[niles]

DV:

As usual these thing are a lot more complex that mets the eye: I still stand by the gear handbook that says any material in ok as long as the hardnesses are compatible. Normally you don't want two very smooth steel gears of the "same" hardness cause they tend to develope atomic attractions to each other like Jo block's do.
You dispense with this my different surface finishes or harnesses.

Cranes own manual down plays the material with there own words:

A quick read of the Crane literature for the cam I use says "8620 STEEL ROLLER CAMS ... WE HIGHLY RECOMMEND THE USE OF AN ALUMINUM BRONZE GEAR OR A SPECIALLY PROCESSED IRON OR STEEL GEAR SPECIALLY MANUFACTURED FOR STEEL CAMSHAFTS, ON THE DISTRIBUTOR" (the caps are theirs ).

this strongly implies that many different combo are compatible.

Two things are going on here in my opinion: 1. Crane wants you to buy Crane gears; not real concerned which type.
2. Crane, or any other manufacature for that matter, may have tooth profiles that are unique to there gear hob machines and "sets" from them are more compatible.

The selection of gear material from an engineering viewpoint is driven by what do you want to wear; hard always wears less than soft. So if you want all the wear to occur on the distributor gear use bronze. If you long wearing but have "some came protection, use iron. The longest of all would be steel; the down side is that the cam may actually wear out first. Never simple; always many layers before you get the bottom.

gn