| So I saw KKroft (OH)'s write-up on the oil pressure sending unit replacement and decided to give it a whirl. Unfortunately things didn't turn out the way I'd hoped. In the end the replacement sending unit absolutely would not fit in the hole into the block! I was attempting to use the same sending unit that KKroft used below
 It was probably my first and biggest mistake not to check the fitment of the new sending unit before proceeding any further but that was a purposeful decision as well. Since I'd never taken the sending unit out before I wasn't sure if in the process I would lose my newly exchanged oil and make an unnecessary mess. So I proceeded in calibrating the aftermarket sending unit before checking the fitment. Calibration of the aftermarket sending unit was no small feat for me. Here are the details of my experience. I had to borrow my friend's air compressor to start with. I tore apart my Actron fuel pressure gauge connectors to enable the calibration set up shown in Kory's FAQ post. Luckily the aftermarket sending unit fit nicely into the T-connector that comes with the fuel pressure gauge. The threads don't match exactly but were close enough to get it on solid with an airtight fit. Next I went to numerous hardware stores trying to find the quick release nipple that matched both my friend's air compressor quick release coupling and the T-connector diameter and thread pitch. I ended up at Home Depot and got what I thought was a standard nipple. I brought it home and easily threaded into the T-connector but for some reason I wasn't able to mate it into the compressor coupling. After 15 minutes I realized the quick release nipple I had bought was 1/8 of an inch too long. No problem I thought, and sawed an eighth of an inch off the end with a hacksaw. I put everything together and although the nipple and the coupler fit together easily the coupling system wouldn't hold pressure. Looking closely I realized that the hacked nipple actually had a very slightly different profile than the typical nipple used off the compressor. GRRRrrr! So off I go to another hardware store trying to find the nipple that not only has the correct diameter and thread pitch, but also has the exact same profile as the ones used for this air compressor. With some luck I finally found one, brough it home, and installed it as shown below. See the difference? The silver one is the one I hacked. So great! The system is airtight. 
I connected the calibration system to the car with long wires and a system of alligator clips so that I could calibrate the potentiometer (final resistor) while looking at the oil pressure gauge in the instrument cluster. Well, the system was extremely finicky because it was very difficult to both hold 60 PSI for very long with the course air compressor adjustments, and the old potentiometer I was using was swinging the oil pressure gauge without me even touching it! Eventually I measured the equivalent resistance to calibrate the system at 4.2 ohms. Below check was done at 30 PSI. 
Unfortunately I don't have any resistors at home. Realizing that I probably wouldn't be able to get exactly a 4.2 ohms resistor I did some math to determine what system of resistors would get me close enough. So off to RadioShack I went. Their selection of individual resistors is poor so I ended up having to buy a $16 kit of like 500 resistors from which I could string together the appropriate resistors to match the 4.2 ohms value. 10 minutes of soldering and shrink wrapping later I installed the finalized sending unit in the car and turn the key. The damn thing is off by 10 PSI! So I re-calibrate the system by trial and error connecting the small resistors and series until I get the needle to line up. 
The new finalized resistance is now 6.6 ohms. Part of the difference is most certainly due to me not accounting for the resistance of the long wire in the calibration. 
Now that my replacement oil sending unit is finely calibrated I take out the OEM piece. A little bit of oil comes out but then stops. I try for a good hour trying to "jimmy" the new oil sending unit into the block to no avail. I compare the aftermarket and OEM sending units and the diameter seem identical and the thread pitch differences are hardly discernible. In fact, both the OEM and aftermarket sending units fit nicely into the fuel pressure gauge set up but only one goes into the engine block! GRRrr! I ask another friend of mine to try and use his tap and die set to re-thread the aftermarket sending unit to better match the OEM piece but his set does not include the exact die we need to get the aftermarket sender into the block. He measures the OEM major diameter to be 0.391" whereas the aftermarket piece as 0.393". In my desperation I tried to use a Dremel to slightly taper the nose of the new sending unit. No matter what the new sender will not go into the block. Now the only options I have left are to re-tap the block (bad option), look for or construct an adapter (which might break due to fatigue problems over time), or find and buy the correct die and hope that there is enough meat on the shaft that using it won't destroy the threads altogether. No doubt this is a lot of effort to be going through to simply swap out the oil pressure sending unit. Based on KKroft (OH)'s write-up I had no idea that this was going to require so much effort. Hindsight is always 20-20. But I checked out my OEM sender and it was 20 PSI off! And the OEM ones don't last for more than a couple of years. So in terms of getting the sending gage working you're damned if you replace with OEM, and you're damned if you don't. Now you know. FAQ?? P.S. Before starting this project I asked KKroft if he could tell me the resistance I needed for this unit and thereby avoiding all this calibration B.S. He replied: Unfortunately there is no way it will work unless properly calibrated. Nissan chose to make the circuit very sloppy since in reality as long as there is any oil pressure at all things are probably OK... The 300zx circuit is variable in the [instrument] cluster. The wiring to the cluster varies and of course the sending units are all over the place. My process corrects for all the sources of error but it has to be done in place for it to work. Why is the system so sloppy/sensitive anyway? A little math shows the gage reading is proportional to 1/(R^2). dI/dO=(-V)/R^2 So changing the resistance one ohm makes a big difference. KKroft noted that a more accurate answer can be goten by calibrating with the engine running but this is not worthwhile. This is because there must be a very effective voltage regulator upstream of the oil sender, plus the sensitivity to voltage changes is not as strong: dI/dV=1/R Calibrating with the car running had no noticeable change. oil sending sender unit oil sending sender unit oil sending sender unit
Zarcasm(Hfd, CT)
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Oil Pressure Gage Sending Unit Replacement Woes - 
