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Embarrassing admission:
Ran into an issue I have not ever experienced before. Had bad missing occurring in the engine and got her running long enough to get the hood open. There was blow-by coming out and around of a number of sparkplugs! I had not tightened the plugs in tight enough!
With the blow-by carbon also reduced the ground and fouled the plug further. I took them all out, cleaned them up, then replaced them doing the final tightening with a torque wrench. All good now.
As an experiment I tightened them to what I felt was 30 ft/lbs on the socket wrench (25-35 ft/lb spec range on a 14mm plug) and found that I was actually around 20 ft/lbs. I expect the metal ring gasket is what fooled me, it took a few arcs of the torque wrench to crush that seal down and start gaining torque. I am always afraid of over-tightening as you can change the gap and worse, break the porcelain (I think the lower torque let them loosen themselves out after a bit of driving, as there was no issue upon first driving the fresh re-build.)
Just thought I would share to help those also not as used to these style plugs.
Bob J.

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The area of the head around the threads where the plug gaskets seat needs to be clean metal. You don't want to completely crush the gaskets, but the plugs do need to be tightened firmly. Having the seating area clean will assure proper heat transfer from the plug to the head and should eliminate any leaks.
Over-tightening the plugs is to be avoided. The tiny plugs (10 MM?)used on the pre-war cars need hand tightening with a short wrench only.

Perhaps you'll find this useful, from a vintage AC spark plug catalog.

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Printed and up on the shop wall! Thanks!
Found some more information too in my brother's trade school book under 'Installing Plugs':
"If a torque wrench is not available, run plugs up until the gasket is just contacted (new gasket). Give the plug 1/2 to 3/4 turn beyond this point"
Have fun!
Bob J.

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That's more or less what I've always done and, of course, it is necessary to use a new gasket.