Small Engines (Lawn Mowers, etc.): Motor wont turn, briggs stratton, piston area

Question
I have an older (1970's) Briggs & Stratton 5hp motor.  I changed the oil, put in fresh gas. I also removed the head and gasket and cleaned the underside of the head and the top of the piston area.
When I put it all back together and tried to start it, after about five pulls the motor "locked" and I could not pull the rope.
I notice now that when the head bolts are loose everything cranks just fine.  When I tighten them, the motor locks up solid.
Any advice?

Answer
Hi Brian,

On some engines the bolts are different lengths.  Check to make sure they are not in the wrong holes.  Other than that, maybe the piston is hitting the head or the valves are obstructed.  Other than that, I can't think of anything else.

Let me know how you make out.
Michael