Automotive vs. Marine Engine Parts
by Bob Van Brunt, C.P.O USCG, Ret.
It is understandably tempting for people to purchase automotive parts instead of marine parts for their boats. Not only are automotive parts easier to obtain than marine parts but they are often substantially cheaper. (And if you think marine parts are expensive, you should check out aviation parts…)
However, there are some very important reasons for not using automotive parts for your marine application. Unlike cars, marine engines are in an enclosed area where the explosive fuel fumes are not blown away by air rushing around the engine. This is why all marine gas engines have blowers that should be used prior to starting the engine. Every year, up here in Maine, we have a couple of boats explode when the owners forget this crucial step.
In a marine application, both the starter and the alternator are “ignition protected” (also known as spark resistant) and have vents with flame arrestors to prevent sparks. (See my article on flame arresters in Volume 1, Issue 2 of THE EXPRESS.) They are also sealed more completely to reduce corrosion. Automotive starters and alternators have exposed contacts that allow sparks.
A marine distributor is also ignition protected and has a vent with a flame arrestor to prevent sparks. It too is sealed to reduce corrosion. An automotive distributor can create very high voltage sparks.
In automotive carburetors any fuel that is not mixed (called overflow or excess) is allowed to drip out of the carburetor and onto the ground. Ideally the carburetor is tuned so that this shouldn’t happen, but a sudden acceleration or sudden stop can get ahead of the various
adjusting controls. In a car this fuel is blown away or drops to the ground, but in a boat this would cause explosive vapors so marine carburetors have vents to allow any overflow to be consumed by the engine. Under Federal regulation, CFR 45162.042, all marine carburetors must have spark arrestor air filters (made of a metal mesh) instead of the automotive air filter (paper or plastic).
Marine fuel pumps are different as well. If the diaphragm fails on a marine fuel pump, the pump will not leak (it won’t work either, but it doesn’t leak). In a car a fuel pump will allow fuel to leak if the diaphragm fails. It is just a different design.
Marine fuel tanks must meet U.S. Coast Guard and American Boat and Yacht Council standards while there are no agreed upon standards for automotive tanks—remember the Ford Pinto.
Aside from the explosion and corrosion issues, the big difference between automotive and marine engines is in how they are used. Marine engines are always under load. There are no hills on the water to coast down allowing an engine to partially rest nor, under most usage, are there periods of peaceful idling.
To deal with this, boat engine manufacturers contract with the large car engine manufactures to purchase engine blocks (often GM blocks). These bare blocks are then “marinized”. Valves are beefed up, camshafts and valve lift are changed, crankshaft seals are added, manifolds strengthened and all the special corrosion and spark resistance equipment added.
Comments? Questions? Suggestions for topics for our blog or newsletter? Send them to
Marine Parts Express is a division of Water Resources, Inc., a privately held Maine Corporation.
For all your marine engine parts needs, call us toll free at 877.621.2628, or outside the U.S. at 207.882.6165.
May 1, 2010 / mpartsexpress / 0
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Margaret Graham Neeeson
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