American manufactured engines are the mainstay of hot rodding. They also enjoy one of the industry's best reputations for longevity. Thus, when cared for, they do not need rotating assembly services unless a complete rebuild or hop-up is in order. For purposes of this discussion, the rotating assembly of the engine consists of the crankshaft along with the connecting rods, pistons, wrist pins, harmonic balancer, and flywheel or flexplate. Companion components, such as piston rings and both the crankshaft and connecting rod bearings, are included in this classification.
After a good visual inspection, the crankshaft main bearing journals and crank pins will be measured with a micrometer. The crank pins are the portion of the crankshaft the connecting rod attaches to. When machinists check for taper and out-of-round, they compare the micrometer readings they get with the specification size chart for the crankshaft to determine if it has already been ground undersize. The common sizes to which bearing journals are ground are in increments of .010-inch, like 0.010-, 0.020-, or 0.030-inch. Most machine shops sublet out crank grinding to a facility that specializes in crankshaft repair, since the work is so specialized. Even if a crankshaft is not ground undersize, the journals should be polished to a #15 microfinish before entering service. In this photo, the crankshaft is being spun on a balance machine in the same direction as rotation in the engine. A polishing material that resembles emery cloth is held against the journals. The polishing cloth needs to pull the material away from the direction of rotation. Polishing is done to prepare the journal surface for the proper contact with the bearing material, so the oil will be able to work properly and form the necessary oil wedge that separates the bearings from the journals.
Shot peening in a special...
Shot peening in a special cabinet is one method that is used to strengthen stock connecting rods. This procedure is usually skipped if high-performance aftermarket rods are being used.
The rod on the top had its...
The rod on the top had its beam polished. This is done by a drill with a very abrasive sand paper roll attached on an arbor. The sides of the rod perpendicular to the bore are smoothed to remove any stress risers. This adds strength to the rod and is commonly done on performance-bound engines using stock rods.
The Sunnen LBB-1660 is a state-of-the-art...
The Sunnen LBB-1660 is a state-of-the-art connecting rod hone, and is used to resize both the small and large ends of the rod.
After the piston is removed...
After the piston is removed from the connecting rod, it will be measured and re-sized if it is determined to be either out-of-spec (for diameter size) or out of round. During a major rebuild, it is usually necessary to resize the rods to perform a high-quality job. Cutting both the cap and the rod end the cap attaches to completes this process. A special rod-cutting machine is used to do this. The rod then needs to be honed to size on the LBB-1660.
The big (crankshaft) end of...
The big (crankshaft) end of the rod is then measured with a special dial bore gauge that is attached to the rod hone.
The connecting rod of an engine...
The connecting rod of an engine is one of the most abused and least respected parts. The load and stress it experiences are extreme. For example, the connecting rod must support four to five tons of pressure during combustion. Also, the rod is required to reverse its travel many times a second. As an example, a connecting rod in the average engine running at 3,000 rpm is reversed 100 times in one second. Much of this load is applied to the bolts that hold the rod to the journal. For this reason, new high-quality connecting rod bolts should be installed during any performace-based rebuild. The first step is to press out the old bolts, as seen in this photo.
To install new connecting...
To install new connecting rod bolts, a special fixture is used to support the rod while the new fastener is interference fit with a hydraulic press.
A connecting rod that originally...
A connecting rod that originally used press-fit wrist pins can be honed and equipped with a bronze bushing to accept full-floating pins, which are common in performance aftermarket racing pistons. The rod hone is used to size the small end of the rod to accept the bushing. Floating pins reduce both internal engine friction and reduce the natural twisting and bending loads experienced during combustion.
Rotational laws of physics...
Rotational laws of physics are applied to the crankshaft, harmonic damper, flywheel, or flexplate, along with the big end of the connecting rod. The piston and companion components, along with the connecting rod (from the mid-point to the small end), are identified as reciprocating. When balanced, the rotational components will posses no heavy areas acting upon the crankshaft, trying to pull it from its axis of rotation. The identification of mass differential is accomplished on a machine such as the Sunnen DCB-750 crankshaft balancer. It is representative of a hard cradle design.
Before balancing can be performed,...
Before balancing can be performed, all of the reciprocating components must be weighed for two reasons: first to generate bobweight information for the balancing machine, and secondly to equalize all of the mass. This KB piston is being measured on a digital scale for weight-matching. This particular slug came in at 642 grams.
Once all the pistons are weighed,...
Once all the pistons are weighed, they will be brought to the mass of the lightest one by either drilling or grinding the balance pad.
A special fixture is used...
A special fixture is used to weigh both the small and big end of the connecting rod separately. As with the piston, the connecting rods will be brought to the weight of the lightest one by grinding or drilling on the balance pads. The balance pad is the flat area located on the top and bottom of the rod.
Bobweights that simulate the...
Bobweights that simulate the reciprocating mass are attached to the crankshaft on the rod journals. The crankshaft is then spun at low rpm, and the machine shows the operator where the unbalance conditions exist.
If weight needs to be removed...
If weight needs to be removed to bring the crankshaft into equilibrium then a drill press, that is attached to the balancer, is used to remove material from the weights. The depth and size of the hole is determined by a drill chart that identifies the weight of material displaced by drilling. If mass is to be added to the crankshaft to compensate for heavier reciprocating components or increased stroke, then a hole is drilled axially to accept a heavy metal slug made of tungsten. It is customary to press-fit and then weld the slug in place. Heavy metal installation usually shows a 2:1 ratio. If one gram of steel is removed, the area displaced will hold 2.17 grams of tungsten.
When the balancing is complete,...
When the balancing is complete, the pistons and rods can be assembled. Here, a press-fit-style pin was heated in the Sunnen rod furnace and then pushed into the pin bore by hand.