Hello all. I have been meaning to fit the caps to my S for a while now and now that I have some spare time I am ready to go. They are the type with the brass collar (no grub screws) which is supposed to fit over the filler neck of the tank. Any tips please from any of you that have done this job?
Many thanks. Your help much appreciated.

I fitted a pair of alloy monza caps to my car years ago & they are still on it.
I worked out where the brass "interference fit" ring would sit, then wet & dried the paint off the filler necks using heaps of masking tape to ensure I didn't remove paint from the parts of the necks that would be still visible.
After that I used Loctite super glue in fairly copious amounts on the inside of the brass rings only, so that as I pushed them on, the glue would not spread down the filler necks, but fill up the inside part of the brass ring / neck joint that wouldn't be visible.
I let the glue set for several days before then wiping black Selleys windscreen sealant around the bottom side of the brass rings to seal any irregularities in the filler necks. - The residue wipes off neatly with a turps rag.
Then I screwed on the alloy covers with red loctite 262 & set the release button at the rear of the car so the flap would stay open whilst refuelling & not interfere with the fuel bowser nozzle.
Obviously during the whole procedure you would have a bung of rag in each filler neck.

I just screwed the lid onto the sleeve to index it (I wanted it to face in the right direction when installed) and banged the sleeve on using my trusty dead blow nylon mallet and then screwed the lid back on.

From memory there was a gasket that sat between the top of the filler tube and the sleeve. Anyway it has never (sic) leaked. I made up a small bridge piece so that I could use a 3 arm puller to get it off. It won't shift any other way

Cheers, Ian

Thank you both for the information. Much appreciated. Just one thing, did you heat the brass collar before fitting?

No

Cheers, Ian

That's a "No" from me also..
Cheers from the other Ian!