The crankshaft's weakest points aren't the journals themselves — they're the fillet radii where the journal transitions into the crank cheek. That inside corner is a stress concentrator, and it's where fatigue cracks almost always start. A perfectly machined fillet on a forged crank will still fail at 60-70% of the load a rolled fillet can survive. This is why every high-output OEM crank — from Cummins diesels to Nissan's RB26 — gets fillet rolled as a final manufacturing step.

The process is beautifully simple. After heat treatment and machining, the crank is mounted in a fixture and a hardened roller (typically tungsten carbide, profiled to match the fillet radius) is pressed into the corner with 5,000-15,000 lbs of force while the crank rotates. The roller plastically deforms the surface layer maybe 0.010-0.020" deep, creating three effects that all fight fatigue:

• Compressive residual stress — the surface is squeezed into a permanent state of compression. Fatigue cracks need tensile stress to propagate, so the crack has to overcome this compression before it can even start growing.
• Work hardening — the cold plastic deformation increases surface hardness by 15-30%, raising the local yield strength.
• Surface smoothing — machining marks that would act as crack initiation sites get literally squished flat.

Real-world example: Chrysler's 6.7L Cummins crankshaft is forged 4140 steel, but the difference between the 350 hp base tune and the 400 hp HO tune isn't just fuel — it's the fillet rolling process pressure. The HO cranks get rolled at higher force, producing deeper compressive layers. Cummins rates the same forging for 30% more torque based solely on the rolling parameters.

Rule of thumb: Fillet rolling typically increases crankshaft fatigue life by 2x to 3x compared to the same crank with only machined fillets. Nitrided cranks add another 20-40% on top of that. This is why a stock Duramax crank survives 1,500+ hp when the block and rods fail long before the crank does.

The catch: fillet rolling only works on the specific radius it was designed for. If you undercut the journal during a regrind — even by 0.010" — you cut through the compressed layer and expose fresh, unhardened material at the new fillet. This is why rolled-fillet cranks should never be reground unless the machine shop can re-roll them afterward, which almost none can.