* Connecting Rod Bearing Clearance: The most probable cause is incorrect connecting rod bearing clearance. If the bearings are too tight, the crankshaft will be difficult or impossible to turn. Even a slightly off spec can cause significant drag. This needs to be checked with plastigauge or a similar measurement tool.
* Connecting Rod Bolt Torque: While you mentioned checking torque, ensure the bolts were torqued in the correct sequence and to the *precise* specification. Incorrect torque or sequence can lead to binding. Also, check for any stretched or damaged bolts.
* Connecting Rod Alignment: If the rods weren't installed perfectly aligned with the crankshaft journals, it could cause binding. Misalignment can also damage the bearings.
* Rod Bearing Damage: During installation, the bearings might have been damaged (nicks, scratches). Even small imperfections can create significant friction.
* Crankshaft Damage: While you mentioned checking the crankshaft, a possibility remains that damage occurred during the rebuild process, perhaps unnoticed initially but exacerbated by the rod installation. A visual inspection with a magnifying glass might help.
* Interference: Although less likely, something might be interfering with the crankshaft's rotation. This could be a slightly bent connecting rod, a misplaced component, or an issue with the oil pump or related parts.
Troubleshooting Steps:
1. Remove the oil pan: This will allow for easier visual inspection of the connecting rods and crankshaft.
2. Check connecting rod bearing clearance: This is the *most crucial step*. Use plastigauge to measure the clearance. If it's out of spec, you'll need to replace the bearings with the correct size.
3. Inspect the connecting rod bearings and crankshaft journals: Look for any damage, nicks, or scoring.
4. Verify connecting rod bolt torque: Double-check the torque specification and ensure proper tightening sequence.
5. Inspect for any interference: Carefully examine the assembly to make sure nothing is obstructing rotation.
6. Check crankshaft end-play: Excessive end play can also contribute to difficulty turning.
If you lack experience with engine rebuilding, it's best to seek the help of a qualified mechanic to diagnose and correct the problem. Continuing to try to turn the engine over with excessive force could cause further damage.
