[Sams66]

My 327 keeps building to much back pressure and purging all coolant from radiator . I am searching for a cure . Machine shop had to bore it .60 over attempting to keep original motor in my 66 elcamino . Any suggestions would help before I pull engine . It doesn't matter if it's at a idle or traffic speeds , and I am still in my first 250 miles of having engine redone .

 

[Crosley]

My 327 keeps building to much back pressure and purging all coolant from radiator . I am searching for a cure . Machine shop had to bore it .60 over attempting to keep original motor in my 66 elcamino . Any suggestions would help before I pull engine . It doesn't matter if it's at a idle or traffic speeds , and I am still in my first 250 miles of having engine redone .

Welcome to Hotrodders. Engine Cooling problems are discussed here in Basics forum. I edited the thread title so members have an idea of what information you are looking for. thanks.

 

[1979Malibu]

Welcome aboard!

What coolant temperature are you seeing with this?

 

[RWENUTS]

Could be as simple as wrong coolant level to a cracked block or warped heads and blown gasket!
pics of your engine bay could help.

 

[2old2fast]

Did you put the thermostat in upside down ?

 

[75gmck25]

How well did the cooling work before you did the rebuild, and exactly what parts did you replace during the rebuild? The plumbing of the SBC cooling changed over the years, and maybe parts are mis-matched (wrong water pump, etc.).

 

[ExcellenceAuto]

How well did the cooling work before you did the rebuild, and exactly what parts did you replace during the rebuild? The plumbing of the SBC cooling changed over the years, and maybe parts are mis-matched (wrong water pump, etc.).

Yes, did it overheat before? Any changes at all besides the rebuild?

 

[BogiesAnnex1]

This can also be a sign that compression is leaking into the cooling system. Usually a head gasket problem.

Also, typical of a plugged radiator especially if this still the OEM style vertical core. Reduced flow through the core leads to a lot of pressure in the top tank as the pump is feeding the return side faster than it’s receiving cooled fluid so the upper tank gains a lot of pressure blowing the cap to vent. It will do this till there basically there is no coolant left in the system. This is less common to cross flow radiators as the cap is on the cool tank side, but vertical flow has the cap on the hot return side tank.

Bogie

 

[ExcellenceAuto]

My 327 keeps building to much back pressure

Bogie! I sped right past this very important detail. And you have made a valid observation.
So, fortunately this is a very easy test to perform.

Block Tester BT-500 Combustion Leak Test Kit - Made in USA https://www.amazon.com/dp/B06VVBSFTF/ref=cm_sw_r_awdo_89Y2SE2Q664Y2EP74ZGQ

I got mine at NAPA.