More info,

Just throwing something out there, If this was a full rebuild how did you get the water pump? The reason I ask is I had a situation once where I used the casting number to find a match, However the cast # is just that! For the cast. Ended up the guts were different and the impellers were opisite of what they need to be, Other than that I would need to know more, I have also seen air pockets trapped, (not usually in older vehicles) and have had bad thermostats right out of the box.

 

1940chevy engine 216 overheats

The pump is the original with a new thermostat and before i rebuild the engine was running fine ,i ran water through and it has a good flow, It takes about 5 minutes to overheats . The engine block and the head was overhaul by a machine shop .

 

possibly installed the thermostat backwards? it would cause those symptoms and is worth double checking cause its free to fix. other then that a small leak in the head gasket from the cylinder to the water passages could cause those problems. take some of the coolant out and see if its oddly discolored or smells funny. i would bet on a thermostat issue more then anything else, for the 8 bucks its worth just getting another one cause you could have a defective one also

 

1940chevy engine 216 overheats

I ran it with and with out thermostat and still does the same also has a new head gasket and i flushed the radiator .

 

if its doing it even without a thermostat and you know that your water pump is working right, then you almost def have a head gasket leak. i dont really know of anything else that would cause those problems. you can try pressure testing the coolant system and see if you get any results. but sometimes there will be a leak in the head gasket going to the coolant passage but the coolant wont have enough pressure to flow back through the gasket into the cylinders. i would take off the heads and take a strait edge and a feeler gauge to the heads and deck and see what the specs are cause there maybe be a machining issue form where you got it done. also check head bolt torque.

 

now that i think of it, do you know if your torque wrench is accurate? a little side thing but if the wrench wasnt handled proporly you may have over or undertorqued your head bolts. if you take your heads off check the thickness of your head gasket in several areas and see if its within crush specs of that particular gasket

 

Check to see if you are getting oil to the top end . The one i had i had to run a oil line to the head were the oil sender was to get enough oil. Just my two cents.

 

I have just finish rebuilding a 216 6 cyl chevy and now overheats, before was ok . Why ?

You don't say to what extent it overheats. I have rebuilt engines that ran a little hot for a few hundred miles until things loosened up somewhat. Tight bearings and rings might just be the reason unless your overheating is extreme.
Terry