Engine won't turn

Hi all.

I'm new, looking for help. I've been working on my 96 3.0 Caravan, had to remove a head.

I can't turn the engine over more than half a turn. It won't go to TDC or BDC. The timing belt is off, both cams are lined up so the pistons cannot hit the valves. What else could keep the engine from turning?

Thanks

Are the spark plugs installed? If yes, remove them.
Why did you have to remove the head?

There are no spark plugs installed. I had to take a head off to repair a spark plug hole. The #3 spark plug hole has been partially stripped for at least a year and I didn't realize it. The spark plug in the hole recently failed - threaded half blew out and the guts of the spark plug disentigrated. No visible damage inside the cylinder.

Before I took the head off I checked each piston under it for movement while I turned the crankshaft. All three pistons would go up and down. The #1 piston stops at the top and bottom of travel though.

The crankshaft stops just before TDC when I try to turn the engine forward, and stops just before BDC when I try to turn the engine backward. This was the case before I took the head and timing belt off.

Was you working around the flywheel and left a pry bar or something to block the engine. If a standard did you leave it in gear. I think you left something drop and block the engine. Drop a socket in the sparkplug hole I've fished them out with a small magnet.

There are no tools keeping the engine from turning.

Pull the valve cover and check if a valve spring broke and a valve is bent.
If it turned over before you pulled the head it isn't locked up so something has to be wrong. I would definately pull the valve cover and look.

The rear (right?) head is off. That is where the spark plug hole went bad.
All the springs and valves are intact on that head.

The front head is still on. I will remove the valve cover and check the springs.

Upon another check, the engine will rotate slightly bast BDC, turning the engine backwards. All the pistons are moving with the engine rotation so I don't think its a thrown rod.

So I checked the front pistons for movement when I turn the crankshaft. All are moving with engine rotation, though none are at compression where the engine is binding.

With the destruction of the plug hole, it's possible there is a piece of metal in the cylinder that's stopping the rotation. Might be a good idea to take a good look inside.

The front head is off. Nothing in the front cylinders but flakes of char. Now the cranshaft turns fine. The camshaft on the front cylinder head turns fine. All valves appear to be opening and closing okay.

The distributor assembly is attached to the front cylinder head. Could that have kept it from turning somehow?

I will put the front cylinder head back on and see if the crankshaft will turn and check the compression on each cylinder.

the timing belt should be on the engine when you are turning it, because at any cam position there are going to be valves pushed open so im thinking you might be hitting a valve, but hoping you didnt bend it. but since your belt is off and you do obviously need to time it, take off the rocker arms(careful when you do, there are little parts that come out that you don't want to lose) and this is going to allow your cam to spin freely and for your valves to close (assuming they're not bent but i hear they are pretty hard to) at this time you set your timing. After your timing is set, reattach the rocker arms and then give it a try