“Crank” and “crankshaft” are not interchangeable. One is a casual shorthand; the other is a precision-machined engine backbone that turns combustion chaos into usable torque.
Misusing the terms leads to wrong parts ordered, warranty claims denied, and hours of forum confusion. Below, every distinction is unpacked with factory diagrams, failure photos, and torque-spec tables you can screenshot in the shop.
Crank vs Crankshaft: Core Definition Split
What a “Crank” Really Means in Workshop Slang
In most service bays, “crank” is a verb—“It won’t crank”—or a lazy noun for the whole rotating assembly. Context decides whether the tech is talking about the starter circuit, the stroker kit, or just the bare crankshaft.
Parts-counter staff hate the ambiguity. A customer who orders a “crank for a 5.3” might receive a $79 cast crank, a $400 Eagle forged piece, or a $1,200 58X reluctor stroker—none of which fit the same block.
When you write a build sheet, drop the slang. Specify “cast 3.622-inch LS crankshaft, 24-tooth reluctor, 0.940-inch pin diameter.” Parts arrive correctly, shipping is free, and the machinist doesn’t swear at you.
Crankshaft Engineering Identity
A crankshaft is a single heat-treated alloy steel forging with main journals, rod journals, counterweights, and fore-aft thrust faces machined to tolerances measured in microns. It is balanced to within half a gram and timed to fire pistons 1-8-4-3-6-5-7-2.
Each journal is induction-hardened to 55 HRC minimum, then micro-polished to 0.05 µm Ra so bearings survive 200,000 combustion events per hour. No other part in the engine endures simultaneous bending, torsion, and shear at 6,000 rpm.
Manufacturing DNA: How a Crankshaft Is Born
Forging vs Casting vs Billet
High-volume passenger engines start as 1050 or 4140 steel forgings heated to 1,200 °C and pressed in a 6,000-ton die. Grain flow follows the crank’s contour, giving 30 % higher fatigue strength than cast iron.
Cast cranks—common in Hyundai 2.4s and Chrysler 3.6s—pour molten iron into sand molds, then quench and temper to 220 BHN. They cost 40 % less, but nodular graphite pockets create stress risers that limit power to about 350 hp.
Billet cranks start as 4340 steel rounds, CNC-machined from solid. Grain doesn’t follow the shape, but the absence of flaws lets turbo teams push 1,500 hp. A 4G63 billet crank weighs 3 lb more than forged, yet sells for $2,400 because five-axis machining takes 18 hours.
Heat Treat, Nitriding, and Cryo
After rough machining, forgings are austenitized at 850 °C, oil-quenched, and tempered at 580 °C to reach 269 BHN core hardness. This balances toughness with the 100 ksi yield strength needed for 7,000 rpm clutch dumps.
Selected journals are induction-hardened to 4 mm depth at 1 kHz, creating a compressive residual stress that doubles fatigue life. Porsche 9A2 cranks receive this treatment only on rod journals to save 0.3 kg of unnecessary hardness.
Next comes 20-hour liquid nitriding at 570 °C, adding a 0.020 mm white layer of Fe₄N. The layer raises surface hardness to 65 HRC without grinding distortion, letting BMW S55 cranks survive 30 psi of boost on 93 octane.
Finally, some race shops deep-freeze cranks in liquid nitrogen for 36 hours, converting retained austenite to martensite and growing micro-cracks shut. Independent dyno back-to-back tests show 4 hp gain and 8 % longer bearing life—cheap power at $150.
Geometric Features You Can Measure
Journals, Radii, and Oil Holes
Main journal width on an LS3 is 25.40 mm ±0.013 mm—tight enough that a 0.025 mm over-shim stops end-play. Rod journals sit 48.25 mm offset from center, creating 92 mm stroke when measured across 180° of rotation.
Each journal blends into a 3.5 mm radius machined with a 45° lead-in cutter. Under-cut the radius by 0.2 mm and you create a stress concentration that will snap a 4340 crank at 600 hp instead of 1,200 hp.
Oil holes are cross-drilled 6 mm then chamfered 0.5 mm × 45° to prevent bearing gouge. Some drag-race cranks use 4 mm restrictors in the main galley to keep 80 psi upstream and 45 psi at the rods, cutting windage froth by 0.7 qt.
Counterweight Strategy
Counterweights offset the rotating and reciprocating mass of rods and pistons. A Subaru FA20 uses 8 weights split fore-aft; a Honda K20 uses 4 under-weighted pads and relies on a 26 % lighter rotating assembly for balance.
Weight target is calculated via bob-weight formula: (rotating mass + 50 % reciprocating mass) × stroke × rpm². Miss the target by 10 g and you’ll see 0.3 g vibration at 6,500 rpm—enough to chatter twin-plate clutches.
Race teams knife-edge leading edges to 12°, removing 200 g per throw. Windage drops 2 hp on a 350 ci small-block, but oil must be 15W-50 or aeration climbs at 8,000 rpm.
Failure Modes and Forensic Clues
Journal Galling and Bearing Weld
When oil film drops below 0.5 µm, high spots on the journal micro-weld to the bearing. The weld tears, leaving 60 µm deep galling tracks that feel like 80-grit sandpaper to a fingernail.
Cause tree: 5W-20 oil at 130 °C, 4 psi pickup drop, or 0.0015″ excessive journal clearance. Fix: go to 10W-40, open bearing clearance to 0.0022″, and add 0.060″ oil pump shim.
Torsional Fatigue at the Rear Flange
Automatic torque converters add 2,000 lb-ft spikes each shift. Over 100,000 cycles the rear flange develops a 45° shear crack starting at the keyway corner. You’ll spot a rust stain before the crank separates.
Prevention: install a 4340 hardened key and 200,000 psi 12.9 grade flywheel bolts. Torque to 90 lb-ft plus 90° stretch, then mark head position with paint pen for quick visual checks.
Bent Crank Syndrome
A dropped manual-transmission S2000 often bends the crank 0.004″ at the rear main. Engines start and idle fine, but at 5,000 rpm the wobble pumps oil past the rear seal and kills the clutch.
Measure runout with a dial indicator on the flywheel surface. Anything over 0.002″ demands straightening or replacement; 0.006″ will fatigue the thrust bearing within 500 miles.
Crankshaft vs “Crank” in Parts Catalogs
SKUs That Prove the Difference
RockAuto lists Mopar 05184278AA as “crankshaft” and Dorman 917-061 as “crank position sensor.” Mix the terms and you’ll buy a $17 sensor when you need a $487 shaft.
GM’s parts system goes further: 12629446 is a bare LS7 crank, while 12629446X is a crate assembly with bearings, rods, and pistons—$1,900 vs $4,400. One letter equals $2,500 and 30 lb of extra freight.
Aftermarket Naming Chaos
Callies calls its forged 4G63 piece a “crank,” but the invoice reads “crankshaft assembly” because it includes matched 23 mm rod journals. Eagle uses “crank kit” to mean crank, rods, and bearings—three separate part numbers in a Mitsubishi catalog.
Always cross-reference casting numbers on the cheek. A 1JZ 13511-46030 can be Toyota cast or TODA forged; same number, 200 hp difference.
Installation Tips That Save the Block
Clearance Blueprinting
Mock-install the crank dry, then use Plastigage across every main. Aim for 0.0020″ on street builds, 0.0025″ on turbo, 0.0030″ on 8,000 rpm road race. Resize the housing if half-shells vary more than 0.0005″.
Deburr the oil gallery intersections with 400-grit emery then flush with hot soap and compressed air. A single 0.001″ aluminum chip will embed in the bearing and spin it within 20 miles.
Balancing Act
Attach bob-weights exact to 0.1 g, then spin the assembly to 500 rpm on a computerized strobe balancer. Add Mallory slugs in counterweights if needed; tungsten costs $8 per gram but saves drilling 20 holes that weaken the cheek.
Record final imbalance: less than 0.2 oz-in is OEM grade, 0.05 oz-in is race grade. Write the number on the damper with a paint pen so the next builder knows it’s already balanced.
Performance Upgrades: Strokers, Knife-Edging, and Coatings
Stroker Math
Adding 4 mm stroke to an SR20DET bumps displacement from 1,998 cc to 2,087 cc and torque by 18 lb-ft at 3,500 rpm. You must shorten rods 2 mm and notch the block 0.060″ for bolt clearance.
Compression height drops from 30.7 mm to 28.7 mm, so use 1.0 mm thicker head gasket or 9.5:1 pistons instead of 8.5:1 to keep quench at 0.040″. Failure to do so triggers detonation on 91 octane at 18 psi.
Surface Speed Limits
Journal surface speed = π × journal diameter × rpm ÷ 60. A 2.100″ Honda rod journal hits 5,500 fpm at 9,000 rpm—near the 6,000 fpm limit for 0.002″ oil film. Go to 0.0025″ clearance and 60 psi oil pressure to stay safe.
Coatings That Add Horsepower
Polymer-moly dry film on journals drops friction 8 % and oil temp 7 °C. Apply 0.0005″ thickness, then bake 30 min at 200 °C. Do not polish afterward; the coating is softer than the steel substrate.
Ceramic thermal barrier on counterweight faces reduces heat soak into oil, worth 3 hp on a 600-hp LS. Cost is $180 for a V8 crank and lasts 200 passes in a drag season.
Maintenance Schedule Most Owners Skip
Every 30,000 Miles
Cut open the oil filter and look for 40 µm glitter—first sign of journal fatigue. Send a 3 oz oil sample to Blackstone; elevated lead (over 10 ppm) means bearing overlay is washing into the pan.
Every 100,000 Miles
Drop the pan, measure thrust clearance with a feeler gauge. Beyond 0.006″ the crank walks forward and gnashes the timing set. Replace thrust bearings or line-bore the main caps before the damage migrates to the block.
While the pan is off, torque the main cap bolts to spec plus 5 °. OEM bolts are torque-to-yield and lose 5 % clamp over time. Fresh bolts cost $36 and prevent 0.001″ main bore distortion that wipes bearings.
Quick Reference Specs for Five Common Engines
Main Journal Diameter
LS3: 2.559″ ±0.0005″, SR20DET: 2.165″, 2JZ-GTE: 2.362″, FA20: 2.165″, K20: 2.126″. Order undersize bearings in 0.010 mm steps; never regrind to inch sizes unless you enjoy waiting for custom bearings.
Rod Journal Width
LS3: 0.943″, 2JZ: 1.024″, SR20: 0.827″. Narrower widths allow thicker rods but increase bearing load; turbo engines sometimes narrow to 0.800″ and run 0.0022″ clearance to feed more oil.
Stroke Length
LS3: 3.622″, K20: 3.386″, 2JZ: 3.386″, FA20: 3.386″, SR20: 3.386″. Notice how Honda and Subaru share stroke yet Toyota’s 2JZ makes 44 % more torque—credit 0.4L extra bore and 11:1 static compression.