need some advice.

I'm working on a conversion engine (25 cc sthil) w/ a 16x8 prop. I don't have any local resource to help me find the right size spinner. I assume that someone here would know.

Thanks in advance.

just for kicks there is a pic of the engine as it stands now.

What plane?
You need to size your spinner to the plane you will be using.

this isn't going on a airplaine. it will be going on an airboat (swamp buggy)

dimentions of the boat are 40" long by 18" wide

will a 3 1/2" do or will i need to go bigger?

Okay then 3 1/2" will be fine.

I'm using a 2 1/4" Tru- turn standard with a 18/6 Mejzlik on one of my planes. Works great. You have to dremel the prop slots to fit but no biggie. oops you said 16/8, It still would work. I'd go for the smallest I could get away with to save weight.