A15 cranks that have had a hard life crack around the rear journals. I bought an engine from a racer in Melbourne years ago & the crank was cracked in all 4 journals. Nitriding stops this from happening, though I've only done it for race engines. Maybe it was abused in a previous life. Daily driving shouldn't stress the crank to this point, but forced induction can, especially if it runs lean & detonates. It's due to the stroke length, the mains & the big end journals are offset enough to be not as tough as the A12 & A14 cranks. The nitriding adds strength to the surface, but it does bend the cranks. The cranks can be straitened easily, Graham Wilkins Performance has done it for me the 3 timesI've had it done. Some friends of mine think it's a waste of money, but I reckon its cheaper to nitride & straighten than finding a new crank.

I have seen this happen on a few different engines. It usually happens when a crank is dropped and then refitted to an engine. Once i broke the crank on a 351 Cleveland that came from HM engines. I fitted a balancer and the boss decided to change it. So I pulled it off again with a rattle gun and the crank snapped behind the timing gears!! Classic.

iv hurd all nissan/datsun cranks are steel forged? but only heard that dont know if its accurate.

vok! how did you manage that?

That looks like it would leave a mark. OUCH!!!