If the turning gear is properly engaged and the crankshaft still fails to turn over, check to see whether the cylinder test valves or indicator valves are closed and are holding water or oil in the cylinder. When the turning gear operates properly and the cylinder test valves are open but the engine still cannot be cranked or barred over, check for a serious problem. A piston or other part may be seized or a bearing may be fitting too tightly. Sometimes you may need to remove a part of an assembly to remedy the difficulty. Troubles that may prevent a diesel engine from starting.
Some engines have ports through which pistons can be inspected.
If inspection reveals that the piston is defective, remove the piston assembly. If the condition of an engine without cylinder ports indicates that a piston inspection is required, you must take the whole piston assembly out of the cylinder. Engine bearings must be carefully fitted or installed according to the manufacturer s instructions. When an engine cannot be jacked over because of an improperly fitted bearing, someone probably failed to follow instructions when the unit was being reassembled. You can trace most of the troubles that prevent an engine from cranking, but not serious enough to prevent barring over, to the starting system.