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Discussion Starter #1
Hello,

New to the site! I have a 66 mustang, initially started life as a 289 2 bl, but was converted at some point to a 4bl, and also redone in a GT style. So at one point it probably had idiot lights, now gauges.

My oil pressure has been pegged out high on the gauge since I have owned it. Already checked the wiring and the signals. (pulsed) I was going to change the sending unit since it is cheap just to rule that out, but when I pulled it out it had 1/2 inch threads. All of the replacement parts available were closer to 1/3 inch. (pic attached of where it screws in). We searched surrounding year models and they were all the same. Do I need to replace the piece of steel (don't know the name) that this screws into? Or is there a compatible part for this setup?

Thanks in advance!
 

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Some one had buggered up the oil pressure extension. You will need a new one that hasn't been tapped out to larger size like yours has been.

NPD and Virginia classic mustang has them.

I would buy the sending unit from them as well, local parts store parts (sending units) are not very accurate
Z
 

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No need to buy a new extension. Just buy a pipe bushing to reduce the threaded hole from 3/8" NPT or 1/2" NPT (whichever size it is) down to 1/4" NPT (the size that the sending unit is).

Also, the sending unit is the ground for the oil pressure gauge. Inside the sending unit can is a variable resistance contactor. As the oil pressure increases the resistance goes down and the indicated pressure on the gauge goes up. If you remove the wire from the sending unit and touch it directly to the engine block the pressure gauge will peg on full High. Maybe the wire going to your sending unit is pinched or otherwise shorted out which would cause the gauge to read High. Does the gauge go to High as soon as you turn the key On? Or does it not go to High until you start the engine? In that case you have a bad sending unit.
 

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Discussion Starter #6
It is a can style. If I screw out the extension the unit will not fit back in that space to check.

Is the pipe bushing something I would get at a hardware store? I don't mind ordering a new one especially if the sending unit will be more reliable anyway.

Thanks for the answers! That is a big help
 

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It is a can style. If I screw out the extension the unit will not fit back in that space to check.

Is the pipe bushing something I would get at a hardware store? I don't mind ordering a new one especially if the sending unit will be more reliable anyway.

Thanks for the answers! That is a big help
You have to remove the sending unit from the extension to be able to remove or install the extension in the engine block.
Any hardware store will sell NPT bushings.
It's your choice- pay $24.95 for a new extension or $1.50 for a bushing.
https://www.npdlink.com/store/products/fitting_oil_sending_unit_repro-107487-1.html
Or
https://www.fastenal.com/products/details/69854
 

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Discussion Starter #10
No need to buy a new extension. Just buy a pipe bushing to reduce the threaded hole from 3/8" NPT or 1/2" NPT (whichever size it is) down to 1/4" NPT (the size that the sending unit is).

Also, the sending unit is the ground for the oil pressure gauge. Inside the sending unit can is a variable resistance contactor. As the oil pressure increases the resistance goes down and the indicated pressure on the gauge goes up. If you remove the wire from the sending unit and touch it directly to the engine block the pressure gauge will peg on full High. Maybe the wire going to your sending unit is pinched or otherwise shorted out which would cause the gauge to read High. Does the gauge go to High as soon as you turn the key On? Or does it not go to High until you start the engine? In that case you have a bad sending unit.
It goes to high as soon as I start the engine. Also the first thing I replaced was the wire extension that goes down and plugs into the sending unit. It had a break in it and I was worried it was grounding to the block.
 
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