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I have a compound shutter which is generally in working order. However, the piston is not doing its job very well, because the caps to the piston body leak. In fact, one of them must have been cross-threaded at one stage because it won't wind all the way to the bottom. So... Do I just insert some gasket material in to the cap and close as tight as possible? If so, what should I use that's air tight? I don't really want to use glue in the threads as that's a little too final. Or: does anyone have spare caps or know a source of such?
Thank you,
Charles

Bondloc's Threadlock and Seal would seem the ideal candidate as it's designed for ill-fitting bolt threads. There are several strengths but I tend to use the general one that B&Q stock. There are specialist tools for 'cleaning' threads but you're unlikely to find the right one these days, I fear. A fine jewellers' screwdriver might help straighten the threads on the cap.

Thanks Sandeha.

A fine screwdriver is a must with these shutters, I have found. And I am not talking Made in China, £1 for a set either.

I have made some progress by myself, by purchasing some o-rings. Even when I sorted the threads out so the cap could screw all the way to the bottom, the speeds were too fast. Interestingly, when I cap both ends of the pistons with my fingers for a very tight seal, the shutter times are too slow. Put the o-rings in without crushing them, and they seem pretty well spot on. Designed to be leaky. I find it amazing that compound shutters rely on a leaky cylinder: not only does the air have to pass by the piston proper, but some air has to be let in so the piston is not working against a vacuum.

So I think that sealing the threads would be counter-productive. It's a good product to know about otherwise and I might just get some for other problems.

Does anybody who invented these shutters?

Thank you.
Charles