessively loose. It should not be tightened with force. It’s nothing too exact. Too loose means noise, too much means bearing wear, and you will have to check the cam timing after tensioning.

Thank you for the comments. I installed a cometic gasket which consists of 3 or 4 layers of standard gasket. That resulted in increased tension and very quickly broke my upper timing chain. Your advice is logical and more specific than the thoughts I had on the matter… thanks again, Happy New Year!!!

You broke your chain?!
And you have to remove the head to change the gasket so… how…?

I ran with a stacked metal cometic gasket for years on a brand new chain. Was your chain somehow tight as a drum head,.

I remember that Big Jim had big problems fitting a laminated gasket as he just could not get the timing chain on. He switched to a stock metal one and the chain fitted OK. I managed to find a NOS genuine Renold chain, not a repro, which fitted just fine with the metal gasket. I don’t think I’d have got it on with a thicker gasket.
I suppose it depends on how much has been skimmed from the block/head. If the car is still standard, as mine was, the chain may be too tight. Most heads have been skimmed at least once by now so the modern joint fits OK.