CAT 15- oil pressure guage dis-assembly question

i got a couple of mine apart, so i could touch up paint the black inside by the numbers.... maybe i was just lucky,,, but i believe it was just by working the outer ring clockwise and ccw while maintaining "pressure" outwards. It came apart, it did not break the glass, and mine were not threaded or crimped too much. I would use much caution using a vise, by staying away from the glass front, i used a channel lock pliers to get the rotation going, then kept at it,,, good luck

could be the same on the D4 the Smiths guage face is a brass ring crimped over a seal. I spent some time prying the brass from the rear outwards until it was clear enought to slip over the seal and glass. Re=crimping is easier just use a screwdriver handle or such like and apply pressure to push the brass back into its crimped shape, checking to make sure it is tight, ie to the extent that it cannot spin on the glass/seal.