Sub Frame Damage

[bergy]

Another thing to keep in mind about cast iron is that it doesn't propagate fatigue cracks like steel does. After cast iron exceeds 1,000,000 cycles (which doesn't take long for a crank shaft) it has unlimited fatigue life. Steel, however, is always suseptible to propagating (growing) cracks. The bottom line is that steel has higher yield strength, but old steel cranks can have some pretty bad fatigue issues - and don't assume that small cracks that show up on crack detection will stop growing.