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Wet Manifold

The plan is replace the Ford 7.3's current cast iron exhaust manifolds with a water cooled aluminum manifolds. (1) Marine exhaust manifolds like the one in to photo on a Detroit diesel are generally boxes built around the exhaust pipes coming directly off the cylinder head. They significantly reduce the exhaust temperature, the engine compartment temperature and decrease the engine warm up time. (2) Marine suppliers like Glen-L carry wet manifolds for Chevy and Ford engines commonly converted for used in jet boats. Among the suppliers listed below I found MesaMarine that could custom build the manifolds but I am too afraid to even ask what that would cost.  Manifold Warehouse had them, but they cost about $2000 for the pair.

(1) Wet manifold or Wet exhaust
with a heat exchanger on a
Detroit diesel.

(2) Wet manifolds
available from








Manifold Suppliers:,,,




(1) Cut-away of a lost foam cast
aluminum wet manifold.


(2) Lost foam manifolds with
pour spouts and vents before
encasing them in mud.

How NOT to Build a Wet Manifold

I started by doing some testing with lost wax casting and then switched to lost foam. (1) (2) Lost foam is a process where you first create the part you want in styrofoam.  You then encase the foam a shell of mud that is baked into a hard ceramic like crust.  Finally you pour in molten aluminum that burns out the foam and fills the void. Let it cool, knock off the shell of mud, clean it up, and you are done.  The process failed miserably for me, but it was a great education. You can read more about it here: "Lost Foam Casting". 


(1) Bending aluminum pipe needs
lots of heat to prevent tearing.

(2) Exhaust with face plates and
adapters for the check valves.

(3) Casting an adapter from the
manifold to the check valve using
aluminum and the check valve as
a mold.

(4) Pressure testing. Face plates
are bolted together and a pump
attached to apply 40 psi.

How to Build a Wet Manifold

The idea now is to bend, cut, and weld a wet manifold together. Harbor Freight put their pipe benders on sale again so I now have a 12 ton pipe bender and I made a couple of practice bends.  (1) Preheating the pipe is the trick to bending aluminum, otherwise it tears open once you reach greater than a 20 degree turn.  The pipe in lower part of the photo tore open when forced to bend without sufficient heat. Too hot is and the pipe melts without much warning because unlike steel, aluminum does not turn red before it melts.

(2) Each manifold is built from 1 3/4 inch pipes welded to a 1/2 plate that bolts to the exhaust head. The pipes immediately direct the flow downward.  About six inches from the head the four pipes merge into two pipes, and these continue down in parallel then turn 90 degrees toward the rear where they will join together.  I tried to keep the turns gentle and maintain as much volume inside the pipe as possible in order to reduce the drag and pressure.

At the bottom end of the exhaust, the pipes connect into a cast aluminum adapter with pipe tread for a 3 inch brass check valve.  (3) To cast the adapter, a simple aluminum mold was fashioned from scrap aluminum and the check valve.  A plate on the end of a short piece of pipe was inserted into the check and this kept the passage open and only allowed the aluminum to cast the threads of the check valve.  Coated the part and the valve threads with a layer of soot allowed the pipe to be removed and the cast part unthreaded from the check valve.

The check valve will prevent water from flowing back up into the exhaust when the boat is submerged.  Sea water flooding in through the exhaust ports is something to be avoided and the check valves are an added level of safety to prevent this from happening. If everything works perfectly they are not needed. The engine compartment is compensated with ambient air, but there is no way for the ambient pressure easily reach the exhaust side of the engine because the exhaust side is closed off the the valves and piston rings so the check valves ensure that water will not rise up through the exhaust. To keep the check valves cool, the raw water supply from the jet pump will be piped in and injected into the check valves so that it is directed onto the valve gate.  The raw water will also further cool the exhaust before it passes through a flexible exhaust pipe that connects the check valve to a solid aluminum exhaust pipe with exits the transom below the water line. 

(4) Ken "Welderman" who is one of my welding and casting resources had been following my work so I sent him the photo of my manifold and he responded that it looked like I had pin holes and that I should pressure test it.  I thought it looked pretty good, but I sealed up the manifold, connected it to a 12 volt tire pump, brushed on some soapy water, and...  Have you ever seen one of those bubble machines?  Some of the holes were easy to fix but others were persistent.  I could grind the weld out, fill it back and the leak would be right back.  I burned through and patched a couple of spots so I was sometimes working backwards.  After 16 hours of trail and error I finally got all of the holes patched so that the pump reached 40 psi before too much air was leading out the temporary seal between the face plated.  The best approach was to lay down parallel welds, and use my stainless steel brush to remove the soot between each pass.  The pin holes most often were at the end of the welds, so I started running a final weld across all of the ends, or hooking the end of the weld so it crossed over the previous weld.






(5) Bolt passages added.

(6) Bolt holes widened in the
face plate so the bolt passages
could be welded completely to
the face. The holes were
completely filled and the drilled
out again for the bolt.

(7) One side of the water box is
added along with baffles to direct
the water flow.

(8) Final side of the water box
is welded on.

(9) Surfacing the face plate.

(10) Test fitting the completed
wet manifolds and deciding on
plumbing to connect it to the
water pump.


(11) Piping to connect the
wet manifolds to the coolant

(12) Photo taken at final
installation shows the exhaust
hose joining the check valve to
the exhaust pipe and the raw
water line going to the valve.

(5) The next step was to weld on bolt passages.  These are make from 1 inch rod and drilled with a 7/16 hole that will allow a 3/8 inch bolt to pass through giving the manifold a little bit of room for thermal expansion.  Each passage was positioned and then spot welded onto the 1/2 face plate. (6) There was no way to maneuver around the exhaust pipes so 7/8 inches holes were drilled thought the face plate to exposed the bottom end of each passage way.  The union of the passage way and the face plate were then welded from within the hole, and the hole was filled with a couple of additional passes.  Finally the 7/16 hole for the bolt was drilled out again.

(7) (8) The exhaust pipes next are enclosed by flat sheets in order to form a water box. Engine coolant water will flow into each manifold at the top front and then be directed past each of the exhaust ports before flowing down the manifold where it will be connected with a flexible coolant hose the heat exchanger build into the hull. The flexible coolant hose and flexible exhaust hose will allow the engine to move due to vibration and tork without damaging the manifold.

With the water box completed I pressure tested the box and once again worked hours to fix the pin holes. Wanting a better solution, I turned to the magic of chemistry and ordered a gallon of CPS-128 Seal All Polymer Sealer from The directions for use are: "Just pour Seal-All into the water jacket. Wait one hour, or apply 10 PSI for 1 minute to penetrate the leak. Pour excess out and store in the original bottle to reuse. Cure for 1 hour at 200F or 8 hours at 72F." It was just that easy too. I will use Seal-All in all of the parts that will hold pressure as well as the speed up gear box.

(9) All of the pipe welds to the 1/2 inch face plate are all on one side of the plate and unfortunately is was enough to cause the plate to bow about 1/8 of an inch. So with the welding completed it was time the resurface it. I fashioned a jig build from 2 inch square tubing and used with with the router to remove bow.

(10) With the manifolds fitted into place it is time to decide how to best pipe the coolant from the water pump to each manifold, but first the rebuilt fuel pump and 8 new injectors were installed, because the coolant pipes will pass very close to the fuel pump.

(11) With the fuel pump installed, a reservoir box was fashioned to hold coolant, provide for expansion, and also serve as a "T" junction for the flow from water pump directly beneath it. A fill spout was welded to the reservoir and short sections of 1 5/8 inch water hose joins the reservoir to each of the manifolds through 1 1/4 inch pipes that were welded to the manifolds.

More chemistry magic is required to seal and tighten the cast treads on the adapter to the check valve. For this joint I used Duralco 4703 a high temperature epoxy from recommended for wet environments.  After testing, the only problem was the weld between the aluminum coolant reservoir and the brass filler neck. Even when using "Super Alloy 5" which is a flux rod material designed for welding dissimilar metals.  "Welderman" told me this stuff was worthless and now I believe him.  I do not recommend "Super Alloy 5".  Instead I purchased a $17 cast aluminum filler neck and welded it on. I also only used a piece of 3/6 inch aluminum for the flag plate that bolts onto the engine block. There are only two mounting bolts and the 3/6 aluminum was too thin to keep it from warping so I had a slow leak past the seal. I replaced the flange with a 5/8 thick hunk of aluminum (3/8 would have likely done the trick) and the problem was solved.

(12) Once the engine was installed for it final fit, a 4" flexable wet exhaust hose was installed to join the check valve to the exhaust pipe and 1/2" hose delivers raw water from the jet pump to the check valve.