Log exhaust manifolds, the most common exhaust system found on jet boats and v-drives. These exhaust systems aren't known for producing any big dyno numbers, but their simple design is very effective and functional. The rugged construction of these manifolds generally produces a long life with trouble free usage. Regular maintenance of the riser gaskets should be performed to keep the manifolds from leaking and potentially causing engine troubles.
We ran across this set of manifolds on a boat that came into BJ's Marine for some pump work. To say they have been neglected is an understatement. These manifolds are an example of a worse case scenario. Once the manifolds get this bad, there's no easy answer. Fixing them properly is going to be tough. Patience is key, this set almost saw the recycle bin a couple of times, but at $1200.00 for a new set, repairing them was our only option.
We start by removing the manifolds and risers from the engine. Remove all water hose connections, mounting bolts, and loosen the exhaust hose clamps. With the manifolds on the work bench, you get a nice close view of the corrosion. This set has been used in salt water, not heavily, but they have more corrosion than normal. These manifolds were assembled with steel studs between the manifolds and risers. In an effort to save a couple of dollars, the decision to use steel studs instead of stainless steel will certainly add to the fun.
Steel studs corrode inside the aluminum and swell, so getting them off is a fight. I'm very nervous working with these studs, they can snap off very easily. I take every effort to not break any studs. I applied heat to the nuts right out of the gate.
Once heated, I turn off the torch and apply WD-40 to the studs.
With the nuts and washers removed, it's time to separate the risers from the manifolds. Because the manifolds are water jacketed, the manifold flange can easily be damaged or crushed in. This set of manifolds is equipped with (2) riser gaskets and a stainless steel block plate. These are stacked in between the manifolds and risers. Not all log systems are equipped with this type of gasket arrangement.
I use a rigid putty knife to start. This particular putty knife didn't make it through the repair if that gives you any idea of how long I had to bang on these manifolds. Eventually I got enough of a gap between the manifold and riser to use my aluminum wedges. With 4 of these wedges I can equally drive the riser off of the manifold. It took roughly 2 hours to get to this point.
With the manifolds and risers separated, you can see how rough these manifolds were.
In order to re-surface the manifolds, the riser studs must be removed. Don't expect these studs to come peacefully. I don't attempt to remove them without getting the torch out again. I put the manifold in the soft jaws, heat the manifold around the stud area and use vice grips to remove the studs.
This set of manifolds suffered 2 broken studs during this process. I had to drill and tap the broken studs out.
With the riser studs removed, I re-surface the manifold and riser flange. I chase the riser holes with a 5/16' drill bit and use my deburring knife to clean up the edges.
I also re-tap all of the threaded holes in the manifold (5/16-18).
When reassembling the manifolds we'll be using high quality riser gaskets, new stainless steel plates, stainless steel fasteners, and we've chosen a copper SCE exhaust manifold gasket.
Log manifolds are known for blowing exhaust manifold gaskets. The manifold mounting flange that bolts to the head moves around quite a bit with the heat from the exhaust. It's not uncommon for these flanges to warp. Rather than re-surface that section of the manifold, I chose to spend a few extra bucks on some good gaskets. The SCE gaskets have individual port compression rings to absorb surface differences as needed. Re-surfacing the manifold flanges will only thin the flange, making the potential warping problem worse.
Now that everything is cleaned and tapped, we're ready for assembly. Before you install the riser studs, apply RTV silicone inside the threaded holes and on the threads of the studs. Some of these threaded holes go into water jackets and we don't want leakage from anywhere. I use black RTV silicone to outline the water jackets on the manifold flange and in between the gasket surfaces
When re-assembling the manifolds and risers, be sure to get the clocking of the riser in the correct location. Before dis-assembly, you can mark the two surfaces so when reassembling no mistakes are made.
I let the manifolds sit overnight to allow for the silicone to dry. I pressure test the system before we install them on the engine. I simply plug off one of the water line connections and connect my leakdown tester to the other. I run 15 lbs of air pressure into the manifold. Both manifolds check out perfect and are clear for installation.
We re-install the manifolds on the engine with our new SCE copper exhaust gaskets. A quick double check on all the hose connections and we test run and inspect for any leaks. Although this was far from my favorite job to do, this set of manifolds has been salvaged and will provide my customer with many more years of reliable service.
Written by Bob Jeanblanc - BJ's Marine
Marine Industires West LLC
1260 N Redgum St
Anaheim Ca 92806