Crankshaft: ADI cast iron, and judging a used crank

A crank is judged on journal size, taper, roundness and cracks — not on how it looks. The material matters because it sets how much you can grind before it is scrap.

ADI instead of forged steel

Austempered ductile iron gives a cast crank strength and toughness close to forged steel, without the forging cost. It resists journal wear, bending and cracking better than ordinary ductile iron.

It is not indestructible. ADI still has to keep oil clearance and alignment; a bent crank will crack regardless of material.

Undersize grinding has a limit

Each grind removes the hardened layer and reduces the journal. Every manufacturer sets a minimum diameter and a maximum number of undersizes. Past that, the crank flexes more and the oil film cannot be maintained.

If the journal is already at the last undersize, or has a crack at a fillet, replace it.

Crack testing is not optional

Magnetic particle inspection on the fillets and oil holes, every time the crank comes out. A crack you cannot see will open once the engine is loaded.

Check runout between centres as well. A crank that is straight but has oval journals will still hammer the bearings.