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Discussion Starter · #1 · (Edited)
Yesterday I drove 1-1/2 miles to my highway mailbox and when I got back home I decided to check my coolant level ahead of my 500 mile round trip on Friday.

When COLD I keep it 5-1/2" below the top of my radiator's right tank and during my short 3 mile trip the coolant level rose to 2-1/2" below the top of the tank.

So it expanded a LOT in my 15 minute trip. When coolant expands it compresses the air above it so a radiator that starts off COLD at 5-1/2" below the top of the tank will be completely filled within 30 minutes or so.

At 212 degrees 5 gallons of a 50/50 mix will expand 1-1/2 gallons or almost 19%. If you have a big block with a 5-quart coolant recovery reservoir the reservoir will be mostly full when the coolant's temperature exceeds 212 degrees because it has exceeded your cap's 16# rating. No problem as the coolant will be sucked back into the radiator as it cools.

Big blocks need 6-quart coolant recovery reservoirs but all of the C3 reservoirs only hold 5 quarts so if the radiator is kept 5-1/2" (one quart) low everything works out.

A side note: My 205 degree thermostat consistently begins to open right at 7/8 mile and will jump to about 215 degrees and then plunge to 205 degrees.
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