Overheating Rebuilt 350

[KDizzle]

Hello. Got an 86 K5. Originally came with a 305/TBI, but a 350/carb was installed back in 97 when I purchased. 350 block came from a 69-79 vehicle, based on my research. Block and heads rebuilt by machine shop this past April. Motor removed and installed by me. First time…somewhat mechanically inclined. Part of rebuild consisted of serpentine belt conversion. Got some pulley brackets from a 90 S10 V6 in a pick n pull along with a reverse flow GM water pump from Rock Auto for a C3500 5.7 pickup. Just attempted “break in” for 20 mins. Only got to about 15 mins before water was steaming/spewing/boiling from overfill and rad cap. Engine sounded fine, it just completely went nuts on the temp. I’m pretty sure I installed tstat correctly, even drilled hole in it as instructed. Not sure about bypass. Considering it was a fresh rebuild, wouldn’t think there would be an issue there. Don’t need a temp gun to know it was hotter than the surface of the sun. Someone suggested belt routing may be the issue?? Sure hope I didn’t screw anything up with the “semi” break in.

 

[Ricko1966]

Are you sure you got a reverse rotation pump? Lot of work, but big risk here if you are trying to break in a flat tappet cam. I'd pull the pump back off and make sure it's right. I'd also take the thermostat out and replace it after break in.Also inspect the pump, just because it's new does not mean it was assembled correctly, impeller not pressed on all the way, wrong impeller etc. If you didnt have overheating problems before. The most obvious cooling system change you made was the serp belt and water pump. What heads are you using? Was there a fan on it? We're you also blowing air into the radiator with a box fan?