Engine Balancing — Internal (Neutral) vs. External (Stock)

Balancing is one of the most common questions we get. Balancing the crank means weighing each part of the rotating assembly and drilling the crankshaft to remove or add weight so the assembly spins in a true rotation. A true-spinning assembly runs without the extra centrifugal forces that cause vibration, bearing wear and, in the worst case, catastrophic engine failure.

Click image to enlarge

How Pontiac Balanced From the Factory

Pontiac traditionally used a semi-internal balance. The crank was not zero balanced — it relied on an offset-weighted flexplate (automatic) or flywheel (manual) on the back of the engine to finish the assembly balance. Because an outside part completes the balance, this is called external or stock balance.

How Butler Balances

We no longer balance the factory way. The cranks in our assemblies are zero balanced — also called neutral or internally balanced. We balance everything internal (neutral) unless the customer specifies otherwise.

The balance is built from the bob weights of the actual parts in your assembly — rods, pistons, bearings and rings. That gives the truest balance: the crank spins true on the exact weights of the assembly, with no outside part needed to finish the job.

Many suppliers “box balance” using the manufacturer’s supplied weights. We don’t. Every rotating assembly we sell, regardless of brand, is balanced in our shop. We weigh and pin-fit the rods and pistons as needed, computer-balance the crank to the rest of the assembly, and document all the weights on a balance card that ships with the assembly. Keep that card — your builder will want it.

Butler assemblies are internally balanced

You must use a neutral-balanced flywheel or flexplate and harmonic balancer. A stock Pontiac flexplate or flywheel is not neutral balanced. Never mix the two — a neutral part on an externally balanced engine, or an offset part on a neutral engine, will shake hard enough to hurt something. If these parts came with your engine from Butler, they are already correctly balanced.

Oil Weight in the Bobweight

When a crank is balanced, a small amount of extra weight goes into each bobweight to stand in for the oil that clings to the rod bearings and journals while the engine runs. Butler uses 5 grams of oil weight in each bobweight — the standard we’ve used for years, with success on a few thousand Pontiac assemblies.

Other shops have their own standard, and that’s fine. This isn’t a right-or-wrong issue — but it matters if your builder re-checks a Butler assembly. Spin it with a different oil allowance and the crank will read out of balance by about the difference, often with the builder finding he needs to add weight. That’s two shops using two standards, not a balancing error.

Real example

A customer’s builder called to say the assembly was out of balance by 8 grams on one end and 5 on the other, and he needed to add weight. We asked how much oil weight he used: 12 grams. We use 5. That difference was the whole story.

Before re-balancing a Butler assembly: check your balance card and ask your builder how much oil weight he put in each bobweight. If it isn’t 5 grams, set it to 5 and spin it again before anyone drills the crank.

Aftermarket Crank With Factory Rods?

Most aftermarket cranks are weighted from the factory for aftermarket rods, which are much lighter than factory Pontiac rods. Put heavy factory rods on one and the crank needs weight added to balance — that means drilling the counterweights and installing heavy metal. The heavy metal and extra labor raise the balancing cost, and in many cases the total isn’t far from a new set of rods. Upgrading 50-year-old rods usually costs little more in the long run.

Heavy metal installed in a counterweight — click to enlarge

Changing Parts Means Re-Balancing

The balance is matched to the exact weights of your parts. Swap pistons or rods, or cut a dish into your pistons, and the crank has to be re-balanced to the new parts.

Which Balance Do I Have?

It isn’t always easy to tell:

Factory original, never rebuilt: it’s externally (stock) balanced.
Rebuilt: ask the builder what balance method was used. Only the builder or seller really knows.
Unknown history: look at the flexplate or flywheel that came off the engine.

An externally balanced part has extra weight added, or material removed — holes or a cavity — in one section, making it offset. A neutral part is uniform all the way around. Compare yours to the photos below.

Internal (neutral)

External (stock)

Original Pontiac flywheel — click any image to enlarge

Still not sure? Take a photo of your old flexplate or flywheel and send it to our tech department before ordering a replacement.

Dual-drilled flywheels: some flywheels are machined to cover both external and neutral balance. Make sure the side you install matches how your crank is balanced.