I lifted his article from Car Craft's web site:
Distributor Gears: What’s The Deal?
Gears Won’t Live A Long, Healthy Life Unless They’re Properly Matched And Set Up.
By Tony Nausieda
photographer: Courtesy of MSD
Look at the effort that goes into a ring-and-pinion installation, yet no one thinks twice about the distributor gear/cam gear relationship unless performance starts to suffer.
Distributor gears are designed to be sacrificial. They’re easier to replace than cam gears, so they’re engineered to go first in the event of failure. Properly chosen, your distributor gear and cam gear will differ in hardness. But since a flat-tappet cam is made to a different hardness than a roller cam, you have to do your homework and pick a compatible distributor gear. We spoke with MSD engineer John Clark to get the lowdown on distributor gear science. Properly set up, these gears will give you one less thing to worry about.
Gear Science
Stock flat-tappet cammed engines usually use a ductile iron distributor gear. A standard iron gear usually works well with a flat-tappet cam grind, but remember that stock gears were designed around a stock cam. Because most modern performance flat-tappet cams are ground on better quality (harder) cores, MSD’s iron distributor gears are surface-hardened a few points higher than stock.
Softer bronze gears are typically used when running a billet steel roller cam, because the roller cam cores aren’t as hard as those on flat tappet cams. Bronze gears have a bad rap for wearing very quickly, but that’s in part due to inferior yellow-brass gears sold by some companies. Quality nickel/bronze alloy gears like MSD’s are made from a tough alloy, so they should live for years provided you don’t beat on the motor before the
oil’s hot. Sometimes your first bronze gear may wear out quickly as it massages the cam gear. John mentioned that if the cam gear is poorly made, the first bronze gear may destroy itself deburring and reshaping the cam gear, but the second bronze gear should wear in faster, work more smoothly, and live longer. Bronze gears wear more predictably since they’re the same hardness all the way through, whereas iron gears are only hardened on their outer layer (0.0005- to 0.0015-inch deep). Remember to check ignition timing; as the distributor gear wears, the timing will retard.
Ford developed mild steel distributor gears for use with factory hydraulic-roller-cammed engines. This steel is softer than the ductile iron gears, but harder than bronze, designed for longer life necessary on a factory engine. Relatively harder Chevy hydraulic rollers use a standard iron distributor gear, but at the expense of shortened gear life. The Ford gears, though, are pretty tricky to install since they’re both press-fit and pinned to the shaft.
The metal’s only part of the story, though. How the distributor and cam gears mesh is just as important. The pitch diameter is a measurement of how closely the cam gear and distributor gear mesh. Excessive clearance between the gear teeth shouldn’t be fixed by shoving the distributor farther into the hole; this screws up the proper wear pattern. The right way to take care of excessive backlash is with an oversized distributor gear. Currently, there’s no practical tool for measuring pitch diameter, but John told us a 0.006-inch oversize works great in 99 percent of small-block and big-block Chevys. For the “other guys,” MSD is working on a tool to correctly measure pitch diameter so a proper oversize gear can be chosen.
Crane Cams recommends using moly disulfide assembly lubricant and
zinc dithiophosphate anti-wear additive before firing up your engine with your new, expensive camshaft installed. I hope this helps a little. Rich