Your 1968 mustang engine will turn by hand without the spark plugs but with in it not hand?

If your 1968 Mustang engine turns freely by hand without spark plugs installed, but won't turn with them in, it strongly suggests a problem with internal engine compression. The spark plugs aren't the direct cause, but their presence reveals the issue.

Here's why:

* Compression: When the spark plugs are in, the cylinders are sealed. If the engine has good compression, turning the engine requires overcoming the pressure built up in the cylinders. If it won't turn, it means there's significant resistance – possibly from:

* Hydrolock: Liquid (water, coolant) in the cylinders. This is a serious problem and requires immediate attention to prevent damage.

* Seized piston(s) or rings: A piston or piston ring could be seized, preventing movement. This often requires engine rebuild.

* Bent connecting rod(s): A bent connecting rod could be binding.

* Valvetrain issues: A sticking valve could be preventing compression release.

In short, the inability to turn the engine with the spark plugs in indicates a significant mechanical problem requiring professional diagnosis and likely repair. Do not force the engine to turn. Doing so could cause further damage.