Oil Temp Probe - How much torque?

[N4316B]

Hi --

I used to have an F&M oil adapter on my 170B, but removed it following an oil leak which caused an off-field landing. I'm now back to the oil screen. I now follow the "KISS" rule.

One of the drawbacks to the screen is that the oil temperature probe must be removed at each oil change. Now given my off-field landing, I have a nasty pyschosis about all gaskets, fittings, etc in the oil system. My question is simple. How much torque do people use when tightening the oil temperature probe into the oil screen assembly? The oil temp probe has a flange that appears to be designed for a tight fit against the screen assembly. This tight fit is required to form a proper seal.

I tighten it until it won't go anymore, and then give one more tug on it for good measure. The oil probe has a few marks in the flange (small rings, like it was in there tight), but nothing bad. Is this procedure in line with everyone else?

BTW, does it bother anyone that the oil temp fitting doesn't have a hole for safety wire? (Ihave the scott probe). Has only drilled such a hole? or is that a bad idea?

[N3243A]

BTW, does it bother anyone that the oil temp fitting doesn't have a hole for safety wire? (Ihave the scott probe). Has only drilled such a hole? or is that a bad idea?

Yes, it has bothered me too. If that nut ever vibrated loose and the probe backed out under oil pressure, you would loose all your engines lifeblood in minutes. I safety the probe by wrapping a tight loop of wire around the probe shaft just behind the nut then twisting the tails up to the corner of the big square nut on the screen housing which has two safety wire holes on opposing quarters. If done right, the safety wire will stretch right across one of the flats of the probe nut. Make sure this safety wire connection is very taught and it will prevent the nut from rotating.

Would you care to elaborate on your forced land experience and problems with the FM Filter?

Bruce

[zero.one.victor]

Bruce,the lack of a way to safety the temp probe also bothers me. But I'd be kinda worried about doing it the way you describe--seems like the safety wire (tough stainless) might tend to cut into the probe tube (soft copper),especially over time. It'd probably be a good idea to eyeball that probe "shaft" pretty close at every oil screen inspection. I wonder if I just tightened the safety wire around the probe nut itself and then safetied it to the hole in the screen housing,if that would do any good?

Eric

[Dave Clark]

If you're paranoid about it drill a hole in the nut for safety wire. Overtightening the nut can break the probe flange and cause a leak. It's probably got a torque spec but I don't have that. I just tighten it till my wrist clicks and then it feels right. I'm not aware that they have been a problem coming loose in service without the safety wire.

[N4316B]

I can understand that you should tighten it until it "feels right", but that's difficult for us newbies. Does anyone have a torque value for the probe? or would someone be willing to tighten theirs until it "feels right", then hit it with the torque wrench and see what value comes up?

I'm thinking that I'm not the only one who tends to over tighten.

[mit]

If it bothers you, just make it a preflight item. Look at it and feel it. I can't tell you how many times I have found things that where just hanging by the safety wire that was suppose to keep them from backing off.

[GAHorn]

Aluminum, Brass and copper fittings do not normally carry torque values. Those type fittings are soft and when tightened they deform and hold position. They are friction-fittings. In other words, they do not vibrate loose as steel fittings do. (They work similarly to hydraulic and fuel hose fittings.) Safety wire is unnecessary.
The oil temp probe was a subject I wrote about in the old Yahoo club, telling about my experience when it was overtightened on my Aeronca C-65 engine (same set up as these C145/O300's). On the next flight following an inexperienced mechanic's overtightening of my oil temp probe nut, the copper skirt was cut by the overtightened steel nut, and the probe seperated at the skirt, fell out of the engine, and leaked ALL the oil out of the engine in less than 10 minutes of flight. It was a 150 hour SMOH engine and it was completely ruined except for the case and the connecting rods and gears.
The correct method to tighten the nut is to snug it up with finger pressure or light wrench pressure, then tighten it on a new probe/nut ONE-ADDITIONAL-FLAT on the nut. On a used probe/nut tighten finger tight and then 1/2 flat only. Do not overtighten this fitting or you WILL suffer complete oil loss.

[N4316B]

Thank you George. You're a great resource!

I went out to the hangar, marked the position of the nut, loosened it up and re-tightened it according to your rule (1/2-3/4 flat turn (probe is relatively new)) and it's in the exact position as before. Feels secure.

This is exactly the validation I needed. Makes me feel much more comfortable. Thanks again.

[N3243A]

Bruce,the lack of a way to safety the temp probe also bothers me. But I'd be kinda worried about doing it the way you describe--seems like the safety wire (tough stainless) might tend to cut into the probe tube (soft copper),especially over time. It'd probably be a good idea to eyeball that probe "shaft" pretty close at every oil screen inspection. I wonder if I just tightened the safety wire around the probe nut itself and then safetied it to the hole in the screen housing,if that would do any good?

Eric

Eric, in several years of doing this, I have not noticed any sign at all of cutting into the probe tube with the loop of safety wire. It's quick to do and I like the peace of mind.

Bruce

[wa4jr]

I don't know, but it sounds to me like there are far more potential problems waiting with the original oil screen configuration. Once the oil filter head is properly installed with good seals, there should not be any further problem and the danger of probe flange fatigue and failure becomes virtually a non-issue. Perhaps you would be happier with the F&M reinstalled with the new type seals? I'd worry about the "just feels right" and "pull til it stops" methods of tightening. In my early days, I used to subscribe to the "pull til I can't pull anymore" method. I really hated the sickening feeling as the nut or bolt tightened and then got easier to turn as the threads failed. Dealing with the internal threads in these aluminumn castings, I'd be really cautious about stripping them out with the various "armstrong" methods. Somewhere there must be a reliable source for proper torque values.

[zero.one.victor]

Hi --

I used to have an F&M oil adapter on my 170B, but removed it following an oil leak which caused an off-field landing. I'm now back to the oil screen. I now follow the "KISS" rule.

You never did fill us in on the oil leak that caused your forced landing. How about filling us in--inquiring minds want to know. Since me & several others still run the F&M adapter,maybe you can save us some grief.

Eric

[GAHorn]

I don't know, but it sounds to me like there are far more potential problems waiting with the original oil screen configuration. Once the oil filter head is properly installed with good seals, there should not be any further problem and the danger of probe flange fatigue and failure becomes virtually a non-issue. Perhaps you would be happier with the F&M reinstalled with the new type seals? I'd worry about the "just feels right" and "pull til it stops" methods of tightening. In my early days, I used to subscribe to the "pull til I can't pull anymore" method. I really hated the sickening feeling as the nut or bolt tightened and then got easier to turn as the threads failed. Dealing with the internal threads in these aluminumn castings, I'd be really cautious about stripping them out with the various "armstrong" methods. Somewhere there must be a reliable source for proper torque values.

The FM Enterprises oil filter adaptor is what I installed on my engine. I ordered it directly from them and followed their installation instructions which include tightening the unit to the accessory case using 60-65 ft-lbs of torque. I used a Pratt & Whitney square socket to tighten it with a calibrated torque wrench. The kit included FM's latest fiber-type gasket between the adaptor and the accy case, and a copper crush-gskt at the threaded transfer cylinder-to-adaptor joint. It has not leaked in 350 hours.
A major reason I selected the FM unit was to get away from oil screen cleaning and the potential for eventual failure of the oil temp probe. (Having had a previous bad experience with that probe on another airplane, it was uppermost in my mind.) I'm very satisfied with this setup.
Back to torque-values: The 60-65 ft-lbs torque for the FM adaptor should not be used on the oil screen. That would be very much overstressed and unnecessary for the screen. The C145/O300 Overhaul manual does not address the issue (in fact, following the manual exactly would have you start the newly overhauled engine and run it up without oil in the engine....There's considerable levity allowed the assembler to make errors if he has no common sense).... it only states to "tighten securely" such items as the accy case, etc. The only torque values given are for threaded fasteners such as bolts, studs, etc.

[zero.one.victor]

Since I don't know anybody who wrenches on P&W PT-6's,I borrowed a friend's 1" square crow's-foot & used that with my torque wrench. Worked fine.

Eric

[N170BP]

So, where's the best place to buy a new thermocouple assy (line,
the whole works) and possibly the receiver fitting that screws into
the screen housing?

Flying my 170 200+ hours per year (25 hour oil changes) have
taken a toll on the original parts, and I now have a slight leak
where the thermocouple enters the screen fitting.

I'd hate the slight leak to turn into an "awe ****" when the
thermocouple breaks off!

Bela P. Havasreti
'54 C-170B N170BP

[GAHorn]

The entire unit (gauge, capillary, probe, and nut) is sold as a single unit by most suppliers including Spruce. (877-477-7823).