My thermostat housing leaks ONLY with the stat in it.
 

I dont get it, what am I doing wrong? I have a new gasket and a new housing. It only leaks with the thermostat in. I had assumed it was the gasket, I bought a new one, went to put the t-stat in, and the housing cracked. I bought a new one. I'm still running into the same thing. NO leaks at all without the t-stat, but once I put the t-stat in it pukes everywhere.

The surfaces are clean, new gasket, new t-stat (tried with 2 different t-stats just in case I had the wrong one), and a new t-stat housing. Anyone else ever run into this?

You are getting the thermostat cocked when you are putting the housing back on. Put the thermostat into the block first and glue the gasket over it. The housing needs to go on as close to straight as you can get it.

9/10, this comes back to operator error (not what you wanted to hear, I'm sure...)

1) Make sure you're getting the thermostat & gasket on in the right order. There should be a recess in the head to set the thermostat into, then the gasket goes overtop of it.

2) Make sure the thermostat is staying in its recess - a bit of RTV usually helps this.

3) Make sure the gasket isn't slipping and getting torn - the layer of RTV you put on it not only helps it seal, but offers some lubrication if you have to move it.

The usual cause if it's not operator error? Make sure that the housing isn't cracked near a screw hole - you may have to drill and tap a metal plate to accept the screws (I eventually did, I was running into this fairly often...) and torque it down to a plate and inspect it. Sometimes, a crack will only show up when the part is strained - I've had the same thing happen with starter motor nosecones as well.