As far as the grease goes there are a couple of things to consider. One, like I mentioned above, is compatibility with o-rings. The o-ring in the actuator is the only o-ring on the whole airframe that is in contact with grease. Interestingly, the parts manual doesn't even show the o-ring. One has to go to the Service Letter or a later C-172 parts manual to see the parts within the actuator. Another consideration is compatibility with other greases. Mixing greases made with different thickener types may cause a breakdown of the thickener structure. Grease is thickened with either clay or soap (lithium complex). Normally this is not an issue on things like wheel bearings because we clean off all the grease to inspect the bearings before regreasing them. But in the trim actuator grease compatibilty might be something to think about. Placing a note adjacent to the actuator indicating the type of grease used would be a good idea for the next guy who greases it years later. Be sure and use a grease pencil!
Seriously though, don't use a regular pencil--the graphite will cause corrosion on aluminum.As a side note, I was under the impression that the reason the jackscrew failed on Alaska Airlines 261 was because they mixed Mobilgrease 28 and Aeroshell 33. Aeroshell 33 uses a lithium complex thickener while Mobilgrease 28 uses a clay-based thickener. But in fact it was lack of lubrication that caused the jackscrew to fail. http://seattlepi.nwsource.com/business/ ... ka11.shtml
George you wrote, "Cessna and most other light-plane mfr's allow any good quality grease produced by major refiners that meet automotive standards in similar service." Where did you find that? I happen to agree with you that the marine grease you use is a very good choice for the C-170 including the trim actuator but was wondering about approval basis. The loads and temps. placed on the various C-170 bearings and the actuator are minimal--the real concern when choosing a grease for the 170, in my opinion, should be corrosion resistance. The following pages from the 1962 and prior 100 Series Service Manual are all I could find on Cessna's recommendations:MIL-G-7711 has been superseded by MIL-PRF-81322. Both Mobilgrease 28 and Aeroshell 22 meet this mil-spec.
Thanks for finding that! I wonder if Cessna changed it's mind on what grease to use for the trim actuator though. Rather than calling for the general purpose grease (or equivalent) listed in the service manual, service letter SE73-25 is quite specific: "MIL-G-21164C High & Low Temperature Grease (Molybdenum Disulfide Grease)".
