Caps are necessary to protect scope lenses and keep them clean.
Flip caps save so much time when seconds matter.

This, size 18 1.7in 43.2mm, fit the eyepiece portion of my Nikon ProStaff Rimfire 3-9 x 40 Black Matte Riflescope (BDC 150) pretty snuggly. It was tight and I'm not sure if I went a size larger whether it would be too loose so I just kept it. Like other reviewers said once you put this on you can't fine tune the focus element on it since you can no longer spin it so get it where you want it first. As for the noise it makes, it's very minimal. I went squirrel hunting with this and I would say it makes less noise than a typical foot step so it wasn't bad at all. If you want to be silent you can use a finger to slow it as it rises up but I didn't feel like I needed to.
One weird thing that happened is when I tried to close it once it felt lose and it wanted to pop open on me but a little pressure around the entire ring and not just one side and it securely closed. I guess one side wasn't closed so it offset the balance so the catch wasn't able to hold it well.
Overall a very nice piece and an absolute improvement over the elastic end caps that come with that scope.

You need the 18 eye piece and the 33 objective cover. They work great, perfect fit. Do not accept anything but Butler Creek. I have used them for many years, they are the best flip up scope covers on the market.

Size 18 is a good fit on the Vortex Diamondback 4-12X40 eyepiece end.

I use them in on my scopes. They hold up well and keep your scope lenses clean.