EAA Sport Aviation - March 2011

CONVERTING AN A50 TO AN A65
The A50, like the other A-series engines that
were to follow, was type certificated in many
different configurations. There were single-
ignition, up-exhaust, dry-sump, fuel-injected,
and starter versions. None of the dry-sump
versions are candidates for upgrading. The
increase in power from 50 to 65 hp results
from increasing the compression ratio from
5.4-to- 1 to 6.3-to- 1 and operating the engine
at 2300 rpm instead of 1900 rpm. The com-
pression ratio change is achieved by using
different pistons, pins, rings, and rods. If an
early A50 engine is converted, other changes
(including crankshaft) may be required.
Other minor changes such as timing and car-
buretor jetting are needed when converting
the A50 to an A65, but most other parts such
as magnetos, ignition harnesses, carburetors,
propeller flanges, and fittings are inter-
changeable between the engines, as are all
hardware items and external fittings such as
intake manifolds, exhausts, gaskets, oil
sumps, and engine mounts. In many respects,
the A65 is the parent to its predecessor
and its later descendants, since it was pro-
duced in greater numbers and is more
familiar to pilots and mechanics than the
other A-series engines.

THE NEXT STEP: A65 TO A75

In order to obtain 10 additional horsepower
by turning the A65 at 2600 rpm rather than
at 2300, Continental changed several things
related to cooling and lubrication. The cap
ends of the rods were drilled with 1/16-inch
holes to provide additional squirt lubrication
for the cylinder walls, the exhaust valves
were changed to Stellite-faced (a cobalt-
chromium alloy with increased hardness
and a high melting point), and the
undersides of the pistons were made with a
waffle pattern for additional cooling. The
wrist pins were also changed to a smaller
diameter, although it is possible to create a
pseudo-A75 from an A65 by simply changing
the valves and drilling the connecting rods
to allow the engine to operate at the higher
rpm without changing pistons, rods, or wrist
pins, but engine life and cooling may suffer.
Once again, minor changes to ignition
timing and carburetion differentiate the
higher-horsepower engine from its
predecessor. The external appearance is
unchanged, and other than by examining the
engine data plate, it is not possible to
distinguish one engine from the other. While
many A65s have been made to operate at the
higher engine speed by simply changing
propellers to obtain A75 performance, one
must understand that the Continental
engineers had reasons for implementing the
various changes to the engine to achieve
certification at the higher horsepower.
Countless hours of operation in all sorts of
aircraft and configurations brought to light
the weaknesses that led to the changes in
later engines.