Rust in Marine Engine Water Jackets

| June/July 1993

7964 Oakwood Park Ct. St. Michaels, Maryland 21663

Most modern marine engines have fresh water cooling. That means that fresh water or a solution of fresh water and glycol is circulated through the cooling jacket and it, in turn, is cooled by sea water by means of a heat exchanger and a sea water pump.

Old marine engines that interest collectors were directly cooled by sea water, leading to rusting of the inside of the cooling jackets if cooled by salt water. Collectors who have engines from fresh water areas such as the Great Lakes are most fortunate.

First, a word about the two kinds of rust. Red rust, which we see on exposed iron and steel, is ferric oxide which is formed when there is plenty of oxygen present. Black rust is ferrous oxide; it is formed where there is a shortage of free oxygen. Much of the rust in cooling jackets is black rust.

All too often, an old engine is removed from a boat, the cooling jackets drained, and the engine stored. In most cases, there is some sand and mud in the jackets. As the jackets dry out, the salt is concentrated in the mud and rusting is more rapid than it was while the engine was in use. The rust builds up on the outside of the cylinder barrel and on the inside of the jacket. When these two rust layers grow to the point where they meet, tremendous forces develop and pieces are pushed off the jacket casting.

The object of this article is to tell how to reduce jacket rusting in a stored engine and how I have repaired a damaged cylinder.


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