Engine rotation question

[Jeff McCall]

While putting my 6.0 back together the rotating assembly, crank, piston and rods, cam, timing chain, all installed and the motor turnes over without much effort with a wrench on the crank. After installing the passenger side bare head (springs only, no rockers, no push rods, no spark plugs) i noticed while it would still turn over it is noticably harder and at every 1/4 turn there is a "sticking" point. Thinking this is nornal i bolted on the 2nd head, well this made it very hard to turn over with the same 1/4 turn sticking points. No heads turns smooth minimal effort, heads on very hard to turn over. Any ideas???