This page describes the various fittings required to connect factory original mechanical oil pressure gauges as well as keeping an "OIL" warning light functional when adding a gauge.
Factory GM Mechanical Oil Pressure Gauge
A mechanical oil pressure gauge was included as part of the optional gague package on Chevy/GMC pick-up trucks up until 1977 (78 & later are electric). Mechanical oil pressure gauges were also included with gague packages on some GM cars as well. This section will detail the fittings required to add one of these GM gauges to a vehicle that didn't originally have one or to repair an existing factory installation.
The factory installation on most vehicles used 1/8" OD steel tubing. This can be rather difficult to locate but 1/8" OD copper tubing (shown on the left in the above photo) is readily available and makes a good substitute. I have seen some factory installations that have a rubber sheath over the steel oil line for additional abrasion protection. That can easily be duplicated by slipping a length of vacuum hose over the steel or copper line. The oil pressure line is also subject to vibration and flexing from movement between the engine and body so it is a good idea to incorporate a few loops or coils in the tubing. That can be done by carefully winding the tubing a time or two around a section of pipe or other round object that's somewhere around 1-1/4" or so in diameter.
The GM mechanical oil pressure gauges utilize a special type of compression fitting to connect the 1/8" OD tubing to the port on the back of the gauge. A couple examples of these fittings are shown in the upper right of the above photo. They require the use of a 5/16" wrench.
Weatherhead, Edelmann, and Everco all offer replacements for these special fittings. Weatherhead calls them "threaded sleeve" fittings and part number 6100x2 is the one that fits 1/8" tubing. It differs slightly from the original GM fitting in that it has larger wrench flats (requiring a 3/8" wrench). A couple examples of these Weatherhead fittings are shown just below the originals in the above photo. Edelmann calls them "double compression" fittings and their equivalent part number is 111200. The Everco is #11A.
Factory installations also used the same "threaded sleeve" / "double compression" fitting described above on the engine block end of the tubing as well. They used a male connector with 1/8" pipe thread (Weatherhead #6200x2, Edelmann #112220, Everco #12A) to fit the threaded oil pressure port in the block. An example of one of these male connectors is shown at the middle/right in the above photo.
Small block V8's have the oil pressure port at the rear of the block (near the distributor and behind the rear edge of the intake manifold). Using a regular male adapter (as listed in the above paragraph) could make it difficult to reach the compression nut since it would end up somewhat recessed down behind the rear edge of the intake. So factory installations used a special taller version of the male connector. An example of one is shown second up from the lower right in the above photos. I'm not aware of a Weatherhead, Edelmann, or Everco equivalent of this tall male connector. If a good used one can't be located, a functional replacement could be formed by stacking a regular male connector (WH #6200x2 or equivalent) into an 1/8" to 1/8" NPT adapter (Weatherhead #3200x2, Edelmann #220220, or Everco 120A).
Many factory installations also made use of a two piece oil line connected together with a special union that also uses the "threaded sleeve" / "double compression" style fittings. An example of one of these unions is shown in the lower right of the above photo. Weatherhead makes a 1/4" version of this union but I'm not aware of any replacements in the 1/8" size necessary for this application. I believe the union was only used to make installation of the tubing easier on the assembly line (two shorter sections as opposed to fishing in one long section). So this part isn't absolutely necessary unless you're doing a 100% factory correct restoration.
Keeping the "OIL" warning light functional when adding a gauge
In many cases, swapping a factory gauge cluster in place of one that originally had warning lights will eliminate the "OIL" warning light. However, some factory gauge clusters still include an "OIL" light. When that is the case, I prefer to have both the gauge and warning light functional. Similarly, when adding aftermarket gauges I like to keep the warning lights functional as well.
Using a Tee fitting as shown in the above photo makes it possible to connect an oil pressure gauge while keeping the stock "OIL" warning light functional. Sometimes it is necessary to use a pipe nipple to space the Tee fitting out away from the engine for clearance. The example on the left in the above photo shows a 1/8" NPT x 1-1/2" brass nipple (Weatherhead #3327x2, Edelmann #213210, Everco #113A1) along with an 1/8" NPT brass Tee (Weatherhead #3700x2, Edelmann #201200, Everco #101A). A relatively short style oil pressure switch (for the "OIL" light) is also shown in this example. It is equivalent to a Standard Motor Products #PS-15 which will activate the "OIL" light if the pressure drops below the 3 to 9 PSI range.
In some cases, an 1/8" street Tee (Weatherhead #3750x2, Edelmann #227200, Everco #127A) can be screwed directly into the oil pressure port on the block. An example of this is shown in the middle of the above photo. Also shown is a stock long type oil pressure switch (for the "OIL" light) that is typically used on small block Chevy V8's. This switch is Standard Motor Products #PS-12 and also activates the light when pressure is below 3 to 9 PSI.
Some engines have 1/4" NPT pipe threads instead of 1/8" NPT for the oil pressure port. For those, it will be necessary to use a reducer bushing (Weatherhead #3220x4x2, Edelmann #210420, Everco #110C) as shown in the upper right corner of the above photo.
Shown in the lower right corner of the above photo is a typical 1/8" NPT to 1/8" compression male connector (Weatherhead #68x2, Edelmann #168220, Everco #68A) that can be used to join 1/8" steel or copper tubing to the Tee fitting when adding an aftermarket oil pressure gauge. This type of fitting can also be used on the engine side of a factory GM oil gauge installation but the special "threaded sleeve" / "double compression" connector described up near the beginning of this page must be used on the gauge end.