Strange Engine Knock -> Crank Pulley

[apbright]

Hey folks,

I have experienced - on two separate engines now - a worrisome engine knock; it appeared only for a narrow range of rpm's (I'm guessing 1500-1700), and only under load (i.e. knock audible under acceleration or cruise; not audible with throttle closed / descending hill for same engine rpm). I first suspected loose main bearings - fearing long hours underneath engine with oil dripping in face - not happy.

However, each time (on each engine) I finally noticed that the crank pulley was a bit loose. The first engine on which I noticed this I have since pulled from the car. The second one, however, was on a brand-new rebuild, recently installed in the car. On that one, tightening the crankshaft ratchet nut solved the problem, temporarily. After a few 10's of miles, the nut and thus also the pulley came loose again, and the knock returned. I tried adding a 5/8-inch washer (between the nut and the pulley) and tightening the unholy heck out of it. That worked... for about 100 miles, and then the knock returned again. I tried re-tightening, and repeated this limp-home procedure for.... ooh... 450 miles for a long Sunday home. (Also refilling a leaky radiator every 60 miles, but I digress...)

Post-mortem shows a deeply cracked crank pulley (see pictures, crack highlighted by red oval). I consider these scrap, and have ordered replacements along with a new ratchet nut.

Question: Anybody seen this, and have tricks to prevent recurrence?

Safe driving,
Andrew