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How can I make my crankshaft more durable?

Unless you are very lucky, your 216/235/261 crankshaft will need regrinding and straightening. GM allowed a .030" undersize, which means that at least some of the hardening exists at this depth but, even at full diameter, these shafts are not particularly hard.

If extreme durability is the goal, you might talk to your machinist about nitriding. This process case hardens the crank to a depth of .020" or .030", without affecting the characteristics of the base metal. Surface hardness doubles from 30 or 35 Rockwell C to 60. This technology was primarily limited to military and commercial aircraft until the 1960s when Chevrolet specified nitrided crankshafts for the 327 Corvette. "Tufftrided" crankshafts were also installed in extreme-duty Chevrolet truck engines. The largest users of such crankshafts today are Mercedes-Benz, Perkins Diesel and Honda.

The earlier method of nitriding involved immersing the part in a high temperature cyanide-based salt bath, which reacts with the steel to release free nitrogen. The nitrogen penetrates into the surface of the steel and combines with it to form a matrix of hard, wear-resistant compounds. The salt bath process, which gave Chevrolet Tufftrided cranks their characteristic dull green color, is now obsolete because of environmental concerns.

How do you get rid of cyanide salts? The grey, sometimes almost black nitrided cranks encountered today were heated in an oven to 950 F in the presence of gaseous nitrogen. Light grey cranks were subjected to ion nitriding, a process that uses microwaves for heat generation.

In all cases a nitrided crankshaft will "grow" about .0005" on the journal diameters and go out of round by an insignificant .0002". The machinist normally grinds the crank .005" under to allow for the swell and lets the out of round go. Of course, if you want absolute perfection, the crank can be ground .005" over and taken down to the desired size after hardening. Cranks will remain straight if process temperatures are held below 1000 F.

Heat-treat services typically charge about $50 to nitride a 70-lb crankshaft.

A special thanks to Paul K. Dempsey for this article. He's currently writing a book on the Chevy Inline engine.

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