The International Cessna^® 170 Association

Hi All -

I'm on about hour 12 after getting an engine overhaul on my '55 C170B. I bought the plane with a burnt engine, so this is about hour 15 on the plane (for me). Three questions for you guys:

1) My oil temperature gauge seems to be "reverse sensing". When I start the engine, it starts at redline and slowly gets cooler as the engine warms up. The probe is installed in a newly installed FM oil filter assembly. As far as I remember, the gauge worked well before we tore the engine apart. Any thoughts on why it might be reverse sensing? or is it just an indication that the sending unit is shot and needs to be replaced?

2) If the sending unit is shot, then I'll probably just get a whole new gauge/sending unit as I currently have a "Rochester Instruments of Texas" temp gauge and cannot find any information about them on the web. Anyone know where I can get such info? If you were to buy an inexpensive (non-EI) gauge for your 170, which would you get?

3) My oil pressure rides right at the top of the green to the redline (40-60lbs). I'm a little concerned that it's a bit high. Where does your oil pressure normally lie (for an O-300A) @ 75%?

Thanks much for the help. I'll be working through these problems with my A&P, but thought I would try to get some help here first!

Dennis Keller
N4316B

It sounds as if you have an electrically sensing oil temp guage, rather than the original capillary type. If so, then your wires are probably hooked up in reverse, or your gauge/instrument panel has lost it's electrical ground.
The original type guages were mechanical (capillary) are are available from Aircraft Spruce 877/477-7823. Scott and Rochester are the two mfr's and I can't tell the difference between them except that one says "Scott" on it's dial and costs twice as much. So, I bought it and put my failed Rochester dial-face in it.
The TCM overhaul manual specifies that more than 50 psi of oil pressure may cause collapsed lifters. My engine consistently produces 55 psi. (I've checked the guage calibration.) So I called TCM field rep, and he said "no problem. Don't worry about it." He indicated that the original lifters (which are no longer out in the field) might fail in such a manner, but not those produced/remfr'd in the last 50 years.
The only spec to worry about is the oil pressure relief spring. It should be compressed to 1.56 inches and the pressure should be 6.06 to 6.31 lbs if new, and 5.75 lbs minimum if used. I would not recommend shortening that spring to lower pressures. Just enjoy the high pressure.

My engine was taken apart and checked out recently.
The mechanic told me that basically the lower half was overhauled.
Everything seems to be wonderful, however the Oil Pressure reads 7 psi higher. Used to read 45psi, now reads 52psi.
Flown about 60 hrs now. No change.
After reading these post, I guess it's nothing to worry about?
Dave

David Laseter wrote:My engine was taken apart and checked out recently.
The mechanic told me that basically the lower half was overhauled.
Everything seems to be wonderful, however the Oil Pressure reads 7 psi higher. Used to read 45psi, now reads 52psi.
Flown about 60 hrs now. No change.
After reading these post, I guess it's nothing to worry about?
Dave

I wonder what the mechanic meant by that? If it was "basically" overhauled was it signed off as such? Why not? (Probably because it wasn't anywhere near "basically" overhauled, which would include magnafluxing cranks, gears, and connecting rods, checking them for twist and straightness, and re-assembling using new hardware and bolts, and grinding/nitriding cams, lifters, etc. Why was the engine taken apart? Was it due to a crankcase crack or prop strike, etc? I'm only guessing that some gaskets and bearings were replaced. If so, that's not an overhaul by even a "basic" measure. It's a repair to "service" limits for continued time in service.) In some cases it's important to be exacting when accepting mechanics explanations, ...in others it's less important. But if it's important to you that your lower unit be "overhauled", then make it clear to the mechanic that you expect him to put in writing whatever he's charging you for.

My oil pressure and temperature are not marked. What should the markings be? Haven't found this in the TCDS or the flight manual.

Look in the TCDS for the engine.
Shawn

I did look in the TCDS and didn't see it. am I missing it.

The TCDS (E-253) for the O-300, Note 1., states "Maximum permissible cylinder head, barrel, and oil inlet temperatures, 525 F, 290 F, and 225 F respectively. Maximum oil inlet temperature limit is 240 F when using Spec MHS-24 SAE No 50 avoe 40 F ambient an d SAE No 30 or 10W30 below 40F ambient."

All modern ashless-dispersant aero oils meet MHS-24 specs.

Where they marked from the factory? If so, exactly what are marks supposed to be for oil pressure and temperature. How would you mark the oil temperature since max temp is dependent on type of oil used?

All that the aircraft limitations section state is that oil temp shall not exceed "red line". The original guages had no temp markings,...only a green arc to a red line (set at 225 F).
Since I only use MHS-24 oil, SAE 50 wt, I had my guage remarked with a redline at 240, and entered that information into my aircraft logs. I'm surprised that the FSDO did not insist upon a field approval for this, but they were happy with that. (Presumeably due to the TCDS referenced in my log entry.)