mag cover plate screw backed out, discovered at oil change

[Vance Harral]

My airplane partner graciously performed an oil change today, and called to say that when he removed the oil filter, he found a machine screw sitting on the "shelf" directly under the filter.  On further investigation, he traced it to the ignition wire cover plate for the left mag, as shown in the attached photos.

Neither of us are A&Ps, and we're not sure of the implications of this.  My partner re-installed the screw, used his general mechanical experience to torque it to what he felt was "about right", and performed a post-oil-change run-up and mag check with no issues.  But we're between A&Ps at the moment (our on-field shop recently closed), so I'm asking the collective wisdom of Mooneyspace for further guidance.

The screw in question doesn't have any kind of lock nut that we can tell.  Seems like it's secured only by friction, which is a little creepy.  Other than checking all the other screws on both mags, anything else we should do before flying again?  Any guess what might have caused it to back out?  This is the first time we've had a problem like this in 17 years of ownership, so I have no prior experience.

 

 

[Guest]

There is not locking device on the screw other than torque.  From the MM,

Clarence

[EricJ]

Replacing or adjusting non-structural fasteners is one of the tasks defined specifically as owner maintenance, so you can put that back and snug all the rest of them and be perfectly legal.

[M20F-1968]

I just recently sent my Bendix mags out.  As I recall, they have lock washers that are part of the screw assembly are are about $8 apiece to buy. 

Are your Slick mags.  It would seem that there should be a locking device.  Keeping those 4 wires in place is pretty important.

John Breda

[KSMooniac]

My Bendix Siamese mag has split lock washers on those screws IIRC.

Sent from my LM-V405 using Tapatalk

[Guest]

Bendix magnetos generally have lock washers as part of the screws.  Slick magnetos generally have lock washers on any of the screws used internally, the 3 screws which hold the wire cap on do not.  The current version of the screw is a countersunk screw with a combination torx/ slot drive.

Clarence

[Vance Harral]

Thanks for the replies so far.  These are Slick 4347/4370 mags, sounds like no lock washer.

These are overhaul/exchange mags, installed 4/26/2019 when the prior slick mags failed inspection.  They have about 250 hours of service to date.  They were timed at the annual about 8 months and 80 hours ago, by the shop A&P; but I don't know if they were actually "touched" at all then (I wasn't there, sometimes the timing checks fine and no adjustments are necessary).

So no very recent maintenance, and I don't have any particular reason to believe the screws were installed incorrectly last time they were touched.  Perhaps this is just one of those things to check every time the cowl is off.

[MB65E]

I’ve found a few caps loose Over the years. I change all mine to the new torx type screws. This Helps get proper torque better than the plane slotted screw. I have never had one back off when torqued correctly. 
-Matt