[Flathead Fever]

I was a phone company fleet mechanic for 30-years. If the thermostat slips out of the groove it will get caught under the housing on one side and create a small gap on that side and the housing will crack when you tighten it down. It's very common for this to happen when your first learning. I always used a contact gasket sealer like Gasgacinch, I put it on the gasket and the part with the recess for the thermostat and let them both dry until it's just a little sticky. When they are stuck together the gasket will not move, and it keeps that thermostat in place. Sometimes it's the intake that has the recess for the thermostat and sometimes it's the thermostat housing. If you use a silicone gasket sealer on both sides of a gasket the gasket will want to squeeze out of place with the silicone, especially on water pumps. I use the Gasgacinch on one side of the gasket so it cannot move and then just a tiny bit of silicone like a 1/8" bead on the opposite side. Be careful when you are scraping the gasket surfaces not to deeply scratch the parts which can cause a leak. Make sure the thermostat sensing bulb is pointed toward the intake. You might want to test it in some hot water first to make sure it opens all of the way. A lot of new thermostats are defective, always test them before you install them.

Sometimes when you first start an engine after replacing a thermostat air is trapped in the block behind the thermostat and it will not open. Most thermostats have a little air bleed hole that lets air out so the block can fill up but some do not. On the ones that do not I try and fill the block with coolant first by pulling a heater hose or coolant sensor, something that will let the air out of the top of the engine so the coolant can fill it up from the bottom hose.

Grab the upper hose and you will fill that's it's cold. When the thermostat opens you will feel the hose get hot.

Once it does open it will purge any air out, but you will need to top off the coolant. It's better to top off stuff when the coolant is up to operating temperature, but you need to be careful not to be burned. If you fill everything to the top when it's cold. when it heats up and expands its going to puke some out. That is normal. That's what I know about thermostats after replacing hundreds of them.