Piston Engine Connecting Rosds

The connecting rod is the link which transmits forces between the piston and the crankshaft.  Connecting rods must be strong enough to remain rigid under load and yet be light enough to reduce the inertia forces which are produced when the rod and piston stop, change direction, and start again at the end of each stroke.

There are three types of connecting-rod assemblies:

• The master-and-articulated-rod assembly
• The plain-type connecting rod
• The fork-and-blade connecting rod

  The master-and-articulated-rod assembly

The master-and-articulated rod assembly is commonly used in radial engines.  In a radial engine the piston in one cylinder in each row is connected to the crankshaft by a master rod.  All other pistons in the row are connected to the master rod by an articulated rod.  The articulated rods are constructed of forged steel alloy in either the I- or H-shape, denoting the cross-sectional shape.  Bronze bushings are pressed into the bores in each end of the articulated rod to provide knuckle-pin and piston-pin bearings.

The master rod serves as the connecting link between the piston pin and the crankpin.  The crankpin end, or the ‘big end’ contains the crankpin or master rod bearing.  Flanges around the big end provide for the attachment of the articulated rods.  The articulated rods are attached to the master rod by knuckle pins, which are pressed into holes in the master rod flanges during assembly.  A plain bearing, usually called a piston-pin bushing, is installed in the piston end of the master rod to receive the piston pin.

Plain-type connecting rods

Plain-type connecting rods are used in in-line and opposed engines.  The end of the rod attached to the crank pin is fitted with a cap and a two-piece bearing.  The bearing cap is held on the end of the rod by bolts or studs.  To maintain proper fit and balance, connecting rods should always be replaced in the same cylinder and in the same relative position.

The fork-and-blade rod 

The fork-and-blade rod assembly is used primarily in V-type engines.  The forked rod is split at the crankpin end to allow space for the blade rod to fit between the prongs.  A single two-piece bearing is used on the crankshaft end of the rod.