16h spark plug cap

What does Lucas recommend?

If it's a WD16H, then there is very little room. The frame tubes are 1" shorter and you have the flat-bottomed fuel tank with just a tiny dome above the plug.

Unlike the BSA M20 which came with the nice screw top KLG caps, the WD16H was always supplied and used simply with the brass tag on the HT lead, retained by the KLG spring clip...

With a smaller modern one-piece plug, you could probably get a rubber Lodge type cap in there, but with a mica KS5, there is no room at all. I have a rubber sleeve over the brass sleeve.

No plug cap on mine. Ignition lead has a crimped ring held on by the screw collar on top of the plug. Some plugs have a dummy collar...