I use them. I think they're metric and don't fit US-standard 3" or 4" holes. I'm quite sure that the factory holes in my boat were made for them without a liner ring. I tried to fit Nicro and found I'd have to enlarge the holes to fit. So I stick with Sunforce.

I have a spare in my basement and will measure tonight just to be sure (if my old feeble brain remembers to). Will post a follow up for you.

I think the quality is about the same as Nicro. They work and they don't leak. One negative comment is that it's tough to replace batteries without breaking open the plastic. They're not made to be easily serviced.