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Old 01-20-2019, 03:37 PM
Kevinstone Kevinstone is offline
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Mechanical oil pressure

Hi guys, does anyone know what the correct type of tubing for a mechanical oil pressure gauge should be? Can I use rubber fuel line? What's the maximum length for the tubing? I think 5-6 feet would get the gauge where I want it.
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Old 01-20-2019, 06:48 PM
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It depends on the rating of the fuel hose. When I relocated my oil pressure senders/switches off the engine to a bulkhead I used hydraulic hose, 300 psi rated available at NAPA. Be sure to use a proper hose barb fitting in the block.
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Old 01-21-2019, 03:13 AM
Marty Levenson Marty Levenson is online now
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copper

I tried hydraulic (grease gun?) hose (can't recall psi rating) that our NAPA store swore would work. Tested fine initially but on a long motor got false low reading as the oil temp rose. Went back to copper.

We run at 180 degrees.
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Old 01-21-2019, 12:22 PM
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Al Schober Al Schober is offline
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When I moved my OP sender off the block, I used a tube from Home Depot meant for a grease gun - about 12" or 18" long. End fittings were 1/8 NPT so no issues. Not expensive.
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Old 01-21-2019, 03:06 PM
Marty Levenson Marty Levenson is online now
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hose length

Al: I'm wondering if hose length accounts for your grease gun hose working and mine not working. I'm guessing that for me the run from the block to the meter was about 3.5 feet. Kevinstone estimated he needs 5-6 feet.

Also, how hot do you run?
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Old 01-21-2019, 04:11 PM
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Quote:
Originally Posted by Kevinstone View Post
Hi guys, does anyone know what the correct type of tubing for a mechanical oil pressure gauge should be? Can I use rubber fuel line? What's the maximum length for the tubing? I think 5-6 feet would get the gauge where I want it.
You need something rated for at least 300 PSI. Copper is common, but you need a flexible section between that and the engine. As mentioned, grease gun hose is a cheap way to acquire such a thing. Gauges usually come with cheap nylon hose I do not trust at all.
5-6 feet should be no issue. I could see a really long line on a really cold day giving false low readings.
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Old 01-21-2019, 05:02 PM
Kevinstone Kevinstone is offline
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Thanks guys, I'll
Check out what auto parts have to offer. My gauge has a nipple which would require a flexible hose and hose clamp. Engine block end will
Take an npt barb
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Old 01-21-2019, 10:26 PM
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You may want to find a better gauge. The connection should be a flare or maybe compression.
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Old 01-21-2019, 11:20 PM
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I hooked up an oil pressure gauge on my old VW bus. I ran that plastic extension tubing from the engine (in the back) all the way to the dashboard. It was at least a 10 foot run. I bled the tubing and had oil all the way to the gauge. It was so accurate I would see the oil pressure drop as I shifted.

That was for a 67 bus a long time ago.
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Old 01-22-2019, 01:33 AM
Marty Levenson Marty Levenson is online now
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heat and length

Trying to understand why other hoses have worked.

Found the hose that I initially used for the oil pressure gauge: 4 feet long, 300psi grease gun hose. There is no doubt it caused the gauge to receive misleading pressure. Running at temp for a couple hours on a hot day the reading fell steadily from 20 to 8. (Same gauge directly on the block worked perfectly, and when reinstalled with a copper tube works fine.)

I do believe that running at 180 with a long run caused the hose to get too soft to maintain the pressure the rest of the system has. perhaps others are running at a lower temp or have a shorter run? Or my hose was defective (unlikely)?

There is no flexible connection at either end.
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Old 01-22-2019, 09:13 AM
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Quote:
Originally Posted by Marty Levenson View Post
Trying to understand why other hoses have worked.

Found the hose that I initially used for the oil pressure gauge: 4 feet long, 300psi grease gun hose. There is no doubt it caused the gauge to receive misleading pressure. Running at temp for a couple hours on a hot day the reading fell steadily from 20 to 8. (Same gauge directly on the block worked perfectly, and when reinstalled with a copper tube works fine.)

I do believe that running at 180 with a long run caused the hose to get too soft to maintain the pressure the rest of the system has. perhaps others are running at a lower temp or have a shorter run? Or my hose was defective (unlikely)?

There is no flexible connection at either end.
Even if the hose blows up like a balloon the pressure should be the same at the other end. This does not make a lot of sense
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Old 01-22-2019, 02:50 PM
Kevinstone Kevinstone is offline
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Thanks for all the tips. I am leaning towards copper from engine with a short piece of fuel line at the gauge. Still
Need to see what I can find. Currently drifting around the keys, will check next time we land at a town. Is it necessary to bleed the tubing up to the gauge?
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Old 01-22-2019, 03:38 PM
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Quote:
Originally Posted by Kevinstone View Post
Thanks for all the tips. I am leaning towards copper from engine with a short piece of fuel line at the gauge. Still
Need to see what I can find. Currently drifting around the keys, will check next time we land at a town. Is it necessary to bleed the tubing up to the gauge?
I would *not* use fuel line. It normally runs at about 5 PSI. No need to bleed the gauge, it doesn't really matter if oil or air gets shoved up to the gauge.
You need to find a good hardware store, hydraulic shop, or perhaps NAPA. Grease gun hoses work and our local hardware store makes up all kinds of high pressure hydraulic lines with various fittings. You need something that can deal with hot petroleum products at 100 PSI.
This is also likely why electrical oil pressure gauges are common

Last edited by joe_db; 01-22-2019 at 03:43 PM.
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Old 01-22-2019, 04:16 PM
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KS, An auto parts store should have a copper tube kit for exactly this purpose. I think I got one at an Advance Auto store. The copper should be more accurate and less prone to failure than plastic that typically comes with the gauge, although I have used both..when I was a kid and broke, it was a plastic tube thru the firewall to my gauge set on the '78 Civic. On my current project car, a '91 Civic, I 'upgraded' to copper when I installed an OP gauge.

And yes, you'll want to bleed it so you don't have air in the line, which could affect accuracy.
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Old 01-22-2019, 05:17 PM
Marty Levenson Marty Levenson is online now
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Hmmmm..

Quote:
Originally Posted by joe_db View Post
Even if the hose blows up like a balloon the pressure should be the same at the other end. This does not make a lot of sense
I'm ignorant but curious about this. If the balloon continues to expand, isn't some of the system pressure going toward strtching the balloon rather than pushing on the meter's sensor?

I'm curious because I can't see another explanation for my experience with the 4' hose, as stated in a previous post.

Question please: is anyone running at 180 with a longish non copper connection?
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Old 01-22-2019, 07:52 PM
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Quote:
Originally Posted by Marty Levenson View Post
I'm ignorant but curious about this. If the balloon continues to expand, isn't some of the system pressure going toward strtching the balloon rather than pushing on the meter's sensor?

I'm curious because I can't see another explanation for my experience with the 4' hose, as stated in a previous post.

Question please: is anyone running at 180 with a longish non copper connection?
When the "balloon" is in transition the pressure will be off, but eventually it will either stabilize or pop
I have hooked up a number of oil pressure gauges and none of them ever needed bleeding. Pressure gauges on airplanes can have a lot of hose between the cockpit and an engine on the wing and they don't have bleeds. If there is 100 PSI oil at one end of a sealed pipe, it will move up the pipe until it meets 100 PSI air
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Old 01-22-2019, 10:14 PM
Marty Levenson Marty Levenson is online now
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pressure

Quote:
Originally Posted by joe_db View Post
When the "balloon" is in transition the pressure will be off, but eventually it will either stabilize or pop
...but isn't the hose ("balloon") steadily inflating as it gets hotter and hotter?
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Old 01-22-2019, 10:17 PM
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I think the gauge I got at NAPA came with 1/8-inch polypropylene tubing. (Not positive - could be PE or nylon, but I think PP is better with heat, oil, and pressure.) It came with quite a lot. At least ten feet. Maybe they have more at NAPA? Hardware stores tend not to stock the smaller sizes. I wouldn't use flex stuff like fuel line.

I use 1/8-inch PE up to 100 PSI for lab gases every day, at room temperature. Hot oil might be another story. I've got 1/16 PP tubing running at 2000 PSI liquid, but it does fail from time to time. Then there's PEEK, but that, and fittings for it, are expensive.

Anyway, my usual source for all that is Cole Parmer, except PE tubing can usually be had at Ace Hardware. May be cheaper to just buy a new NAPA gauge with tubing and fittings included.

Or heck, the whole mess is just sitting in my parts box. Could make you a great deal...
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Old 01-23-2019, 03:31 AM
JOHN COOKSON JOHN COOKSON is offline
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Quote:
Originally Posted by toddster View Post

. May be cheaper to just buy a new NAPA gauge with tubing and fittings included.
...
I tried a mechanical oil pressure from a "boutique" auto parts store (Pep Boys).
It lasted ~ two years before it rusted out. A "marine rated" gauge would have lasted longer.

TRUE GRIT
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Old 01-23-2019, 08:30 AM
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Quote:
Originally Posted by Marty Levenson View Post
...but isn't the hose ("balloon") steadily inflating as it gets hotter and hotter?
It is open at each end. It would have to inflate pretty quick to make a noticeable change and it would then be popping and shooting oil all over. Slowly changing from 0.25 to 0.29 inches would do nothing on the gauge. I am talking something like 0.25 to 6 inches swelling rapidly The hose would pop before the gauge settles down.
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