* Seized or Very Stiff Engine: This is the most probable cause. If the engine is seized (due to a lack of lubrication, a broken connecting rod, or a severely worn piston), the starter motor is fighting against an immovable force. The sudden, violent resistance can easily shear the timing key. A very stiff engine (due to lack of lubrication or extreme wear) might also produce enough resistance to break the key.
* Incorrect Timing: While less likely to directly cause a kickback *and* shear the key simultaneously, if the timing is significantly off (e.g., way too advanced), it could create enough compression resistance to put excessive strain on the starter and timing components. This is more likely to result in a hard start or bent starter gear teeth, but a severe mistiming *could* contribute.
* Broken or Worn Starter Components: A faulty starter motor (especially a weak one) could struggle to turn the engine over, resulting in a situation where the engine might try to backfire through the starter, causing a kickback. However, this is less likely to directly shear the key unless other issues are also present (like engine stiffness). A stripped starter gear or a worn bendix gear could contribute to the problem.
* Low Compression in One Cylinder (Less Likely): While not typically causing a kickback, extremely low compression in one cylinder (due to a blown head gasket, valve problems, or piston damage) might allow the engine to unexpectedly recoil when the starter motor attempts to turn it. It's less likely to directly shear the timing key but could contribute to the overall strain.
* Extremely tight compression (Less Likely): Although unlikely, if something has caused the engine compression to be exceptionally high (perhaps due to a major internal issue), the starter could be overwhelmed, even if timing is correct.
Troubleshooting Steps:
1. Check for a Seized Engine: Try turning the engine over by hand (using the crankshaft). If it's extremely difficult or impossible to turn, you almost certainly have a seized engine. This requires a full engine tear-down to diagnose and repair.
2. Inspect the Starter: Check the starter motor for proper operation. A battery test is also crucial – a low battery could lead to insufficient starter torque. Inspect the starter gear for any damage or stripping.
3. Check the Timing Chain/Gear: Once the engine is accessible (likely after addressing a seized engine), inspect the timing chain/gears for any wear, damage, or jump.
4. Check for Low Compression: A compression test will reveal if there's low compression in any cylinder, hinting at internal engine problems.
In short: The most probable reason is a seized or very stiff engine due to a serious internal problem. This needs immediate professional attention. Attempting to start the engine again without addressing the underlying cause could cause further damage.
