The Protek plugs were fitted when an engine was either new or off overhaul. The sparking plugs not being fitted until the engine was “dressed” ready for installation on the aircraft.

The silica gel crystals mopped up any residual moisture in the air in the cylinder to keep it dry. They were threaded 18mm for the American engines and 14mm for the British ones. We found loads of them on the old Burtonwood airfield dump years ago as they were discarded when the engines were prepared there as it was a massive maintenance base during and after WW2.

Being of spark plug thread size they were used on any engine of the correct thread, not any specific engine type so you would, potentially, be able to find them on any airfield which, at one time or another, re-engined aircraft. As far as I know they were disposable though I would suggest that they could have been re-used if warmed in the appropriate oven to dry the crystals out. But, knowing the way the Yanks operated it was quicker and easier for them to chuck them in the bin.

Anon.