1/4" or 1/8" Thread for Oil Pressure Sending Unit?

[adamj]

For the oil pressure sending unit for the gauge (not the idiot light) what is the thread size? I'm seeing 1/4"-18 and 1/8"-27 at parts stores - both for the gauge. I want to order before I get in there. Also, is draining the oil necessary for this job?

 

[firebane]

Should be 1/4 as that is what I used and no you do not need to drain the oil. Just make sure its cool and sat for a bit.

 

[adamj]

Should be 1/4 as that is what I used and no you do not need to drain the oil. Just make sure its cool and sat for a bit.

Thanks for the response firebane!
Is your oil pressure gauge at the top, near the distributor, or is it lower by the oil filter?

 

[firebane]

Thanks for the response firebane!
Is your oil pressure gauge at the top, near the distributor, or is it lower by the oil filter?

Distributor. I have an aftermarket gauge but most people have the sending unit and a pressure unit.

 

[chengny]

http://www.gmsquarebody.com/forum/showthread.php?t=13924

 

[yevgenievich]

Older blocks it is 1/8 and newer 1/4

 

[firebane]

Older blocks it is 1/8 and newer 1/4

Mine is a pre 80s block and its 1/4"

 

[chengny]

The oil port behind the distributor is drilled - as stock - for a 1/8" - 27 NPT fitting. From the Hotrod Handbook:

Oil Pressure Port

Again, because of the configuration of the TPI intake manifold, it is necessary to use an angled adaptor fitting to gain access to the 1/8" pipe threaded (oil pressure) port at the back of the block near the distributor. The G M Part number for this adaptor which is angled at 105 degrees is #14078987. There are two fittings which can be added to this connector which will allow the installation of two switches or sending units, as necessary - GM part #14047850 or part #14078988.

Part # 14047850 is that special fitting that you always see (on most late 70's early 80's SBC's). It is used whenever the vehicle came equipped with factory gauges and an electric choke. It is basically a brass nipple with a tee pressed on to the top.

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The oil port above the oil filter is drilled for 1/4" - 18 NPT:

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[adamj]

Thanks for the link chengny, I have referenced your helpful images before in my searching. Let me back up here for a sec. I am upgrading to the gauge cluster from the idiot lights, and need to put in an oil pressure sender for the gauge. Do I have the option of putting it in either location? Is one better over the other? (maybe one's easier)

Also, I checked out my block and found this:

By dist:

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By oil filter:

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top view:

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dangling connector (I'm assuming):

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[chengny]

The sender is an electro-mechanical device that is used to position the needle - of the pressure gauge - in response to changes in oil pressure. It does this by providing a variable resistance (which controls current flow) to ground in the sensing leg of the associated gauge:

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The coolant temperature sender/sensor does the same thing for the temp gauge - but it's resistance is varied by temperature rather than pressure.

Both of those sensors have only one lead connected to them. The ground path circuit is completed through the threads of the sensor body and into the engine block.

This is why the use of teflon tape as a thread sealant is discouraged - the teflon will add increased resistance to the sensing leg and the gauge will not read accurately.


On the other hand, the body of a control switch (like the oil pressure switch) is not connected to ground. Also instead of producing a variable resistance/current flow, it is either open or closed. Consequently, it is has only two resistances - infinity or zero.


The point of all this is that the oil pressure sender can always be identified by the fact that is has only a single terminal.

The switch can have any number of terminals (and wires connected to it).

Bottom line - install the sender in the port that has a single wire going to it. And install the switch in the port near the multi-lead connector. That is, unless you have a lot of time on your hands and want to re-wire the engine harness.

One problem might be that you have trouble finding a sender with the correct sized threads for the port you want to use. A bushing okay is if the hole is oversized, but if the sensor's threaded connection is bigger than the hole it's going in, you'll have to use a bell reducer (or drill and tap to the next biggest size).

 

[yevgenievich]

I will add here that in my case, I have 70's block with 1/8th tap and had to use 90* 1/8th to 1/4 adapter for 91 style sender to work with 91 harness

 

[adamj]

Harness wiring is not an issue, I will be running a single new wire myself from the sender directly to the gauge for the resistance reading. I'm wondering why I would have two though... My 350 is TBI and doesn't have a choke. Shouldn't I just have one for the gauge/light? ECU does not read oil pressure, does it? (similar to separate CTS for gauge and ECU) The other strange thing is that I have that single orange wire in my harness even though I have idiot lights currently.

 

[Georgeb]

One is used for the light and the other is used to maintain current to the fuel pump.

 

[chengny]

Yeah, not really to maintain current to the fuel pump. The oil pressure switch used in the fuel pump relay circuit of a TBI system is only there to provide power to the relay in extended crank situations.

When the starter is cranked the ECM will power the relay directly - but only for 2 seconds. If, after that two second interval, the engine has not fired - the power supply from the ECM to the relay is shut down.

Obviously, there are many times when it takes longer than 2 seconds of cranking before the motor fires up. That is where the oil pressure switch comes in.

The pressure switch for an engine equipped with TBI is set to close at a much lower pressure than the switch used in a carbureted engine.

A TBI switch closes at 4 psi and a carbureted engine switch it closes at 10 psi.

So, if the engine doesn't fire up after 2 seconds (and the ECM power supply is lost) the oil pressure switch takes over. Just the cranking speed provided by the starter motor is sufficient to develop over 4 psi in the lubrication system. If a 10 psi switch were used instead of a 4 psi switch, the oil pressure developed during cranking would never get high enough to close the contacts. Consequently, the relay would not pull in and the fuel pump would stop.

At that point (when oil pressure exceeds 4 psi), the oil pressure switch closes and begins supplying an alternate source of current to the relay. This allows the fuel pump to continue to run and supply the TBI with fuel.

Eventually the engine fires and the ECM also begins to supply power to the relay. From then on - until the engine is shut off - power to the relay is available from both sources (pressure switch and ECM).

A couple of pages from the 1987 Light Truck Driveability & Emissions Manual - to help explain what I mean:

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Oh yeah, the point of all the useless information above is that I wanted you to be sure that the switch you install has a set point of 4 psi.

 

[Georgeb]

Yeah, not really to maintain current to the fuel pump. The oil pressure switch used in the fuel pump relay circuit of a TBI system is only there to provide power to the relay in extended crank situations.

When the starter is cranked the ECM will power the relay directly - but only for 2 seconds. If, after that two second interval, the engine has not fired - the power supply from the ECM to the relay is shut down.

Obviously, there are many times when it takes longer than 2 seconds of cranking before the motor fires up. That is where the oil pressure switch comes in.

The pressure switch for an engine equipped with TBI is set to close at a much lower pressure than the switch used in a carbureted engine.

A TBI switch closes at 4 psi and a carbureted engine switch it closes at 10 psi.

So, if the engine doesn't fire up after 2 seconds (and the ECM power supply is lost) the oil pressure switch takes over. Just the cranking speed provided by the starter motor is sufficient to develop over 4 psi in the lubrication system. If a 10 psi switch were used instead of a 4 psi switch, the oil pressure developed during cranking would never get high enough to close the contacts. Consequently, the relay would not pull in and the fuel pump would stop.

At that point (when oil pressure exceeds 4 psi), the oil pressure switch closes and begins supplying an alternate source of current to the relay. This allows the fuel pump to continue to run and supply the TBI with fuel.

Eventually the engine fires and the ECM also begins to supply power to the relay. From then on - until the engine is shut off - power to the relay is available from both sources (pressure switch and ECM).

A couple of pages from the 1987 Light Truck Driveability & Emissions Manual - to help explain what I mean:

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Oh yeah, the point of all the useless information above is that I wanted you to be sure that the switch you install has a set point of 4 psi.

Thanks for this! I needed a better understanding of how that all worked.