Bendix Dual mag timing check — how?

[BaldEagle]

I have a J with the Lycoming A3B6D and Bendix dual mag.  Ghastly thing it is too.  My A&P chased a small oil leak down to the top nut being loose, which I understand could have ultimately lead to a catastrophic engine failure.  Anyway, the question concerns an owner-assisted annual.  My understanding is that it’s perfectly legal for the owner to check mag timing, but an A&P is required to adjust the timing if necessary. I have a dual mag tester from Spruce.  However it’s not possible to attach the tester to the P leads as they are completely shielded, and I can’t believe you have to pull these to expose the connectors to check the timing.  Run it from the ignition switch?  My A&P/IA is unfamiliar with the Bendix dual mag.  Was wondering other folks are doing.

[N201MKTurbo]

Take an AN 3 bolt and wrap it with tape to insulate the sides but leave the end exposed. Clip the mag tester lead to the head of the bolt and push it into the p lead hole on the mag. Hold it in while you are checking the timing. Time it to the most advanced of the two. If there is a differential between the two mags, you have to fix it by adjusting the points.

[EricJ]

Checking the dual mag timing is the same as checking two Bendix mags:   pull the p-lead on each side and either put a bolt in as suggested or the p-lead adapters:

https://www.aircraftspruce.com/catalog/eppages/magnetoadapters12-03524.php

Then check the timing as you would with two mags.   It's the same process as with independent mags unless there is a difference in timing between the two mags, in which case the points have to be adjusted as Rich mentioned above. 

There's not really anything mysterious or detrimental about the dual mags, imho.   They get a worse rep than deserved, I think.

 

[PT20J]

Important to make sure you have the new style clamp, proper gasket and correct torque. See Lycoming SI 1508C.

Skip

[carusoam]

And if any star washers become loose... do not re-use...

Be sure to use fresh ones....

The star washers are hardware related to mounting....

PP thoughts only, not a mechanic...

Best regards,

-a-

[EricJ]

So far every IA I've talked to personally about the washers strongly suggests just using split washers.   That's what's been on my airplane since I bought it, so they have remained.

One highly experienced authority even said, "You'll find a lot of split washers where they're not supposed to be." 

Don't try this at home, ymmv, etc., etc.

[N201MKTurbo]

The lock washer is just there to slow down the nut when the torque strain fails.

[PT20J]

1 hour ago, EricJ said:

So far every IA I've talked to personally about the washers strongly suggests just using split washers.   That's what's been on my airplane since I bought it, so they have remained.

One highly experienced authority even said, "You'll find a lot of split washers where they're not supposed to be." 

Don't try this at home, ymmv, etc., etc.

You really should use the hardware as specified by Lycoming. This clamp is critical since it is a single point failure for BOTH mags.

For info on split vs. star washer applications, see pp 9-10 of https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009424.pdf

Skip

[xcrmckenna]

Checking the dual mag timing is the same as checking two Bendix mags:   pull the p-lead on each side and either put a bolt in as suggested or the p-lead adapters:

https://www.aircraftspruce.com/catalog/eppages/magnetoadapters12-03524.php
Then check the timing as you would with two mags.   It's the same process as with independent mags unless there is a difference in timing between the two mags, in which case the points have to be adjusted as Rich mentioned above. 
There's not really anything mysterious or detrimental about the dual mags, imho.   They get a worse rep than deserved, I think.
 

Eric, any chance those work for the 4000 series as well? At some point my dual mag was upgraded to the 4000 model. When we installed it after the last 500hr check we just used a bolt to time it. These studs would help out a lot.


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[Guest]

You can always ad a Pal Nut on top of the Lycoming nut for additional security.

Clarence

[PT20J]

2 hours ago, M20Doc said:

You can always ad a Pal Nut on top of the Lycoming nut for additional security.

Clarence

Well, if it makes you feel better 

I've heard that PAL nuts aren't really very effective, but I don't have any data. However, I ran across this interesting link and it says a jam nut is more effective if it goes on first. 

https://engineerdog.com/2015/01/11/10-tricks-engineers-need-to-know-about-fasteners/

Every source I can find though agrees that split lock washers are useless in any application where the fastener is torqued. The pressure flattens them and they effectively become flat washers with no holding ability. For a split lock washer to work, it has to be under low compression so that there is still some offset in the ends to grab the adjacent material.

Interesting how complicated apparently simple things can be 

Skip

[N201MKTurbo]

1 hour ago, PT20J said:

Well, if it makes you feel better 

I've heard that PAL nuts aren't really very effective, but I don't have any data. However, I ran across this interesting link and it says a jam nut is more effective if it goes on first. 

https://engineerdog.com/2015/01/11/10-tricks-engineers-need-to-know-about-fasteners/

Every source I can find though agrees that split lock washers are useless in any application where the fastener is torqued. The pressure flattens them and they effectively become flat washers with no holding ability. For a split lock washer to work, it has to be under low compression so that there is still some offset in the ends to grab the adjacent material.

Interesting how complicated apparently simple things can be 

Skip

The article you posted was talking about double nutting. The pal nut shown above has a spring action that probably gives it an advantage to double nutting.

[Guest]

The only magnetos that I’ve seen come loose were never installed right in the first place.  Wrong gaskets, wrong mounting clamps and  wrong mounting hardware are at the top of the list of causes.

Clarence

[EricJ]

16 hours ago, PT20J said:

You really should use the hardware as specified by Lycoming. This clamp is critical since it is a single point failure for BOTH mags.

For info on split vs. star washer applications, see pp 9-10 of https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009424.pdf

Skip

As previously mentioned, the utility of the lock washer comes in after it breaks torque or if it is not sufficiently torqued to begin with.   At that point, most lock systems just try to slow down how fast it loosens. 

Similarly safety wire often doesn't prevent fasteners from breaking torque, and yet it's used all over the place to safety fasteners.   I think most safety features, self-locking techniques, safety wire, lock washers, jam nuts, pal nuts, etc., are there for the cases where torque strain has already failed.    Safety wire, cotter pins, tabs, etc., mechanically limit loosening, but none are used on these magneto fasteners.   Other effective techniques, like locking compound or proprietary washer designs, aren't allowed.    Bolt fitment (tolerance) goes a long ways toward reducing the transverse vibration that often loosens fasteners, but between internal star washers and split washers, there's evidence that split are slightly better under transverse vibration, but not by a lot.   I suspect the typical fastener loading on GA reciprocating engine is perhaps not so much transverse, given the general preference among many experienced practitioners for split washers.   As is often the case, a variety of opinions exist.
 

 

14 hours ago, xcrmckenna said:

Eric, any chance those work for the 4000 series as well? At some point my dual mag was upgraded to the 4000 model. When we installed it after the last 500hr check we just used a bolt to time it. These studs would help out a lot.
 

No idea.   Aircraft Spruce also sells the little kit with "all three" styles of Bendix p-lead adapters.   I don't know which works with the D4000, but you might be able to sort it out from the diameters and details in the kit:

https://www.aircraftspruce.com/catalog/eppages/magnetoadapters12-03525.php

[PT20J]

4 hours ago, EricJ said:

As previously mentioned, the utility of the lock washer comes in after it breaks torque or if it is not sufficiently torqued to begin with.   At that point, most lock systems just try to slow down how fast it loosens. 

Similarly safety wire often doesn't prevent fasteners from breaking torque, and yet it's used all over the place to safety fasteners.   I think most safety features, self-locking techniques, safety wire, lock washers, jam nuts, pal nuts, etc., are there for the cases where torque strain has already failed.    Safety wire, cotter pins, tabs, etc., mechanically limit loosening, but none are used on these magneto fasteners.   Other effective techniques, like locking compound or proprietary washer designs, aren't allowed.    Bolt fitment (tolerance) goes a long ways toward reducing the transverse vibration that often loosens fasteners, but between internal star washers and split washers, there's evidence that split are slightly better under transverse vibration, but not by a lot.   I suspect the typical fastener loading on GA reciprocating engine is perhaps not so much transverse, given the general preference among many experienced practitioners for split washers.   As is often the case, a variety of opinions exist.

I claim no great expertise in fasteners. My point is simply that fasteners are an engineered component and there is more complexity there than one might think. If a particular set of hardware is specified, it's safest to assume that some engineer specified it that way for a reason.

A properly torqued fastener shouldn't come loose in a vibration-free environment. The value of the locking device is vibration resistance and, as the documents and video show, different locking devices have different resistance to vibration.

Skip

[EricJ]

6 minutes ago, PT20J said:

I claim no great expertise in fasteners. My point is simply that fasteners are an engineered component and there is more complexity there than one might think. If a particular set of hardware is specified, it's safest to assume that some engineer specified it that way for a reason.

And my earlier post on the topic was just passing on that several very authoritative and experienced and credentialed individuals had been unanimous in telling me to keep the split washer there.

ymmv.    Even engineering solutions are usually compromises with assumptions that sometimes aren't valid.   This one is definitely not black and white.

 

 

[carusoam]

Sticking with what is proven to work... usually gets predictable results...

The hardware has been proven, the procedure has been proven...

There is some opportunity for improvement... it usually comes from the experimental world first then filters in everywhere else...

 

There are mechanical engineers that really know their fasteners...

There are even MSers that have noticed some standard AN bolts can be in AN spec, but not fit the hole they are mounted in very well...ask Tom about his Lancair’s wing bolts...
 

Who wants to be involved in statistical experimental design of lock washers on Mooney magneto installations?

Sounds like a 50amu documented experiment to improve on the existing lock washer...

The existing lock washers and V clamps seem to work as designed...

re-using hardware, always gets a cursory inspection... re-using locking washers and V-clamps is always a bad idea...

Even the cursory inspection notes the washers flatten out with use...

re-used V-clamps fail miserably...

Got any other examples of crummy re-used hardware failures?

Tinnerman nuts after about 100 uses... less critical where they are mounted...

Best regards,

-a-