A customer came into the boat store the other day with a tale of woe. Seems he had been having his boat, equipped with an OMC I/O, serviced by his dealer for many years. To his dismay, he ended up with water in his engine. This trashed his engine, requiring it be replaced at a goodly sum. The cause of his problem was that the exhaust manifolds looked like new from the outside but had rusted on the inside, allowing the water leaks that ended the engine’s life.  

It is a little known fact that it is a good idea to remove the manifolds every three to five years and check the internal structures out, and this hadn’t been done. Therefore, in the hopes of preventing another such casualty, we have "Manifolds 101".

Manifold Theory

Exhaust systems on watercraft differ fundamentally from those installed on their cousins in automobiles. The engine blocks may be the same old GM 350, but the exhaust systems aren’t.

Boats are equipped with water-cooled manifolds and water injection into the exhaust gases. This cools down the exhaust and allows the gases to be conveyed to the outside world through flexible rubber exhaust hoses. The water is pulled off the raw water-cooling circuit and circulated around the exhaust manifold, spacer (if used) and riser.

From there, it is injected into the exhaust gas flow. The resulting mixture of exhaust and cooling water is ejected overboard. This can be done above the water through transom ports or below water through the outdrive propeller.

On V-6 and V-8 engines, the manifolds are usually just the exhaust manifold. On in-line fours and sixes, the manifold usually has both the intake and exhaust manifold contained in a single casting. The manifolds, themselves are usually cast iron, cheap, easy to cast but susceptible to rust and attack by salt water. Less often, some manifolds are aluminum. There are some high-end cast stainless steel ceramic-coated manifolds, if you have the bank balance to be able to acquire them.

Almost every system also includes some sort of riser. This item bolts on the exhaust manifold and allows an engine package to be custom fitted to a particular boat design by providing for different size exhaust hoses and different hose attachment angles. In some installations, a spacer is bolted between a manifold and riser to provide more height for the exhaust outlet from the riser.

Finally, there are a variety of gaskets, mounting kits, drain taps, drain plugs and hose barbs and hoses to further configure the exhaust system to the specific boat and engine combination. The same basic manifolds, risers and spacers can be used to configure warm manifolds, cold manifolds or freshwater-cooled manifold. The differences are in the details.

The need for gaskets between components is obvious; what isn’t obvious is that these gaskets often come in several different varieties. This is especially true of the gaskets between the manifolds, spacers and risers. Depending on the coolant flow, these may completely block the water flow or have several open passages. Take special note as to the type and orientation of these gaskets when you remove the old parts. Make sure that you have the right ones when purchasing your replacement system.

Use only rubber hoses rated as wet exhaust system hoses when you replace hoses. There are both wire reinforced and no-wire hoses available. Replace yours with whatever was originally used. This may take some effort because four-inch wire reinforced exhaust hose is sometimes like an immovable object.

Some exhaust systems use an exhaust bellows to connect the inboard exhaust components to the outdrive. If you have certain model OMC or Volvo I/Os, don’t be surprised if the exhaust bellows has one side of the bellows trimmed away. This was done on purpose to relieve excess backpressure. Some installations may also use a short length of hose, called a hump hose, to connect components together. These are commonly used where there is a great deal of movement between the riser and the rest of the exhaust system.

Often times you have the choice of a gasket or a mounting kit. The mounting kit will contain the gasket and whatever studs, nuts or bolts needed to mount the unit. Different size riser and spacer combinations will require different mounting kits. If you have the kit, you will usually be able to get the nuts or bolts off okay. If you just bought the gasket, chances are you’ll break the stud or round over the nut. It’s just Murphy’s Law.

Some manifolds have hose barbs for water outlet or inlet; make sure you know the size if they have to be replaced. You may also need several pipe plugs for your manifold. These will fit threaded holes in the sides, top or bottom of your manifold. These holes are often required as part of the casting process and are sealed with the pipe plugs. Make sure these are in the manifold box when you buy them. These plugs also make nice inspection ports IF you can get them out after rusting in place for years.

New manifolds are available from several suppliers. OSCO, Sierra, Indmar and Barr are some of the more popular ones. Typical manifold prices range from $140 to over $350 each, depending on the popularity of the engine and hence the availability. Risers run from $120 to over $200, again depending on engine and its popularity.

Manifold Servicing and replacement

As indicated above, it is a good idea to periodically check your manifolds for excessive corrosion or blocking of the coolant passages with salt and rust build up. Regularly flushing the manifolds with fresh water after every use will significantly prolong the life of the cast iron.

Excessively blocked or corroded manifolds should be replaced. After all, you’ve got them off the engine at this point, trying to prolong the agony and getting a few more months or years out of questionable hardware isn’t worth it.

The actual unbolting and replacing of the exhaust system isn’t rocket science. It can be dirty and messy but is certainly within the ability of the average boater. Remember to drain the cooling system before removing the manifolds. The most common problem areas are dealing with rusted and corroded nuts, bolts and studs. Breaking off a stud or bolt in the engine block will ruin your whole day.

Spraying the fasteners with penetrating oil, such as PB-Blaster or Kroil, will help in getting them off. WD-40 is less effective, as it is a water-displacer, not penetrating oil. Start treating the fasteners well in advance of when you are actually going to replace the manifolds.

If you are installing new studs, use a thread lock compound to keep them from backing out of the block. Tighten the nuts and bolts to the recommended torque specifications in your engine’s shop manual. If you don’t have a shop manual, get one! Don’t over-tighten hose barbs or pipe plugs. They just need to be snug and not leak. Over-tightening may crack the cast iron manifold and you will get to start all over again

Make sure whoever you intend on purchasing the replacement parts from actually has all the parts you need in stock and on hand. We get folks coming in the boat store on a Friday night, wanting to pick up a complete exhaust system for some obscure engine they want to work on that Saturday. Call ahead and make sure the parts are there.

We occasionally get folks in the store who are replacing one engine, usually an I/O, with another. If you are putting a different engine in a boat, make sure that the exhaust manifold/riser combination rises high enough to prevent back flow of water into the engine. Likewise, too low exhaust outlets on the transom may require an exhaust flapper to prevent water entering the exhaust system and engine when backing down or in a following sea.

So, keep an eye on your manifolds, a $750 to $1000 do-it-yourself manifold job beats a $15,000 engine replacement anytime.

Capt. Pauley

Capt'n Pauley writes a column about boat repair weekly at LakeExpo.com