Internal Engine Failures:
* Seized bearings: This is a common culprit. Lack of lubrication (low oil, clogged oil passages, oil pump failure) leads to overheating and metal-to-metal contact, causing bearings to seize. This often results in a sudden, complete lockup.
* Rod bearing failure: Similar to seized main bearings, a connecting rod bearing can fail due to lack of lubrication, excessive wear, or a spun bearing. This will often cause a knock before complete lockup.
* Camshaft failure: A broken or severely worn camshaft can interfere with valve operation and potentially lock the engine.
* Broken connecting rod: A connecting rod can snap, causing catastrophic damage and immediate engine seizure.
* Piston damage/failure: A broken piston, piston skirt failure, or a piston striking a valve (due to valve timing issues) can lead to a locked engine.
* Valve train issues: A dropped valve or stuck valve lifter can prevent the engine from turning.
* Crankshaft failure: Though less common, a broken crankshaft is a devastating failure resulting in a complete lockup.
External Issues (Less Likely to Cause a Complete Lockup):
* Broken timing belt/chain: While this prevents the engine from running, it typically doesn't cause a complete *lockup* in the same way as internal failures. The engine will simply stop. However, if a broken timing chain wraps around the crankshaft, it *could* cause a lockup.
* Binding in the clutch: A problem with the clutch assembly in a manual transmission could prevent the engine from turning, but this is felt more as resistance than a complete lockup.
Determining the Cause:
Pinpointing the exact cause requires a thorough inspection. This usually involves:
* Oil analysis: To check for metal particles indicating bearing wear.
* Visual inspection: Once the engine is disassembled, visual inspection can identify broken parts, damaged bearings, etc.
* Compression test: Before disassembly, a compression test can help identify problems in individual cylinders.
In summary, a locked Chevy 400 small block is a serious issue almost always requiring engine repair or replacement. The most likely culprit is a lubrication-related failure leading to seized bearings. It's crucial to diagnose the problem accurately to determine the extent of the damage and the necessary repairs.
