Tech Talk #50 – Crank Calls

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“These choices have serious consequences when a 75-pound chunk of machined steel is spinning in the heart of an engine.”

Once upon a time, a 454-cubic-inch engine was considered a big motor, and anything over 500 inches was referred to in awe as a “Mountain Motor”. But like Big Gulps and the budget deficit, everything is bigger in the 21st century – especially drag racing engines. While we’ve run 500-cubic-inch engines in Pro Stock since 1982, the rest of the drag racing world has adopted 600ci, 700ci, and even 800ci motors.

The one essential ingredient for a big-inch engine is a long-stroke crankshaft. The availability of reasonably priced aftermarket cranks has fueled the displacement inflation. When the only alternatives were factory forgings and high-dollar billets, racers had few choices. It’s a different world now with high quality crankshafts available with strokes that range from 4.250 inches up to 5.750 inches. And with the growing popularity of Top Sportsman and Pro Mod classes, the trend toward bigger and bigger engines is gaining momentum.

A long-stroke crankshaft is a highly specialized component, and not all cranks are created equal. When racers pulled crankshafts out of junkyards or bought over-the-counter factory forgings, they took what they could get. Now with the advent of affordable aftermarket cranks, racers face a bewildering assortment of materials, counterweight styles, oiling systems, and options. These choices have serious consequences when a 75-pound chunk of machined steel is spinning in the heart of an engine.

Balancing is a serious issue with long-stroke crankshafts. As the stroke increases, it takes larger and heavier counterweights to offset the weight of the rotating and reciprocating assemblies. I’m not an advocate of external balancing, which puts a portion of the required counterweight on the flywheel and balancer. I’m also not a fan of counterweighted flywheel flanges on crankshafts. While external weights do balance the crankshaft assembly overall, they also introduce torsional forces into the crankshaft by positioning some of the weight at the extreme ends of the crankshaft. Although a steel crankshaft seems quite rigid, in fact it twists and bends in response to power pulses and torsional forces. A crankshaft doesn’t have to fail to make the effects of this stress apparent: Worn bearings and cap walk (fretting of the main caps against the block) are signs of torsional bending.

A production V-8 crankshaft typically has six counterweights positioned at the front and rear of the crank. Many aftermarket crankshaft manufacturers offer “eight weight” designs with two additional counterweights adjacent to the middle main bearing journal. These center counterweights simplify balancing and significantly reduce the torsional loads on the crank.

While external balancing is less expensive than internal balancing, I believe it’s better to balance the crankshaft internally even if it’s necessary to install heavy metal in the counterweights. These heavy metal plugs must be installed parallel to the crankshaft axis by drilling holes through the counterweights and pressing the plugs in place. Plugs installed in the counterweights perpendicular to the crank axis can be dislodged by centrifugal force, turning them into heavy metal projectiles.

When we install heavy metal in a crank at Reher-Morrison Racing Engines, we don’t put all of it in the end counterweights. We drill through the first counterweight and into the second counterweight; in some instances, it’s necessary to drill through to the third counterweight as well. We then repeat this procedure on the other end of the crank. Each hole is progressively smaller, and the corresponding plugs of heavy metal are turned on a lathe to produce the correct interference fit. Distributing the weight throughout the crank in this manner also reduces the torsional loads on the crankshaft.

Hollow rod journals are a real asset for a long-stroke crankshaft. Drilling the crank pins to lighten the throws has the same effect on balancing as adding mass to the counterweights, but it produces a lighter overall rotating assembly. The longer the stroke, the more important it is to drill the crank pins. Most manufacturers offer drilled crank pins as an option, and it’s money well spent. Don’t be tempted to buy an “economy” crank when building a big motor; the cost of balancing with heavy metal can more than offset the low initial cost of an undrilled crank.

The position of the counterweights is also important to proper balancing. Unfortunately it’s difficult to determine whether the counterweights are in the right positions unless the crank is mounted on a balancing machine. If a crank needs a lot of material to be removed from one side of a counterweight and then a plug of heavy metal inserted at the opposite end, it’s likely that the entire counterweight is in the wrong place. It’s possible to balance a crank with this problem, but the fact that the counterweights aren’t indexed properly means that more weight is required to balance it than if the counterweights were in the correct positions.

A crude example of this is an out-of-balance tire. If a tire is mounted on a wheel and it’s way out of balance, often the easiest solution is to rotate the tire to a different place on the rim. Perhaps the wheel and tire are both heavy in one place; if both heavy sides happen to be together, it takes a lot of lead to balance them. But if the heavy sides of the wheel and rim are opposite each other, the overall balance is better and less lead is required. Applying this principle to crankshafts, if the counterweights aren’t in the right place, the balancing job is much more difficult and ultimately requires more weight to achieve a balanced state. An engineer could analyze the moments and angles of force that are involved, but I just know what I see on the crankshaft balancer.

Crankshafts are a complex subject, and I’ve already filled the available space for this column. Next month I’ll get into knife-edged counterweights and why you should never use a cross-drilled crankshaft.