Balancing engine cuts vibrations, gives longer life

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This article was published 02/01/2004 (8055 days ago), so information in it may no longer be current.

THE faster a part rotates, the more critical its balance becomes. If a part is out of balance, it sets up a vibration that can damage other parts. An engine has many rotating parts: camshafts, sprockets, oil pump, crankshaft, front engine damper and flywheel or torque converter. Parts such as sprockets and camshafts ate relatively easy to balance. Find the heavy spot and either remove weight from that heavy side or add weight to the other side. When tires are balanced, weight is added opposite the heavy spot, but on internal metal engine parts it is usually easier to remove weight. If you see small shallow holes drilled into one side of a part, this is where metal has been removed to balance it.

Balancing crankshafts is a little more complex because the connecting rods and pistons are bolted onto the crankshaft. Pistons move up and down instead of rotating, so this type of movement needs balancing, too. Four-cylinder engines would seem to be the easiest to balance, because as one piston moves down in its cylinder, another piston is moving up, so the movement of the pistons balance each other. However, if the counterweights on the crankshaft are made so they match the weight of the piston and rod, then they are out of balance when the piston is part way up the cylinder. To reduce the vibration, engine designers use a mathematical formula to determine the weight of the crankshaft counterweights.

The basic formula is relatively simple. Take half the weight of the piston, piston pin, rings, and the top part of the connecting rod. Add this weight to the weight of the lower part of the connecting rod, the rod bearing and the oil in the crankshaft lubricating passage. This is the weight that must be balanced by the counterweights.

If you take your engine to an engine rebuilder to have it balanced, they would start by weighing all the pistons. Most piston sets are within three or four grams of each other or less, but sometimes a replacement piston can weigh considerably more. The machinist will find the lightest piston and grind material off all the other pistons so they match the lightest one.

Next, the connecting rods are weighed. The bottom end of the rod is supported with the rod hanging parallel to the floor. Now the top end of each connecting rod is weighed and again material is removed to make all rods weigh the same. The procedure is repeated with the bottom end of the rods. Now, the other components such as rings and bearings are weighed and all the measurements recorded. Using the formula, weights are clamped onto the crankshaft rod journals and the crankshaft is placed into the balancer.

The balance spins the crankshaft at a few hundred rpm while sensitive electronic pickups measure any vibrations. The machine shows where weight needs to be added or removed on the crankshaft. Drilling holes in the counterweights removes weight. If weight needs to be added, a hole is first drilled and then a heavier metal is added with a steel plug welded on top to keep it in place.

Some crankshafts are externally balanced, which means the front damper and flywheel have weights on them that help balance the crankshaft. These parts must be installed while the crankshaft is being balanced.

Some engines are naturally better balanced. In-line six-cylinder engines and twelve-cylinder engine designs have a natural crankshaft balance where there is always a crankshaft counterweight in the correct position to offset the weight of the piston and rod. V6 and V8 engines are not naturally balanced, but more cylinders make for a smoother running engine. Sometimes V6 engine balance is improved by using a balance shaft.

A balance shaft is actually unbalanced and turns at half crankshaft speed. The location and size of the unbalance counters engine vibrations so the result is a smooth-running engine. General Motors is introducing new four- and five-cylinder engines in the Colorado and Canyon that use balance shafts, too. Like some earlier Chrysler products, two balance shafts are used. The GM four-cylinder engine has both shafts rotating the same way, while the five-cylinder uses two counter-rotating shafts to keep the engine smooth.

Balancing an engine won’t give it more horsepower. It can make it last longer, though. Factory engines are usually balanced fine for everyday driving, but if you like to keep the tachometer near redline or even beyond, having the engine balanced can reduce vibrations and help it live a little longer.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca