Most have heard this but I think it's an interesting and unsolved mystery. My last Nissan had 2 alternators, one was a 150amp add on that required a third pulley on the drive shaft.

When after installing new injectors, it smoked, mostly black unburned fuel. But the general prognosis was, it's beat. In looking for a rebuild, I found a top running replacement.

I was about to take the old block to the metal bin at the dump, when I decided to pull the front cover on the timing gears. Long story short, I removed the crankshaft nut etc, slid off the gears, and this is what I found.

At some point, the main shaft nut either loosened or something moved. At any rate, the aft timing gear was being held only by the Woodruff key(incorrect). So in time(years, the 150 was put on decades ago), the Woodruff key and the milled slot in the crankshaft, had slowly rocked a new over sized slot and distorted Woodruff key. So the timing gear to the IP would change timing on each stroke! It was subtle, but getting worse.

In the end, the engine which was perfect except for the worn slot, would have required a complete re-build to fix the shaft keyway. So not much lost. But I will NOT put another pulley on this engine extending the leverage on the shaft, nor will I go to a bigger alternator. I'll change boats before I do that.

Look closely, you can see the perfect fit of the forward Woodruff key, and the oversize key and key way, aft. Good engine, toast.