Oil line for gauge

[Grant Bowker]

Originally Posted by Bruce Parker

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.

[Bruce Parker (RIP)]

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"

Originally Posted by Grant Bowker

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.

[rob love]

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.

[Richard Farrant]

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.