78 with a stuck engine

[Easter]

Stuck, not frozen. I picked up this 78 last weekend and discovered pretty quickly that the engine would not turn over. Knew it was a possibility but the bike has a lot of good parts and although a lot is missing it doesn't have much rust or corrosion and it was cheap.
Anyway, pulled the plugs and filled the cylinders with penetrating oil and let sit for a couple of days. Tried to turn it over and was momentarily elated to have it brake loose and start to turn over, but unfortunately it only turned about 180 degrees before it came to a solid stop.
So this morning while hanging around before my sometimes "annual physical", I pulled the heads, first one then the other thinking I would find bent valves or something. Cylinder number four had some red crud in it, rust, water, old gas, penetrating oil but nothing sufficient to prevent the piston from cycling. It all wiped out with a rag and to my inexperienced eye, doesn't look that bad. But the engine still will not turn past the half-way mark.
I suspect I can go no further without pulling the engine but i am curious if anyone cares to venture a guess about what is going on.? Where should I go next?

[desertrefugee]

Sounds like bad mojo. Bottom end issues. Rod let go maybe? If it's truly a "solid stop", with the heads off, it can only be bottom end.

Not sounding good. Maybe spin it in the other direction and see what happens...

[TNTbefree]

No idea, but good luck.

[Whiskerfish]

pull the stator cover and make sure that bolt is in there properly.

[Easter]

It moves freely both directions till it hits the stop. Will check out the stator next, (sooner or later) when I pull the engine. Got a couple of other projects to finish first.

[Easter]

Just a quick update. Pulled the engine over the holiday, and tore into it tonight. Started with stator cover and all was good. Pulled the heads back off and took a more critical look at cylinder number 3 where all the crud was collecting. My first thought was that it wasn't sufficient to keep the piston from moving but after looking at it more carefully I began to suspect that was the cause of the problem. It was odd though that the piston would move freely until it reached a certain spot in the cylinder and would then stop as if it had encountered a solid object. Anyway, I ran a wire wheel on the drill motor and cleaned up the cylinder and sure enough the piston would move easily through its whole cycle. So now I have a totally disassembled short block to clean and put back together. I suppose it only makes sense to do the valves while I am in and of course all new gaskets, water pump and probably clutch. I do have a '76 frame that can be titled but it is pretty rough. So I am not sure what to do with this engine. I doubt the bad cylinder will ever have full compression but hopefully the fresh valve job will help.

[TNTbefree]

That's good to hear. Hope I have as good of luck when I get to the stuck engine I have out back.