You can see in the illustration that this is a five-cylinder engine -- radial engines typically have anywhere from three to nine cylinders. The radial engine has the same sort of pistons, valves and spark plugs that any four-stroke engine has. The big difference is in the crankshaft.
Instead of the long shaft that's used in a multi-cylinder car engine, there is a single hub -- all of the piston's connecting rods connect to this hub. One rod is fixed, and it is generally known as the master rod. The others are called articulating rods. They mount on pins that allow them to rotate as the crankshaft and the pistons move.
The master-and-articulating-rod assembly
is used on X-type engines, radial-type engines, and on some V-type
engines. The master rod is similar to any other connecting rod except that
it is constructed to provide for the attachment of the articulated rods on
the big end.
The articulated rods are fastened by knuckle pins to a flange around the
master rod. Each articulated connecting rod has a bushing of nonferrous
metal, usually bronze, pressed or shrunk into place to serve as a
knuckle-pin bearing. The knuckle pins may be held tightly in the
master-rod holes by press fit and lock plates or they may be of the
If the big end of the master rod is made
of two pieces, the cap and the rod, the crankshaft is made of one solid
piece. on the other hand, if the rod is made of one piece, then the
crankshaft may be of either two-piece or three-piece construction.
Regardless of the type of construction, the usual bearing surfaces must be
It should be understood that the type of connecting rod used in an engine
depends largely on the cylinder arrangement. If the cylinders are arranged
in a line parallel to the crankshaft, the connecting rod is similar to
that used in most automobile engines. However, certain types of aircraft
engines have a system of connecting rods connected to the same crankshaft
bearing, called an articulating connecting-rod assembly. The main rod or
master rod joins one of the pistons with the crankshaft, and the other
rods, called articulating rods or link rods, connect the other pistons to
this same master connecting rod.