I believe that is where a road draft tube would go. I had a '65 200 that had one in it. However, my '66 170 and '69 250 just have that hole plugged. I do not know the purpose of a road draft tube though... someone else might be able to help with that.

 

The road draft tube, which IS what one might find there, is the crank case vent on cars before the PCV valve was used. The car in the photo should have a "freeze plug" in the hole, because as a 1966 Mustang, it should have a PCV valve.

The air cleaner is also installed wrong, an easy fix.

 

Road draft tubes were used prior to PCV emission systems. Allows the engine crankcase to breathe. The PCV valve allows a vacuum to be placed on the internal engine and runs the fumes into the intake to be burned along with the fuel-air mixture.

Road draft tubes just dumped below the engine and the motion of the vehicle created a vacuum or draft to the tube to help remove the air in the crankcase.

Also used on airplane engines of the era, called a pitot tube if I recall correctly

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Road draft tubes were used prior to PCV emission systems. Allows the engine crankcase to breathe. The PCV valve allows a vacuum to be placed on the internal engine and runs the fumes into the intake to be burned along with the fuel-air mixture.

Road draft tubes just dumped below the engine and the motion of the vehicle created a vacuum or draft to the tube to help remove the air in the crankcase.
...

Yep - had one on mmy 1960 Lincoln. Relieved air pressure inside the engine -- but often SMOKE came out of the tube. Blocked it off at the engine; then put on a PCV routing to the air filter so pulled back into the combustion chambers and out the exhaust.

 

Thanks for the info. The picture isn't my engine but now I know what the hole is for.

 

On my 67 there's a freeze plug there and then the power steering bracket bolts over it.

 

Here is a "what's this" for everybody ...

I removed the upper radiator hose outlet to change out the thermostat, and I found this little piece lodged in the bypass port of the outlet. Does anybody recognize this piece or have any idea where it might have come from?

View attachment 759208

 

Maybe it's just me, but clicking on your attachment keeps taking me to "The Track" section of the forum.

I have no idea how you managed to get part of the forum stuck in your thermostat though.