[sunrise7]

I have a small block Chevy engine that is currently mounted on my engine stand. The oilpan, intake and exhaust manifolds, and valvecovers are off.
Heads are on with sparkplugs out. From below, all looks good with pistons, rods and crankshaft. Here's my problem.... I can only turn the crank one revolution ( by hand ) and then something jams and stops it from turning any further. I loosened all the rockers to rule out a stuck valve. Distributor is not in either, so nothing can be jammed on that end of the camshaft. Any ideas ??
I was thinking maybe something got into one of the two cylinders that have the piston coming up to the top at the point where it always stops turning or maybe something wrong under the timing chain cover, it is still on, like a problem with the chain ... but how likely is that since it does turn quite effortlessly for one turn ..... What should I check ? Thanks....