The thing that the water pump bolts onto

Furthermore; The cylinder undergoes a very wide pressure change from a partial vacuum on the intake stroke to 100psi or more on the compression stroke
to 600/800 or more n the powerstroke, So you can bet that pressure will blow your rad cap off.
But you might not ever get to this point, cuz what Mike said, usually happens first.
You may hear it running into the pan when you start filling the rad back up. And you'll say bad words cuz that ain't supposed to happen.