Internal exhaust flappers?
 

Are these the ones I want to use or is something better? 3 1/2 in

There was just a similar thread recently. Personally I don't like them and you'd think they'd ulter/restrict exhaust flow some. I can't say for sure however I just don't like anything in my exhaust. Ext flappers way to go unless you have under 112 lsa.

I've seen a lot of 4in stuff but not much on 3 1/2.

For externals I like these from Harden but I can't find them in 3 1/2 inch

Just did a search myself and appears your correct. Those newer internal ones with the stainless / rubber evidently hold up much better the older ones. Not a lot of options.

IMCO make a 3 1/2" internal flapper. There are not a lot of options, if any for clamp on ones.

I have these on my boat and I love them. Way better then the rubber ones

[IMG]http://i1149.photobucket.com/albums/...psvbj77ds7.jpg[/IMG]

Buddy shattered a piston and punched a hole in the cylinder running these rubber internal flappers. After a bit of use they do not want to lay back flat. Stainless butterfly with the silicon boots for internal. Just need to peak up the tips occasionally as even those can toss a flapper without you knowing it.

Did the same on our past 2 boats
[ATTACH=CONFIG]567300[/ATTACH][ATTACH=CONFIG]567301[/ATTACH]

Why don't you take the tips off the boat and have a local custom exhaust fabricator swage the ends to 4"? That way you will have options.