You should be able to pick one up for a few bucks at HD or Lowes. For anything under the water line you want minimal gap around the fitting - don't just rely on the 3M sealant to fill in any extra space from too big of a hole. Well you could but I wouldn't.

For above the water line a little oversized on the hole won't hurt. It's a low pressure system and once you goop it up with sealant, as long as the thru hull has a decent sized outer lip, you'll never know there was a tiny gap after you tighten it down.


Too small of a hole and you're forcing the fitting through or going after it with a file to open it up - too easy to chip the gel.

I may have one or a 1-3/8" you can borrow if you don't find what you're looking for.


Of course I'm a GOOD influence. As far as you know...