Thru transom exhaust flange questions... with pics

TBarCYa

Senior Chief Petty Officer
Joined
Apr 13, 2005
Messages
781
The port side exhaust flange was leaking so now that it's out of the water, I started digging into it and found some interesting stuff.

First off, the port side flange is significantly smaller diameter than the starboard side. 3 1/2" on one and 3 1/8" OD on the other. The exhaust pipe is the same side on both and on the port side, it's a 3.5" ID rubber hose that slips over the exhaust pipe and was just clamped to the 3 1/8" flange. That to me seems to be a potential source of the leak.

Here's where it gets ever weirder. The hole in the transom is only 3 1/8" so it seems like the origianl flange size was 3 1/8" but the starboard side was changed to 3 1/2".

So my plan is to try to find a 3 1/2" OD flange for the port side, cut out the transom to the right size and replace the rubber hose. The problem is that everywhere I call tells me that they're either 3" or 4" and that 3 1/2" is an odd size so first question... Is 3.5" an odd size? Am I better off trying to find a 3.5-3.125" adapter and using the flange I have? It would be less work, but I'm pretty sure the exhaust should be the same size on both sides.
 

smokeonthewater

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Dec 3, 2009
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9,838
to plug the thru hulls and route the exhaust through the prop if it was an option.it doesn't much matter from a functionality standpoint but it would annoy me that they didn't match...

IF it were my boat I'd run whatever size hose fits the mufflers (it DOES have mufflers doesn't it) to 2 matching thru hulls that fit snugly inside the hose.


EDIT:
wait, I lied.... If it were my boat the above would be my second choice... my first choice would be to cap off the thru hulls and run the exhaust through the prop if it was an option.
 
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bobdec

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Aug 12, 2010
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I'm going to assume you talking about your '71 Concord , single inboard ? ... A 3-1/2" inch flange is used for a '4" wet exhaust hose' that normally has a 3-1/2" ID. That 3-1/8 OD flange on your port side was made for a 3-5/8 exhaust hose. The 3-1/2 OD flange is more common and available, the 3-1/8 is more of an odd ball available via custom order. search on " Transom Exhaust Fittings " take a look at go2 marine sight for some sample prices. I looked at Iboats first and they did not have that size.. .
 
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TBarCYa

Senior Chief Petty Officer
Joined
Apr 13, 2005
Messages
781
Ok, so yes, it does have mufflers and being an inboard, there's no way to route the exhaust thru the prop.

When looking online, if I see a 4" flange, that should be the 3.5" OD that I'm looking for? Kinda sucks because I just ordered a pair of the only 3.5" flanges I could find online. At least they're cheap enough that if they're wrong, it's not a huge loss if I can't send them back. I'm not thrilled that they're both plastic and flush to the transom so maybe it's not terrible if they're the wrong size.
 

Bondo

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Apr 17, 2002
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70,468
Ok, so yes, it does have mufflers and being an inboard, there's no way to route the exhaust thru the prop.

When looking online, if I see a 4" flange, that should be the 3.5" OD that I'm looking for? Kinda sucks because I just ordered a pair of the only 3.5" flanges I could find online. At least they're cheap enough that if they're wrong, it's not a huge loss if I can't send them back. I'm not thrilled that they're both plastic and flush to the transom so maybe it's not terrible if they're the wrong size.

Ayuh,.... To simplify things, use the same diameter outlets, as whatever's up-stream, I'm guessin' the mufflers,....
Then straight wall rubber exhaust hose can connect 'em,....
 
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