1997 Mazda MX-5 Miata: Mil.Spec Connecting Rods
August 16, 2012
There's more to them than simply additional length, though.
Mil.Spec's connecting rods are machined from billet chromoly steel and are made in the USA. They're designed to be lighter and stronger than the stock rods despite being 7mm longer from the centerline of the big end bore to the wristpin bore. How is this possible?
It comes down to material selection and geometry. "Chromoloy" typically refers to 4340 steel, which is a well-known but very general term. Mil.Spec's rods are made of 4340AQ steel (produced in USA by Timken), where the AQ denotes Aircraft Quality. This is an important distinction over run-of-the-mill 4340 -- the AQ variant contains lower concentrations of phosphorus and sulfur which embrittle steel.
I'm stressing the USA part because offshore steel remains questionable, in my opinion, and connecting rods are probably the last place you want to skimp on a high-performance engine build.
The AQ steel comprising the Mil.Spec rods undergoes a remelt process... twice. Remelting casts off trapped gases and allows for a more controlled rate of solidification. It's an expensive and time-consuming process that produces a cleaner steel (factoid: 4340AQ is often chosen for aircraft landing gear).
After manufacture the rods are cryo treated and shot-peened. The upshot to all this leg work is elevated tensile and modulus properties that, in turn, allow mass to be removed relative to stock. According to my scale the Mil.Spec rods each weigh 516 grams, compared to 550 grams for a stock Mazda BP rod.
This photo has not been retouched -- the parting line where the cap meets the rod is essentially invisible. This is the sign of some pretty impressive machining capability.
The H-beam construction was intentionally selected by the rod manufacturer as preferable for a boosted application. H-beam versus I-beam is a long-contested debate, but in my view on it goes like this -- I-beams are less costly to manufacture, making them popular among OEMs. H-beams however offer inherently higher resistance to lateral torsional buckling and have lower stress concentrations at the shank-to-small end transitional areas. This characteristic allows mass of H-beams to be reduced -- or margin of safety to be increased -- relative to I-beams.
ARP Custom Age 625 rod bolts secure the cap to the rod. It's been said that rod bolts are among the most highly stressed components in the bottom end of an engine.
The stock BP rod (right) is forged, but you can see the difference in beam section (and length) here. Surface finish of the stock rods exhibits a moon-like characteristic. Stock rods work fine at stock power levels, but are the first thing to fold when you start feeding a Miata boost. Also note the generous fillet radii in the Mil.Spec rods.
E aho laula. I'll go ahead and point out the obvious -- these Mil.Spec rods aren't drop-in replacements for the stock rods, onaccounta the additional length. They're to be used in long-rod configurations with, say, the JE Pistons FSR pistons. They're a premium product for a specialized application. Also, Mil.Spec are probably willing to work with you if you've got a unique application in mind.
Jason Kavanagh, Engineering Editor
Photos by Kurt Niebuhr
Mil.Spec - http://www.mil-spec.org/home.php - connecting rods