All great info. I have 5 through hulls below the water line. From your descriptions, 4 seem to be relatively straight forward (just tempted fate with that statement), the 5th sounds to be the problem child. It's the water intake for the engine and as toddster points out, it is a 1/2". And as Loren points out, it's probably going to need to be a 90deg because of tight clearance with the cabin sole. So I am thinking BA for the 4 due to the chamfer compatibility. For the 5th, I did notice that there is a 1/2" Forespar with a screen.
Loren - when you say they ground off the outside, this was on a chamfer Forespar through hull (and it was still slightly raised)?
Brad
Raised as I recall, but not really relevant, because this part was ground off smooth. One other wrinkle was that the factory location for the raw water intake on the Olson was under the galley counter and then the hose wandered aft through some holes in the "grid" to the pump on the engine. I had that new intake moved aft and close to the centerline, under the front of the aft berth. Lots more room there.
Old hole was patched with layers of glass and epoxy after edges chamfered.
(Winterizing the engine has been 1000% easier since making that change, also!)
One thing about those external "screens" - they provide a place for trash to clog them and have to be cleared by diving. Not good. :0
Strictly IMHO, but I would much rather have stuff go on into my (added) Groco strainer where I can get at it on the inside of the boat.
And... even one more thing, if you search the thru hull threads here there is one with a great installation of new bronze thru hulls on an E-35-2 belonging to John Moses. Lots of pictures.
As to the old 1/2" thru hull -- just over-bore to the modern standard "small" standard of 3/4".
Regards,
Loren