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Re: Engine repairable? - Mercruiser 470 Blowing oil out vent - should I donate
Nobody cares who made the motor. The 470 gets knocked because of a very basic design flaw. Mercury wanted a cheap motor so they could make more money. To keep costs down Merc went with an "open deck" block which allows very simple and cheap tooling for casting. You can cast it with re-usable hard tooling as opposed to the one-use sand molds required by a "closed deck" engine. The hard tooling also allows much closer casting tolerances which minimizes machining costs.
Ever wondered why there's no "freeze plugs" (really casting plugs) on a 470? The "freeze plugs" on a closed deck block are there to get the sand mold out of the water jacket after the block is cast. On the 470, the water jacket mold just slides out the top of the block to be re-used.
Unfortunately, while saving lots of money on tooling and manufacturing, it leaves the top of the cylinders unsupported.
Merc 470 block from Mercruiser Manual #8. Notice how the top of the cylinders are NOT connected to the block. The head gasket seals on the thin ring at the top of each cylinder.
Pic of a 5.7L V8 block. You can see how the top of the cylinders are fully supported by being an integral part of the block casting. Every marine engine EXCEPT the 470 is built like this with sand-cast tooling and fully supported cylinders.
Combine the 470's open deck design with an aluminum block and an iron head and it makes it even worse. CTE (coefficient of thermal expansion) of aluminum is about 12, cast iron is less than 6. This means that the aluminum block expands twice as fast as the iron head. They move around in relation to each other.
Throw typical boat use and maintenance into the discussion and you can understand why they quit making 470's 20 years ago. Typical boat owners run through mud, grass, etc with the intake clogged until the overheat buzzer goes off. Typical boat owner maintenance is to replace the impellor when the engine overheats.
The 470's design is EXTREMELY intolerant of overheating. The unsupported aluminum cylinders move around in relation to the iron head and blow the head gasket. Once this happens, many owners only know that "it's not running right" and will proceed to keep using the boat or drive it back to the dock. The engine will be totally trashed. While a severe overheat on a closed-deck block will frequently warp a cylinder head, the same thing on a 470 will warp the block because of the open-deck, unsupported cylinders. While it's usually cost-effective to surface a warped cylinder head to get it back into spec, it doesn't work out that way when the block is what warps.
Honda uses open deck blocks on many of their popular engines for the same production reasons. It's cheap. They also use an aluminum head with the aluminum block so everything expands at the same rate. Works great for most people. However, if you want to put a sustained load on these motors (kind of like a boat motor), the first thing hot rodders do is to weld a "deck plate" onto these engines which ties the top of the cylinders to the block. Google "Honda deck plate" sometimes.
From ERL Performance:
If you're happy with your 470 and willing to make sure you never overheat it, it'll work great for you. Of course, you'll also have the alternator and cam seal problems to worry about but those are pretty easy to remedy although they are not cheap if you have to pay someone to do it for you.
To sum it up, under typical consumer use, the 470 was a disaster for Mercury. It can't stand up to the way the majority of the people maintain and use their boats.
Nobody cares who made the motor. The 470 gets knocked because of a very basic design flaw. Mercury wanted a cheap motor so they could make more money. To keep costs down Merc went with an "open deck" block which allows very simple and cheap tooling for casting. You can cast it with re-usable hard tooling as opposed to the one-use sand molds required by a "closed deck" engine. The hard tooling also allows much closer casting tolerances which minimizes machining costs.
Ever wondered why there's no "freeze plugs" (really casting plugs) on a 470? The "freeze plugs" on a closed deck block are there to get the sand mold out of the water jacket after the block is cast. On the 470, the water jacket mold just slides out the top of the block to be re-used.
Unfortunately, while saving lots of money on tooling and manufacturing, it leaves the top of the cylinders unsupported.
Merc 470 block from Mercruiser Manual #8. Notice how the top of the cylinders are NOT connected to the block. The head gasket seals on the thin ring at the top of each cylinder.
Pic of a 5.7L V8 block. You can see how the top of the cylinders are fully supported by being an integral part of the block casting. Every marine engine EXCEPT the 470 is built like this with sand-cast tooling and fully supported cylinders.
Combine the 470's open deck design with an aluminum block and an iron head and it makes it even worse. CTE (coefficient of thermal expansion) of aluminum is about 12, cast iron is less than 6. This means that the aluminum block expands twice as fast as the iron head. They move around in relation to each other.
Throw typical boat use and maintenance into the discussion and you can understand why they quit making 470's 20 years ago. Typical boat owners run through mud, grass, etc with the intake clogged until the overheat buzzer goes off. Typical boat owner maintenance is to replace the impellor when the engine overheats.
The 470's design is EXTREMELY intolerant of overheating. The unsupported aluminum cylinders move around in relation to the iron head and blow the head gasket. Once this happens, many owners only know that "it's not running right" and will proceed to keep using the boat or drive it back to the dock. The engine will be totally trashed. While a severe overheat on a closed-deck block will frequently warp a cylinder head, the same thing on a 470 will warp the block because of the open-deck, unsupported cylinders. While it's usually cost-effective to surface a warped cylinder head to get it back into spec, it doesn't work out that way when the block is what warps.
Honda uses open deck blocks on many of their popular engines for the same production reasons. It's cheap. They also use an aluminum head with the aluminum block so everything expands at the same rate. Works great for most people. However, if you want to put a sustained load on these motors (kind of like a boat motor), the first thing hot rodders do is to weld a "deck plate" onto these engines which ties the top of the cylinders to the block. Google "Honda deck plate" sometimes.
From ERL Performance:
If you're happy with your 470 and willing to make sure you never overheat it, it'll work great for you. Of course, you'll also have the alternator and cam seal problems to worry about but those are pretty easy to remedy although they are not cheap if you have to pay someone to do it for you.
To sum it up, under typical consumer use, the 470 was a disaster for Mercury. It can't stand up to the way the majority of the people maintain and use their boats.
