I'm going to put an aux engine cooling fan and I am debating where to put the thermo switch for the fan.

Which is a better place to put the temperature switch?
1. Screwed into the block (one of the empty threaded holes)
2. Screwed into the coolant passage (bleeder plug)

Which one has a higher temp - the metal block or the coolant?

 

I have installed a few of those. They usually come with one that sticks through the radiator. I didn't like the temps that were factory set so when I installed one in my 62 fairlane, I just installed a toggle switch for it, so that I can turn it on when I want it...Mark

 

It looks like the sensor that I have has a tip that needs to be in contact with coolant, so I'm gonna try that and stick it to the coolant bleeder port on top of the block. i'll be looking for one that is temp-adjustable.

 

All of the ones that I have installed had the sensor that poked through the fins in the rad. They had a clip to hold it in. I just cut it off and ran it to a toggle, of course I couldn't return it at that point. The electric fan on my MC has a sender in the coolant, but I also put a toggle on that one too, because I thought it should be coming on sooner, for what it's worth...Mark

 

I used a Flex-a-lite fan on my '72 Power Wagon mud truck.
It was their Black Magic fan.
It had a little temp sensor that picked up heat right off the radiator and was temp adjustable.
I installed and on and off switch on it as well.
I turned in off for deep water crossings so the fan blade didn't try to act like a boat prop. when the water level rose up into the engine bay.