Once you have drilled the 3/8" holes you need to counter bore into your cabin side exterior 1/2" deep to accommodate the bolt studs on the trim ring. My cabin sides were 1/2 inch deep so I had to go all the way through which is fine.


The NFM counter bore has a 3/8" guide pin, to match the previous 3/8" holes you drilled, and it bores it out to 5/8". It is well worth the $29.00 and is a very high quality & clean cutting tool.