Ok I guess it’s time to bring it to the pros. I’ve been on here for the last 3 days and nights reading every thread I can find about this. Tried a few things I found on here, but no remedy yet. This is a first fire up minus run on a motor stand after rebuild about 9 years ago. Start with: 1957 392 rebuilt basically stock with a very mild cam. Stock valves, not bored over... blah blah Hot heads BBC pump, cross over, intake. Blocked off at rear heads. 180 thermostat with two 1/8” holes drilled Bypass run from water pump to back of cross over (see photo) 16 radiator cap Speedway aluminum’32 radiator HH stainless freeze plugs cause first fire up and one stock one shot out... First problem was I left radiator cap off to burp it and temp got to 140 and radiator shot coolant like a geyser. I put cap back on and it seemed ok, sat at 195 for 10-15 minutes and I shut it off. It did push a lot out of the overflow tube though. This last time it gets to about 185 degrees and then bam shoots up to 220 and climbing and radiator pukes everywhere. thermostat checked in boiling water, opens as it should. Took gauge sender out and checked that in boil water with a thermometer, it’s good. I initially suspected temp sender because it’s very slow to read temp, then it falls extremely quick. Also my electric fan sender does not seem consistent. Like it’s not reading water temp, but maybe steam sometimes and water sometimes? Both senders are in HH manifold ports at the front of the motor. radiator is higher than block and I had the front wheels up on stands. Is it possible to have this much air in the system? where should I go with this next?? thanks!