It’s happening again – my posts just disappear. Don’t let the eighth inch throw you – that’s the INSIDE diameter, the outside is much larger. Because of 80+ years of in and out ware (despite being coated with grease) the hole in the aluminium tends to get bigger, allowing the plug to go in farther. We made our own for racing by using the eighth inch pipe die to thread a brass rod. this was threaded into the gear case and carefully measured. it was then removed, cutoff and slotted. It was then put back in and filed smooth. When it started going in too far, we made another. If you retap the hole for a regular screw or bolt, you will have to machine a flat spot on the gearcase for a gasket to seal the opening.
