Stiff petrol cap

Smidge of grease? Vaseline, axel, silicone etc.

I have found in the past just *thoroughly* cleaning both cap and filler thread solved the problem. With a locking cap that included cleaning the lock mechanism. Petrol seemed to do the job and seemed to me a safe bet :)

vaseline

Won't that absorb petroleum vapours and go runny? Also will tend to attract dirt.

Personally I would go for trying to give it a good clean, then spray with silicone spray although perhaps you should stick a cloth down the filler tube first, as you don't want that in the fuel.

Silicone spray furniture polish would probably be just as good as the "plumbing" stuff.

Or WD40 or similar, if you don't have silicone to hand.

That was my thought as well.

You only need the meerest smear of the stuff not half a jar!

Yes, it is "sticky".

Or spray a tiny amount on the cloth and wipe the thread with the cloth. As one would do for plastic curtain track.

The can of Mr Sheen we have only gets used for the dry lubrication of plastics, like curtain tracks.

That'll go even gunkier than vaseline.

Graphite. Rub a soft pencil over the thread.

Yes if you flood it, but a quick spray and a wipe will be OK. I carry a tiny can of WD40 in my main toolbox for when ICBA to locate the silicone.

Yes I agree but only after the threads are cleaned.

ss wrote in news:Nf54y.407109$% snipped-for-privacy@fx36.am:

I am going to try an alternative approach - to lubricate the rubber seal at the joint face between the seal and the cap. It could be friction there that is making it stiff.