Oil Pressure Gauge replacement

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2 days 7 hours ago #2180 by TOM BRAEUNIG
When I talked to the overhauler, he said that the air would (and should) remain in the line, and that the oil pressure from the engine would act on the column of air to give the pressure indication.
I don't feel completely comfortable with that. It seems like air would be more subject to expansion with heat, and might give errroneous indications. (and the overhauler is not necessarily an A&P mechanic)
Right now I'm tending to go with filling the line with light oil.

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2 days 7 hours ago #2179 by STEVE ELLS
Not quite!. Air is compressible, liquid not so much. But I tend to listen to the overhauler and it said hook it up! I'm not sure how the air is bled out, but there must be a way.
S

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5 days 20 minutes ago #2177 by TOM BRAEUNIG
Thanks Steve,
Good information. I'll be installing the overhauled gauge tomorrow, working with my A&P mechanic. I discussed it with him today, and he normally uses a light oil to fill the line. I think I'll go that route since he'll be signing it off.
I guess it really shouldn't matter, pressure is pressure whether it's oil or air.

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5 days 6 hours ago #2175 by STEVE ELLS
Hi Tom;
Either method will work. I have replaced gauges like this--they're called direct reading gauges--without filling the connecting line with light oil or kerosene and the gauge worked.
It certainly wouldn't hurt anything to pre-fill the line but I don't believe it's necessary.
I have never done the pre fill when I replace an engine. In fact, my pre-oil routine prior to the first start consists of removing a spark plug from each cylinder, then spinning the engine with the starter until I get a solid oil pressure indication on the gauge.
I hope this answer helps.
Steve
The following user(s) said Thank You: TOM BRAEUNIG

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5 days 7 hours ago #2174 by TOM BRAEUNIG
I'm going to be replacing the oil pressure gauge on my 1975 172M, and I'm getting conflicting opinions on how to go about it.
According to the Cessna Service Manual, (par 15-36) "The oil pressure line from the instrument to the engine should be filled with kerosene, especially during cold weather operation, to attain an immediate oil indication"
I opened a support ticket with Cessna Pilot's Assn, just to confirm that this is still an accepted practice. They agreed, to use either kerosene or a very light oil such as sewing machine oil to fill the gauge line.

The replacement unit is an overhauled gauge by Air Parts of Lock Haven. According to them, I should blow out the gauge line and have nothing but air in the line. That's how they test their gauges.

I'm a bit confused, and I'd like to hear from someone who has some direct experience with this replacement procedure.

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