Yoke set screw: to Loctite or not

[Vance Harral]

Short version: How the !$#&* do you keep yoke set screws from backing out with normal use, while still allowing them to be easily removed for inspection and maintenance?

Long version:

Our 1976 M20F is a bit of an oddity.  During this last year of the F model, the factory started sneaking in some of the M20J components, e.g. our airplane came from the factory with the rams horn yokes and horizontal bolt/vertical set screw arrangement that people generally associate with J and later models.  Regrettably, there is no exemption carved out in AD77-17-04 or Mooney SB M20-205B that relieves our oddball airplane from the infamous 500-hour yoke shaft inspection, even though many people believe our particular airplane "should" be exempt by design.  So we dutifully pull the yokes off every 500 hours to inspect the shafts, and thus begins a sordid history...

About 12 years and 3 inspection intervals ago, the inspection proceeded without incident, but the pilot-side set screw was chronically loose thereafter, resulting in slop in the yoke and requiring frequent re-tightening.  The problem wasn't dangerous, because the set screw isn't what holds the yoke onto the shaft - it just takes up the small amount of slop that the primary connecting bolt leaves.  But it was annoying.

About 6 years and 2 inspection intervals ago, the pilot side set screw stripped out when attempting to remove it, likely because it had been buggered up from all the re-tightening.  After much cussing, the screw was removed with an extraction tool.  We were unable to procure official AN565-D416-H4 set screws to replace it at the time (factory was in hibernation and none of the usual sources seem to stock this particular flavor of AN565).  So for better or worse, hardware-store set screws of appropriate thread size were installed on both sides.  Almost as an afterthought, blue Loctite was added to prevent the set screw from loosening.  Much satisfaction ensued over the next several years: no loose yokes, no annoyances.

You can probably see where this is going... At the most recent inspection a year ago, the mechanic doing the work found the co-pilot side set screw "glued" in place, and attempts to remove it effectively destroyed the screw in-situ.  Maybe it was too much Loctite, maybe dissimilar metal corrosion from incorrect hardware, maybe bad luck, or a combination of all three.  After much cussing, drilling, and other failed attempts, we all cringed while the mechanic pried the yoke off the shaft with pieces of the set screw still in place, drove out the remaining bits, and cleaned the threads with the correct size tap.  Understandably, the mechanic insisted on ordering the correct hardware from Mooney ($$$) for re-installation, and threatened us with our lives if we so much as even considered applying thread locker of any kind.

Fair enough, but for the past year, the set screws are back to being chronically loose, now on both sides.  We keep a hex driver in the seat pocket and tighten them up basically before every flight, but by the time we land they've backed out enough that the yoke is sloppy again.  And after a year of this, I can already tell the hex sockets in the screw heads are starting to soften up, and will strip out soon if not replaced.  We have extra stock left over from last year's order, so we'll replace them at this year's annual, but new hardware is clearly not going to resolve the problem of the screws working loose.  Hard to say if the threads in the yoke are worn at this point, or the airplane is vibrating more as it ages, or if maybe this just isn't that great of a design in the first place.  But I'm looking for solutions.  Try blue Loctite again, but sparingly?  Some other threadlocker product?  Teflon tape?  I'd like to set screws to stay tight, but I want to avoid another giant debacle at the next inspection interval.  Any and all suggestions appreciated.

[carusoam]

PP guess only...

1) Are those steel screws in a cast aluminum part?

2) Each time the screw gets tightened... the aluminum probably sheds layers...

3) just using it will have increased wear... because the differences in hardness of the different metals...

4) Lock-tite blue is the removable thread lock...

5) Using the smallest amount possible seams to be a good idea...

6) Aluminum threads are often removed by steel screws...

Did you see any curly aluminum or black dust come out when you removed the screws?

Let’s ask @M20Doc his thoughts ... (steel set screw in aluminum casting...)

Best regards,

-a-

[47U]

Teflon tape might help, or a drop of Permatex #2?

As far as retiring the AD, doesn’t the service bulletin include a repair option on the later shafts?  Maybe a drastic step to take as the repair kit (either from an MSC or local manufacture) might not be any cheaper than a set of replacement shafts, salvage or from the factory.  But, if your shaft already has the horizontal bolt, that might muddy the water there.

The newer shafts were also of increased wall thickness (right?).  Is there any opportunity there to establish the AD does not apply to your aircraft, assuming yours are of the newer stock?  If you can establish 100% that you have the newer style (ask a J owner if you can crawl into his foot well for some measurements), I’d think your mechanic could annotate the logbook that your installation is in fact the other part number, and the AD is retired.

Sounds so easy brainstorming in the middle of the night... good luck.   

[Guest]

If the hole is deep enough, add a second set screw to act as a jam nut against the first. 
 

Clarence

[Old Chub]

Have you discussed the possibility with your FSDO for an AMOC?

[Vance Harral]

7 hours ago, M20Doc said:

If the hole is deep enough, add a second set screw to act as a jam nut against the first. 
 

Clarence

That's a clever idea.  One thing I find interesting about the factory-specified set screw is that it's very short compared to the threaded depth of the yoke itself.  The "hardware store" solution was a much longer screw that achieved better purchase (though this probably contributed to the "glued in" problem).  Using a pair of shorter screws based on the jam nut theory uses the factory-specified part in a manner that seems to meet both the letter and the spirit of the law.  Plus, we can try it out right now rather than waiting until the annual.  Thanks for the tip!

[Vance Harral]

1 hour ago, Old Chub said:

Have you discussed the possibility with your FSDO for an AMOC?

We have not.  For most of our 15+ years of ownership, it just seemed simpler to perform the inspection than to fight the bureaucracy to legally avoid it.  I'm feeling differently about it these days, though.  This is a classic example of the preventive-maintenance-induced failure concern: performing this mandatory inspection has unquestionably caused issues and damage, that approach or exceed the risk the inspection itself is supposed to mitigate.

[Vance Harral]

11 hours ago, 47U said:

As far as retiring the AD, doesn’t the service bulletin include a repair option on the later shafts?  Maybe a drastic step to take as the repair kit (either from an MSC or local manufacture) might not be any cheaper than a set of replacement shafts, salvage or from the factory.

The retrofit kit is pretty spendy.  We got a quote from Mooney on just the shafts (we can re-use the existing yokes), and it was $1250 a side, or $2500 total.  We've been casually on the lookout for a quality used part from salvage ever since, but there are many different types of yoke shaft, and the exact P/N we need is hard to come by.

Again, for most of our ownership it seemed simple just to perform the inspection, but I'm feeling less and less like that with every inspection cycle.

[Vance Harral]

On 3/17/2021 at 4:28 AM, M20Doc said:

If the hole is deep enough, add a second set screw to act as a jam nut against the first. 
 

Clarence

Follow-up: we decided to try this "second set screw acting as jam nut" idea.  Installed a second screw in each yoke about 3 weeks ago, seems to be holding nicely so far.

[ryoder]

My IA used loctite. Worked great.

[RobertGary1]

2 minutes ago, ryoder said:

My IA used loctite. Worked great.

Idk. Have you tried to remove it? Would suck if you have to drill it out next time. May have to hit it with a torch for a bit. 

[Vance Harral]

Concur with Robert.  After our original loctite debacle, I'm skeptical of any "works great" story that doesn't include a positive PIREP on the ability to remove it at the next inspection.