Oil line for gauge
 

Hi all

I’m wondering what is the proper size of tube for the oil line that goes to the gauge? Also does anyone have pictures of this lines route?

I’m interested in what was used on a Cab13 Chevy.

Next question...do you bleed the line or does the oil find its way to the gauge despite the air in the line?

Does it matter? The relative densities should keep the oil below any air. As the engine oil develops pressure, any air above it will develop an equal pressure and indicate the pressure on the gauge.


Regarding "proper" size, most (all?) civilian pattern Chevrolet gauges I've seen used a small/capillary line. I'm sure I've seen the round gauges with larger (3/16" or 1/4") OD line (the fitting on the back of your gauge may help on sizing) but I'm not sure if I remember capillary line being used as well.
Parts book C15A-04 section 1.519 lists 1/8" tubing (tubing is normally specified by OD as opposed to pipe which is by nominal ID) for use as "Oil Pressure Gauge Pipe" which is a bit larger than the capillary tube I'm thinking of.

If air in the line the oil would compress it possibly giving a false reading. It seems to me any time I've taken the line off the back of a gauge there's oil in it right up to the gauge. Probably over time the line bleeds itself.

Early square Chev gauges were capillary but the think the fitting on the back of a round gauge is 1/4"

How would air in the line give a false reading? If you put 20 pounds of oil pressure onto the pocket of air, it would compress to 20 pounds.



Re the line, while I can't speak as to the correct size for the Chev (but have usually used 1/8 tube with compression fittings), I would advise that you usually add a pigtail to the line at the engine to absorb the difference in movement from the engine to the cab. Otherwise, the line can, over time, become brittle at the point where it absorbs most of the flex, and break.


The actual thread for a 1/8 compression fitting is 5/18 x 24, the same as used on a 5/16 UNF nut or bolt. If that is the thread on your gauge, then that will be the tube size used.

In particular, the Daimler Scout Car has a long oil pressure pipe, possibly about 10 feet and you have to bleed them off at the gauge in order to get a reading, once done it is OK then. Have had to do this a number times, and only recently on a fire engine with a Rolls engine in it.

If I recall from my high school physics a gas will compress more than a liquid. Otherwise I'd use air in my hydraulics.

Absolutely, but since the supply of oil will be more than required for the line and to do the little amount of work required within the gauge, it will compress the trapped air to an equal pressure, which will be read by the gauge. . There may be a slight to negligible delay in the reading of the gauge as the air compresses or decompresses, but it will be too slight to notice.

Here is a link to the same discussion over on the HAMB: https://www.jalopyjournal.com/forum/...e-gauge.31196/


One fellow mentions something that I was thinking about re air in the line: It can act as a shock absorber if there is pulsing of the supply. Now that is not really a big problem on the engine oil pressure system, but I do run into it on well pumps and in some hydraulic and fuel systems. There are dampers/accumulators that can be installed to counter this.

Rob,
I have to disagree on your last sentence, referring to one particular vehicle, (Daimler Dingo). If you have been working on it and go to start up and nothing showing on the gauge, alarm bells ring. I can tell you that it is much more of a delay than you describe, possibly due to the length of pipe. There is a difference between theory and practice. Having had 52 years on engines you don't take chances!

Richard


In the case of the Dingo, or any other similar transmission of a signal over a long length of tube, I'll agree. But in the case of the usual 2 foot length of tubing as used on normal vehicles, It is not a great consideration.



I had an interesting experience with the plastic tubing for an oil gauge which I would like to relate. It was on a M38 Jeep that had been converted to 12 volt. I was driving it when suddenly I had a bunch of smoke and the oil gauge reading zero. I immediately shut off the vehicle and opened the hood. The oil line had somehow contacted with the exhaust manifold and melted open. It then melted shut. I was able to continue on my way. I'll never use the plastic stuff again.

You were lucky with the M38 incident!
Most of the armoured vehicles of days gone by, had engines in the rear with several Yards of pipe ( Metres to you guys :)). Nowadays it is all done by electrickery sensors.

I'm with you Rob. Many gauges come with a very fine capilliary tube. Some are long. There is no way to bleed the air out of them. The gauge is usually mounted higher than the other end and is not designed to be bled of air. Thick cold oil in the tube would have friction and be slow to move. However air would be instantaneous.

Lynn,
We are not talking sealed capilliary gauges, the ones I were referring to have a pipe nut connection at the gauge, you slacken the nut off and run the engine until oil appears, nip up the nut and it is all sorted.

Richard, My point is:
If you don't have to bleed the ones with a fixed capillary tube, and they work ok with a head of air trapped in them, then why would you need to bleed the others?

Great discussion and all but any pictures showing the route of this line?

:giveup :giveup :giveup Sorry Jordan.

Sorry Jordan.


Two options: take lots of pics as you dis-assemble or buy another wreck that is crappy but original as a reference (and maybe a future restoration).

Or run it where it make sense and replace it when you get better information (and pat yourself on the back if you got it right). The Chev manual unlike Ford is particularly unhelpful.

We will likely have some chevs out in the compound that have the line, but I won't be back to work until Tuesday. By then Jordan will be long past the oil line stage.

If you could check that would be great Rob. I’ve got enough other things to work on that the line can wait until Tuesday. Lol

Today I went down to the hydraulic fittings store for some help in finding something to fit the fitting on the back of the gauge. They told me the fitting on the original is a #4 sae. I ended up getting an adapter that goes from female #4 sae to female 1/8 pipe. Then a male 1/8 pipe to female 1/4 tube nut.

What have others used?

Compression fittings are normally used in that application.

You could use a JIC (?) nut and tail straight onto your gauge and use pushlock hose??? then you don't end up with that conglomeration hanging on the back of the gauge.

I found what I was looking for this morning at Home Hardware in their plumbing aisle.

I went with the long flare nut for the back of the gauge for better line support. The three way fitting will go on the block. That way I’ve got one line for the oil pressure gauge and the other will be for the filter.

I found a chev just outback that still has the line on it, although the engine has been removed.

Use the fitting you have in the photo, along with appropriate brake tubing. Flare the first end with that nut on it. From there the line goes around and up to the tp center of the cowl just beneath the channel under the windshield. At that point there is a regular brake line nut with flare and a union. On the other side, another flare and union. The line drops down the engine bay and is then cut on the example I have here.

We have a bunch more chevs at the compound, along with a few in the building. I'm afraid I can't post a photos on the computer right now as the government has issued me windows 10 and there is a compatibility problem with the camera right now. Hope to have that rectified tomorrow.

The line appears to be 1/4 steel brake tubing, but I'll get the digital calipers onto it tomorrow.

As a hydraulics engineer, that is the correct way to do this, not with the other ‘Christmas tree ‘ fittings from your hydraulic store! I’m surprised you found them in your hardware store, I’m impressed!, :thup2: