“Connecting Rod Balancing”

By Ted Eaton

Fig A.An engines connecting rods exhibits traits of both rotating and reciprocating mass and hence, must be match weighed end for end to insure these two masses are kept independent of each other. As a point of clarification, the reciprocating end is the small end of the rod or the portion of the rod that is representative of up and down motion in the cylinder while the rotating end of the rod is the bearing end which rotates with the movement of the crankshaft. Your balancing shop will have a rod weighing fixture that’s designed for separating these two masses and then being able to have all the rod small and big ends match in weights throughout the particular set of rods being balanced.

Simply finding the lightest rod in a set for total overall weight and then reducing the weight of all the other rods without any regard to which part of the rod the weight is being removed from to match the lightest does not make for a balanced set. This is because the weight being removed is most likely being taken from the wrong spot on the rod and thus actually making the rods even more out of balance than before attempting to weight match them. This method fails to take into account whether the mass being removed is reciprocating or rotating mass which is a major consideration in a dynamically balanced engine.

Connecting rod balancing requires a fixture that allows each end to be weighed independently. There are several different fixture designs available on the market but all utilize the same concept; each end of the connecting rod is isolated from the other for weighing purposes.

Fig A.Similar in concept to the match weighing of the pistons, the ends of the rods must be weighed with the lightest small and big end of each rod within a set being found and isolated. Very rarely will the same connecting rod from a factory installed set have both the lightest small end and lightest big end on it. After finding the lightest ends, it is then just a matter of taking the remaining heavier rods and making the ends match the previously found lighter end weights.

Fig A.Your balancing shop can employ one of several different methods in which to reduce the connecting rod end weights. Typical tools for this operation can vary from using a grinder, belt sander, or a milling machine. The design of the connecting rod in itself can dictate what machining or weight removal operation will be used. Most stock style connecting rods have a balancing pad on each end which is a convenient spot from which to remove material for balancing purposes. Many of the newer aftermarket rods and especially the H-Beam style do not have these balance pads on the ends and do require some forethought before attempting to remove any material from them. For many of these newer designed rods, material from the big end is removed at the rod bolt edge instead of the very bottom. The small ends for rods without balance pads are usually best done on a belt sander using a nice rounding motion in which to remove material evenly from around the pin end. Regardless of the method used for weight removal, it’s important that the metal not be unduly overheated. This may require repeated quenching if excessive grinding must be performed in which to remove the required amount of material. Excessive overheating of the big end can cause out-of-roundness to the big end bore which can prove disastrous to bearing clearances besides affecting the structural integrity of the metal itself on either end.

Fig A.Fig A.After all the connecting rods have been weight matched, the reciprocating and rotating end After all the connecting rods have been weight matched, the reciprocating and rotating weights are then recorded on a balance card or work sheet for the upcoming bobweight calculation.  All that remains at this point is to clean the connecting rods of any debris or grinding/sanding residues caused by this particular balancing step and rebox them until engine assembly takes place.

The next article in this series will cover the nuances involved within the bobweight calculation in preparation for spin balancing the crankshaft. Until then, Happy Motoring.

Originally published in Y-Block Magazine, Nov-Dec 2004, Vol 11, No. 6,  Issue #65