Slightly off-topic--How to change an oil pressure sending unit?

You don't say what vehicle, but generally, no need to drain the oil.

Just get a new sender, unscrew the old with a pair of pliers, wrap a couple of loops of teflon tape around the threads of the new sender, and screw it in as tight as you can by hand (no tools). Start the engine and check for leaks - if so, turn it in a quarter turn at a time until it stops.

Thanks! I forgot to mention it's a 1994 Geo Metro with the 3-cyl engine. That's why it's an OT post. I believe you when you say I don't have to drain the oil, but am curious as to how it can leak oil if it's above the oil level? It's located on the back side, and there's no way the engine could ever generate enough acceleration to slosh the oil back... I suppose it could happen then I apply the brakes while backing down the driveway...

What kind of car is it? I have to replace the one in my Supra, the TSRM shows it at the front of the motor, but I'll be dammed if'n I can find it!

Hmmmm. Ever think what an "oil pressure sensor" is actually sensing? Do you suppose that there might be some sort of pump that puts that oil under pressure?

The oil pressure sender, as the name implies, senses oil pressure. When the engine is running, oil is pumped under pressure throughout the engine. In other words, it is leaking because of the pressure behind it, not because of the oil level.

Yeah, thanks, I am aware that it is under pressure. However if the level of the fluid level is below the hole, how does it get to the hole? Does the pressure cause the oil to flow up the side of the crankcase to the pressure sensor? Does the level rise when it is under pressure?

So, when the engine is off and there's no pressure, I remove the sensor, and no oil comes out the hole it was in, OK, that I've got.

But why, when I remove the sensor, doesn't the oil drain out of the hole it was in by gravity the same way it would out of any other hole in the crankcase? Is the sensor in some sort of oil passageway through which pressurized oil flows in order to get to where it's being pumped, and when the pump shuts off, the oil drains from that passageway back into the crankcase, leaving the sensor above the oil level?

Geo Metro with the 3-cyl. They may be sluggish but they sure are easy to work on. When I look at the engine, sometimes I think that if I was ever going to rebuild it, I'd just carry the engine into the house, wash it in the sink, and work on it on the kitchen table. ;-)

You have correctly answered your own question!

You could pretty much do that! I beleive the engine weighs about 90 pounds.

Or make it 'Newfie tight'...

I have amazed myself! ;-)

Thanks. :-)

Don't break the heavy-duty torque wrench on it then?

Amazing yourself is the best way to be amazed! You never have to worry about what others are thinking.