VCCA Home Forums TECHNICAL - GENERAL Upholstery - Interiors & Glass Air in your lines, Oil Pressure that is.

I’m assuming that this would be the proper place to discuss the subject of gauges?
During engine rebuilding the oil pressure line is disconnected and usually drained of oil. When it is reconnected it is obviously full of air which will end up at the gauge head. Since air is compressible and oil is not, it seems that the gauge reading would not be an accurate display of the oil pressure. Bleeding of the line would seem to be the most logical approach, but I would bet the farm, that it is very rarely done.
So I’d like to hear some of the old timers opinions on this.
Denny Graham
Sandwich, IL

hey denny
since nobody got back to you i`ll tell how i bleed oil lines going to the gauge.
tighten up the line from the oil block on the engine and only hand tighten the line going to the gauge. start the engine and you should see oil drip out behind the gauge. once you got oil leaking from behind the gauge. tighten it up. i have been doing that on all mech. oil gauges for years..

Denny,
Sorry but didn't see your post earlier. It is not necessary and also not recommended to bleed the air from the line. The compression of the air does not change the amount of pressure exerted on the Bordon tube except during dramatic changes or very low oil temperatures. The air cushions the tube from pulsations or shock. Therefore it is far better NOT to bleed the air from the tube as it provides a little protection.

Also the air is not affected by temperature like the oil is. At very low temperatures and high oil viscosity the gauge response is delayed if full of oil. A second reason to not bleed off the air.

Thanks guys, a little food for thought from two opposing views. I’ll have to give that a little more thought Forest. Since the Bourdon tube is simply a hollow coil spring I liken it to a brake system or any other hydraulic system, which has air in it and is biased by a spring. We all know that a hydraulic brake service system doesn’t work very well when full of a compressible fluid and of course, that’s why we bleed it. A hydraulic jack or as far as that goes any hydraulic cylinder operates under the same principals. Therefore it seems more logical that the gauge would read lower than the actual oil pressure if there were air being compressed against the spring pressure of the Bourdon tube. If at the time of manufacture however, the gauge were calibrated using a compressible fluid such as air then what you say may be true. But if the calibration was done with a non-compressible fluid then it would not be accurate with air in the system.
I'll suppose the only way to really prove this one out is to set up an experiment on the bench and get some real numbers. Wish I had an old oil gauge laying around, I’d set up a test just so I would know for sure next time this comes up.
I would agree with you that the viscosity of the fluid would certainly effect the response time, that is a logical assumption.
Well, thanks for voicing you view of the subject, but you know what they say about opinions………”opinions are like @$$ &*!&$ every body has one.” Who knows, maybe we’ll hear from an engineer who has already done the research.

Denny Graham
Sandwich, IL

p.s. By the way, I dint know just where this sort of question would fit in so that's why I figured interiors (like dash, instruments) might be the best place for it.

Denny,
The Bourdon tube is formed in a curve and tries to straighten when pressure is applied internally. It is the deflection of the tube that is mechanically linked to the needle on the gauge face. The needle linkage presents little resistance to the movement of the gauge and when higher pressure is applied the system compensates for the change by changing the dimensions of the gauge.

That is vastly different from the hydraulic jack or brake scenarios. As more fluid is added to the system the system compensates by changing the dimensions the same as the gauge. But once the jack or brake shoes contact an opposing surface which does not easily move then any change in the dimensions of the system are severely limited. It is then that the compressibility of the system contents become an issue. If there is air in the system then it compresses instead of moving the piston but if liquid full the piston moves.

So your theory about the compressibility of the fluid (yes air is a fluid, just not a liquid) effecting the gauge reading is correct. The only problem is that with the gauge construction the difference is so small that it is negligible.

The point about the air cushion is not trivial. Page 46 of the 1928 Chevrolet Repair Manual shows an illustration of the oiling system of a '28 Chevrolet engine. Fig. 44 clearly shows an "Air Cushion" between the Oil Distributor and Oil Pressure Gauge. It is this air cushion that helps protect the gauge from damage. FYI, Later Chevrolet reduced the size of the tubing to the gauge (1931) to help keep oil from the back of the gauge.

One point I forgot the oil is more corrosive to the Bourdon tube particularly when it contains acids from combustion leakage and water. Keep the oil from the back of the gauge!

I hope this helps you understand the difference.

Thanks Chip, I'm open to any and all. I don't have access to a '28 repair manual, haven't looked to see if Keith has one posted on TOCMP but I'll take a look. Right now I'm under the '54 scraping grease and mud so I'm typing one fingered. I'd like to have a copy of page 46 to add to my reference file. I like to be able to present the documentation when I get into one of those debates where every one has their own opinion.
Thanks again.
Denny Graham
Sandwich, IL

Hello Denny,

Keith does not have the '28 Repair Manual posted, but the '28 Owner's Manual is listed. Page 41 shows the oiling system and the "Air Cushion".

http://chevy.oldcarmanualproject.com/chevyowner/28cim41.htm

I learn something new nearly every day from this site. I never gave much thought about reconnecting the oil pressure gauge after the engine rebuild. I just hooked it up and it worked. The Air Cushion was there, and I wasn't even aware it needed to be there.

Happy Motoring,

Dan

Thanks Dan, I looked for the shop manual but didn’t think to look for the operating manual. And that’s documentation enough for me. I’ve added it to my archive and printed out a hard copy for the file cabinet.
Air cushion confirmed, oil pressure line bleeding myth busted!
Thanks again,

Denny Graham
Sandwich, IL

i guess i been doing it wrong all these years.
DOH!!!!!!
you can teach an old dog new tricks. LOL

Yes that's true.
Woof, woof

Denny Graham
Sandwich, IL