How do you use a plug gauge in a groove? A ~ 2mm wide groove? Or do you have a different design/application? This confuses me, please explain. I thought a plug gauge was just a precision pin. I'd need something like an inside micrometer with 1.5mm wide probes to measure the groove, if there is such a thing. Pretty sure it would be unaffordable...I usually cut my thread relief larger than required and have pretty much always relied on plug gages to verify fit. YMMV
Die sinker rifflers?
Yes,I meant Riffling but Auto incorrect new better than me so.....What's a ruffling file? I tried to look it up, but found nothing. Found riffling, but not ruffling. Riffling files look like what you meant, I can see how they could be helpful. There seems to be quite a few to choose from! Could go broke assembling a set...