The Top-End Engine Rebuild Last month (December PropTalk, page 37) the author detailed his journey of buying his dream used boat, only to discover engine troubles as soon as he got home. In this article he begins the process of repairs.
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he first task was to conduct a simple compression test, which showed that two cylinders on the starboard cylinder bank were down to very low compression of around 20 PSI, while two cylinders on the port side were around 120 PSI, indicating a likely blown and/or warped head gasket respectively. On a typical used Mercruiser 454/7.4L big block GM marine engine, one wants to see compression of around 145-150 PSI on all cylinders. With these results recorded, it was time to begin the onerous task of tearing down the engine.
Head Gaskets
The first step was to remove the exhaust manifolds, risers, and all the accessories bolted to them. After this series of tasks was complete, I unbolted the intake manifold and valve covers. Then I disassembled the valvetrain, including the rocker arms, push rods, guide plates, and flat tappet lifters, being sure to store all nuts, bolts, and fasteners in labeled zip lock bags for a much less painful reassembly. With the exhaust, intake, and valvetrain out of the way, I was able to unbolt the exceptionally heavy and unwieldy cylinder heads from each side of the motor, taking care to place them in designated cardboard boxes resting on the swim platform for safe storage and transit to the machine shop,
By Drew Maglio where the heads would be disassembled, cleaned, and rebuilt with new valve stem seals. I also bagged the rocker arms, push rods, and guide plates for transport to the machine shop for a closer inspection by the machinist. Upon removing the heads, my suspicion of a blown head gasket was confirmed! Head gaskets ‘blow’ for various reasons, including age, but more often than not it is due to an overheat event—and John had insinuated that such an event had occurred to Grape Expectations. In our case, the head gasket blew between cylinders four, six, and eight. The piston in cylinder six, once exposed, was completely silver and devoid of all carbon build up. It was clear that a minuscule amount of water had been seeping into the number six combustion chamber while the engine was under load in gear on the water, essentially ‘washing’ the piston clean. With this revelation, the source of the mysterious popping noise was determined to be caused by the number six cylinder attempting to compress water. Since water does not compress, it was reverberating back through the intake valve, into the intake manifold, and out of the carburetor, causing both a backfire as well as a slight misfire. Nonetheless, once the motor was apart, the rotating assembly was carefully inspected and rotated by hand
and it was determined that the bottom end of the motor had been spared any apparent damage. Therefore, all that was needed was a top-end overhaul. With the condition of the motor now known, the cylinder heads were transported to the machine shop to be overhauled while I pursued other tasks.
Wet Exhaust System
With the heads at the machine shop, I began to browse online marine parts retailers for a new wet exhaust system. On the typical marine petrol engine, the cast iron exhaust manifold and risers are the weakest link in what is a fairly simple and robust propulsion system. The way the system works is that exhaust gases are drawn from the cylinder head’s exhaust port into the exhaust manifold after being compressed out of the combustion chamber by the piston. As one may imagine, these exhaust gases are exceptionally hot and must be cooled before being evacuated from the fiberglass vessel. Therefore, the exhaust manifold contains a water jacket where cool water from the sea cools the gases being exhausted from the engine. This system is known as a marine raw water-cooled exhaust system. Bolted to the top of the exhaust manifold is the exhaust riser, which rises the mixture of exhaust gases and cooling water above the static water line before evacuating the gas and water mixture overboard
##Grape Expectations engine before rebuild.
##Starboard cylinder head removed revealing engine block, pis ton s, and blown head gasket .
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