Over the 11 years of this engine being produced, many changes were made. Some of the most important changes and differences are as follows.

The valve seats in the 1961-1965 engines are part of the cast iron head (block). The 1966 (and later) engines have stainless steel valve seat inserts. This was for improved valve seat life and would last longer with the introduction of lead-free gasoline. These blocks are better and can be identified by looking under the 5 digit block part number located port side, just below the distributor. There will be an -A- or an -F- stamped under the 5 digit block number if it is the later style with the valve seat inserts. The valve seats can be installed on the earlier blocks if needed. The -A- and -F- blocks have 30 degree intake valves. The earlier style block has 45 degree intake valves. All exhaust valves are 45 degree.

The 1961-1966 engines had an oil filter. In 1967 Fisher-Pierce decided an oil filter was not needed since there is no dust on the water, and (according to reports) some were vibrating loose. They chose to install a bypass cap on the oil filter adaptor. This is something I never agreed with and I use an oil filter on all mine. The Homelite oil filter does not crush a gasket as it tightens, like an automotive oil filter does. It is very important to tighten (snug-up) the filter with large channel locks to ensure it won't vibrate loose.

The 1961-1965 models have a splined driveshaft on both ends. In 1966 the lower end of the driveshaft was changed to accommodate a threaded drive pinion. This change did not effect the gear ratio. While the individual parts are not interchangeable, the entire lower units are, as long as they are switched as complete units. Several different bearing designs were used in the lower unit over the years also.

The 1961 engine had an automatic voltage regulator. The 1962-1964 engines had no voltage regulator at all. They were wired for a 5-7 amp preset charge rate at the factory. The 1965 and later engines have a manual "hi-lo" charge rate switch to select from, depending on the electrical load required by the boat. Three different style rectifiers were used. The first style is red finned, air cooled, and mounted on top near the distributor. The second and third style is mounted on the port water jacket with either one or two mounting studs.

The 1961-1964 engines have a gear reduction starter motor. In 1965 a high torque direct drive starter moter was introduced. The flywheels are different between the two and not interchangeable. Either style can be used, as long as they stay matched up with their respective type.

1961 and 1962 engines had no thermostat. In 1964 the first style thermostat was introduced. The second (and best) style cooling system and thermostat was introduced in 1966. This system keeps the block full of water at all times. This model has a drain plug if freezing is possible. Manifolds are also different, depending on the cooling system type.

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