When I replaced my thermostat (Gates 33918) recently, the spring side couldn't be placed in the thermostat housing. The spring had to be placed correctly (spring on block side).
That's good to know. I wonder if they did that intentionally to prevent backwards install.
...aftermarket thermo different than factory?
Apparently even the OEM replacements are different. The FSM shows a sketch of putting the stat in with the spring towards the block. Enough people have posted here over the years reporting problems when they did it the other way, and the problems being resolved when they re-did it the right way that the community here pretty much agrees that it needs to be in with spring towards block.
I am pretty sure the spring side goes into the block. IIRC there is also a small relief valve on the t-stat that has to be positioned correctly (on top I think)...
Correct on both counts.
If the t-stat was in the wrong way, I would think it would overheat since it is the engine heat that causes it to open up.
That is what consistently has been reported over the years.
proper bleeding is done with engine off
Can't disagree, but cracking the bleeder with the engine warm and pressurized works too. I find it simpler to do.
Note that there is evidence that some aftermarket thermostats tend to cause slow warm up and low settle-out temperatures - I have one like that in my 3.2. It could be a defect (stuck open), but in the future, I will only be using OEM thermostats on my Concordes.
Always a good idea to test any thermostat in a pan of water on the stove with a good thermometer to make sure it works before installing it.