Good point raised Goober, alluminium will work harden, I should have realised that! Nylon could work I suppose, depends how much the crankcase expands when upto temperature though as the Nylon wouldn’t expand.
I’ll do some research into alloys suitable for the job. I had considered making the hexagonal end of the plug over length though, more like 3/4” so as there is a lot of plug in the socket when removing. I can source hexagonal aluminium quite easily, I’ll speak to the supplier and see which alloy would be best for the job.
We size our ratchets exactly the same funnily enough, it’s quite odd actually as we took on the stupid Metric system years ago! Imperial measurements seem to be creeping back in slowly though, it seems only big engineering companies are sticking to the metric system.