Flexible Oil Pressure Line

[n2582d]

The C-170B IPC fig. 50-31 shows the oil pressure adapter to be an AN 816-2B brass nipple. This is available from B&B Aircraft Supplies for $2.50. Aircraft Spruce has a modified AN823-4 fitting (45 degree) which has a .040" restricted orifice. It's described as a steel fitting used on Lycomings. Wicks has an .040" restrictor to place in the pressure line. Personally, I'd be very reluctant to tap an AN823 or AN816 fitting to install a restricted orifice as the Stearman guy suggests. By tapping the fitting one would now have threads forming stress risers paralleling the inner and outer diameter of this fitting. If the fitting were to fatigue crack it would result in the eventual loss of oil. I would rather braze, solder or press fit a rivet and then drill a steel fitting. This is something that should be done by or with the oversight of an A&P.

[GAHorn]

Thanks, Gary!
(SPRUCE = $31, Wicks = $6, same part)

[bagarre]

A restrictor is a good idea but why doesnt anyone make a flow blocker?
The gauge works off pressure. A properly made check valve could still show pressure but stop any flow if the line breaks.

[n2582d]

Thanks, Gary!
(SPRUCE = $31, Wicks = $6, same part)

While each part has an .040" orifice, the Wicks part is a straight restrictor with an 1/8" female NPT on one end and an 1/8" male NPT on the other. The Spruce part is as depicted in the drawing stolen from them. If one still has the original Cessna part without a restriction adding the Wicks part would be an alternative although I personally would feel more comfortable with steel parts rather than brass.

[blueldr]

Good advice in the above, but a helluva lot more hot engine oil will go through a no. 42 size drill hole than will go through a no. 60 hole.

[n2582d]

Good advice in the above, but a helluva lot more hot engine oil will go through a no. 42 size drill hole than will go through a no. 60 hole.

Yes, but exactly the same amount will go through a .040" hole as one made with a #60 drill bit.

[blueldr]

Gary,
That's a very astute observation. However, when you're at the drill bit store shopping for a drill bit, it is easier to find a no. 60 than a .040 size.

[GAHorn]

I suppose arguments can be made for either type...
Brass fittings are self-sealing.
Steel fittings are harder on aluminum crankcases and more likely to cause cracking.

[n2582d]

I agree George. What is important is that a restricted orifice fitting is in place. To my surprise the IPC does not call for a Cessna p/n (implying a fitting that has been modified with a restriction) but rather a common brass AN nipple. Ironically, after reading Harlow's accident report--the salient part quoted below--I think we all should make sure that our planes have an "improper fitting" on the oil pressure line rather than the one called for in the IPC (with the FAA's blessing of course).

What failed was a simple coupling from the oil pressure line to an illegal transducer/pressure gauge. This was an improper fitting that severed due to vibration. It was not equipped with a restrictor to minimize oil loss and the engine stopped due to oil depletion/starvation.

[GAHorn]

I'm not sure to which "IPC" you refer but, keep in mind that an IPC is a "catalog" for ordering parts, not an approved source of maintenance or assembly data. In addition, such a part may be considered by Cessna (as the airframe mfr'r) to be an engine component/part... while TCM (as an engine mfr'r) may consider it to be an "airframe" component/part... or both of them may consider that item to be a peripheral or accessory part used to mate the two major components (engine and airframe) together and therefore identify that item as neither airframe nor engine, which leaves the selection to the owner/operator. (who must meet whatever rule applies to his certification authority.)
In other words, it's possible that while neither of them will take responsibility for either specifying or supplying that part, Cessna's IPC merely offers a legal suggestion for what may suffice in the form of a standard part. It's up to us as owner/operators to take responsibility for what we use.
Similar situations come to mind when considering things like generators, starters, pumps, etc.., the airframe mfr'r sometimes considers those to be engine components while the engine mfr'r considers them airframe items. Certain responsibility is avoided by each in that fashion, despite the fact the airplane as a whole requires them.

[n2582d]

I'm not sure to which "IPC" you refer ... .

The C-170B IPC fig. 50-31 shows the oil pressure adapter to be an AN 816-2B brass nipple.

The last O-300 powered 172's used a MS20823-2B brass elbow. The C-185 uses a larger diameter elbow with a restrictor (p/n0752037-1). The TCM IPC only shows a 1/8 NPTF plug (p/n 629518-1H) in fig. 1-8.

[gfeher]

Per Del's suggestion, I contacted Precision Hose Technology about a flexible version of the oil pressure gauge line. (My '52B still has the old copper line on it.). I just off the phone with them, and they were very helpful. He said that Sratoflex 193 is the only option, as it is the only -2 hose available. (Given that someone mentioned that PHT is a Stratoflex shop, I did not ask him about Aeroquip options.) He was familiar with the C-170 oil pressure line issue and said that Stratoflex 193 is perfectly fine for a C-170 oil pressure line as it is regularly used for fuel and oil engine hose. The price he quoted to make a C-170 flexible oil pressure line (Stratoflex 193-2, 19" long end to end, straight to straight -2 female fittings each end) was $54 plus shipping (ships the next day). Shelf life is 8 years. I specified 19" for the firewall-to-engine-fitting hose length after measuring in my engine compartment. The length you need for that line may be a little different. I don't think that a small variation in the length will affect the price.

Del, I mentioned that you referred me to them, if that helps you out in any way.