Crankshaft Failure: How it Happens and How to Prevent It
Why Do High-Performance Crankshafts Fail? It’s Not What You Think
When someone invests in a high-performance crankshaft, it’s natural to assume it’s nearly bulletproof. After all, these components are built from premium materials, forged or billet-machined with tight tolerances, and designed to handle more power than their OEM counterparts. It's rare, but sometimes they do fail. Today, we're deep diving into what can happen, so you make sure it doesn't happen to your high performance BMW build.
In short: It’s almost never the crankshaft’s fault.
Most crankshaft failures in performance engines can be traced back to application problems—how the engine is built, tuned, or operated—not the quality of the crankshaft itself. Let’s break down the most common reasons high-performance crankshafts fail, even when the part itself is made to the highest standards.
1. Fatigue Failure
What it is: Just like bending a paperclip back and forth until it breaks, crankshafts can crack over time from repeated stress cycles.
Where it happens: Stress concentrates at specific points—fillets, oil holes, and journal transitions.
Why it happens: This is common when the rotating assembly isn’t properly balanced. Even a precision-made crankshaft can’t survive endless cycles of imbalance or vibration.
2. Overloading / Excessive Torque
What it is: A crankshaft can only handle so much twist before something gives.
Why it happens: Aggressive turbo setups, high-boost levels, nitrous oxide, or sudden traction changes (like launching on sticky tires with a trans brake or anti-lag system) can produce torque spikes that go far beyond the crank’s design limits. The result? Twisting or even snapping the crankshaft—typically near the journals where stress concentrates.
3. Improper Engine Assembly
What it is: Mistakes during assembly put uneven loads on the crank.
How it happens: Misaligned bearings, uneven journal clearances, or incorrect torque specs can introduce stress that leads to early failure. A crankshaft installed in a misaligned block or poorly set up bottom end can’t do its job reliably.
4. Hydraulic Lock (Hydrolock)
What it is: Liquid doesn’t compress. When water, fuel, or coolant gets into a cylinder, the piston stops dead.
Result: That sudden stop sends a shockwave through the crankshaft, which can crack or even snap it. This is a catastrophic failure—and again, not caused by the crank’s design.
5. Bearing Failure
What it is: Bearings act as a buffer between the crank and block. When they fail, the crank is in trouble.
Why it happens: Oil starvation, dirty oil, or incorrect clearances create metal-to-metal contact. This raises friction and heat, which can deform the crankshaft or cause it to break under load.
6. Detonation / Pre-Ignition
What it is: When combustion happens too early or too violently, it sends shockwaves through the engine.
Why it matters: Detonation hammers the crankshaft with force it wasn’t meant to take continuously. Low-octane fuel, bad tuning, or lean air/fuel mixtures are often to blame—not the hardware.
7. Crankshaft Misalignment or Flex
What it is: If the crank doesn’t sit true in the block, it’s constantly being stressed in unnatural ways.
How it happens: Poor block machining, warpage, or improper main bearing setup can cause flex that leads to cracks or failure over time—even on a perfectly machined crank.
8. High RPM Operation Without Proper Balancing
What it is: High revs amplify vibration. If the rotating assembly isn’t balanced, you’re asking for trouble.
Result: Harmonic vibrations build up at high RPMs and eventually cause fatigue cracks. A good crankshaft needs a good balance job to survive a high-revving engine.
10. Sudden Deceleration or Engine Lockup
What it is: When the engine suddenly stops spinning, the crank takes the brunt of the force.
Causes: Broken timing chains, seized valves, or transmission lockups can slam the engine to a stop, potentially twisting or snapping the crankshaft instantly.
The Bottom Line
A high-performance crankshaft is built to survive power levels far beyond what OEM components can handle—but only when the entire system is properly built and maintained. These failures aren’t due to bad materials or poor design. They’re due to real-world stress, oversight in assembly, or tuning issues.
Think of it this way: even the strongest crankshaft is only as reliable as the engine it’s bolted into. The key to crankshaft longevity isn't just in the part itself—it's in the care and precision behind the build.