First few runs after engine rebuild and she over heats

[rindel]

Hey guys,

I spent all day today with my father n law troubleshooting why the engine gets so high when I drive it for about 20 minutes. The needle goes to about 80% or just before high. We checked that the thermostat opens up by removing it and putting it in boiling water. We also switched from a 180 to 160 thermostat so it opens up at a lower temperature. With the 160 thermostat in, we confirmed water pump works by seeing the water move with radiator cap off. We also noticed that its burps (bubbles) about every 3 seconds when she is high. Someone at the auto store told us that it is trapped air and so we removed the temperature sending unit, filled radiator with anifreeze until she spilled out of the sending unit outlet, and then put the sending unit back in. Now she doesn't bubble/burp but the temperature still climbs to about 80% of high. Question is if this is normal after a rebuild? I remember on another thread that someone mentioned their thermostat going past the 50% point as well.