Competition Connecting Rods

December 25th, 2011

Text by Sam Logan

Photos by Moore Good Ink

Nothing undermines the reputation of a connecting rod maker more than a deficient batch of rods. He agonizes constantly about heat treatments, high revs, heavy pistons, heavy pins, the number of race laps between rebuilds, but probably most of all whether or not nitrous is being sprayed. It’s a complicated business determining minimum weight while yielding maximum strength, enough to withstand the abuse sustained by the average race motor.Lunati (http://www NULL.lunatipower overcomes these special problems with its I-beam Signature Series connecting rods by forging them in 4340, a very tough material with high nickel and high molybdenum content. In fact, the chemical constituents of the rods are almost identical to the dies from which they are forged. Probably the chief reason they consistently withstand high impact loads at high temperatures is because they are subjected to a special heat treatment, a painstaking process closely governed in a batch furnace. To this end, controlled quenching and elaborate racking procedures maintain the stability of the connecting rods during the procedure.

Though Magnaflux testing (which uses dust in a magnetic field to reveal cracks on the rod’s surface) has been in use for decades, “It was sonic testing that had the most profound effect on the quality of Lunati’s rods,” says Derek Scott Lunati general manager. In use for most of this decade, sonic testing is characterized by a sound wave transmitted through the metal, revealing any hidden internal inclusions. Under high stress conditions, inclusions or “cold shuts” can be fatal to the connecting rod’s longevity.

For performance, practical and economic purposes, most competition small-block V-8 engine builders use cranks with Chevrolet pin journals. These accept two connecting rods, each .940-inch wide, and a pin diameter no larger than 2.100 inches. Of course, smaller journals reduce bearing surface speeds and also present a lighter rotating mass (crank pin and connecting rod), which requires lighter counterweights and enables the engine to accelerate faster. But perhaps the chief advantage of selecting this crank pin size is that it provides access to an exceedingly wide range of racing bearing under sizes. Also, cranks with 2.100-inch pins offer more useful stroke lengths than any other, and they are usually less expensive.

Here in the following sequence of pictures are the 24 major operations undertaken in the production of Lunati’s Signature Series connecting rods.


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