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Converting an automobile motor to a viable marine engine has been going on for years. Although, going back in memory, many of these conversions were disasters waiting to happen.

Very early conversions simply amounted to lifting the motor with transmission from the car and slapping it in a boat. Several things were quickly learned. The exhaust manifolds became red hot and the circulating water pump didn’t adequately cool the motor as a radiator and rush of air through it was not used. Since the motor was set in at an angle, the carburetor would flood and the oil pickup in the pan would run dry. The transmission was never intended to absorb the forward and reverse thrust of the propeller, and the clutch and very low speed of the reverse made rapid braking impossible. These are but a few of the problems; most were quickly solved.

Today there are conversion parts for popular automobile motors to make them viable marine power plants. But that doesn’t mean that any motor can be taken from an auto and converted for marine use. Conversion parts must be sold in quantity to make a profit. Just the water-cooled exhaust manifolds patterns and tooling for a given motor will run into thousands of dollars. The consumer pays for that tooling in the long run so the more identical motors used in autos the more likelihood that it will be a viable candidate for conversion parts.

A prime example is the so-called small block Chevy, first used in autos in the late fifties and still in use today. No, it’s hardly the same motor, but the basic block pattern for major bolt-on appendages has remained essentially the same. This means there are lots of these motors available and used in boats more than any other. Check the past few years and you’ll find this motor was the basic power plant for many marine motor manufacturers.

There are many motors that would be ideal for boats, but conversion parts are not made. Marine motor manufacturers have used some, but individual parts are not available. It is possible to buy the manufacturers marine appendages and put them on a basic automotive engine block, but the cost would be prohibitive.

Can’t you make the parts? Possibly, but doubtful, or maybe yes and no. Motor mounts, cover plates, pump adapters and many other parts have and can be made. However, making marine exhaust manifolds is another story. A marine manifold jackets the exhaust chamber with water. Making and welding a manifold so each chamber is separate plus, fitting it to the block-bolting pattern is a formidable project. Many have tried but, in our experience, few have succeeded in making a useable manifold.

What about the transmission? Marine transmissions are readily available and, with an adapter plate, will fit to almost all automobile motors. What about using the auto automatic transmission in a boat? It can and has been done, but, with older transmissions and internal changes. Such a transmission will be considerably longer than a marine gear and it won’t take the forward and reverse thrust. Shifting from forward to reverse as a brake may create excessive wear on the auto transmission. Then, too, the unit probably reduces the rpm’s in reverse. It’s difficult to duplicate the marine gearbox by converting an automotive one unless you are particularly adept and very familiar with the inner workings of these units.

Be warned. If you get a chance to get a motor from Aunt Gertrude’s low mileage old car, pause for a moment. Be sure you can get the parts required to marinize it before congratulating yourself.

Footnote: For a detailed description of auto to marine motor conversion see the book INBOARD MOTOR INSTALLATIONS available through GLEN-L. For more details about specific pieces of hardware, see our Inboard Hardware Catalog.


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One Response to Practicality of Converting Automobile Motors to Marine Use

  1. Pam Lassila says:

    One of my favorite childhood memories is going out to the lake with my family and boating around. My dad would take us and just throw us over the edge of the boat! It was so fun. I love boating and being able to still do that.

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