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Magneto Overhaul

My N-registered aircraft TB20 got a new magneto in 2010. It’s closing in on 500 hours. What is the general consensus w.r.t to overhaul every 4 years or 500 hours. Excessive, recommended? Thank you!

No, 500hrs is a good idea. That’s what I’ve been recommended as well. Send them to a good shop that re-magnetises them. If you do, the drop in rpm is almost imperceptible as you switch.

I posted this link to a Mike Busch webinar yesterday in the “Why piston engines fail” thread.

http://www.eaavideo.org/video.aspx?v=1367499350001

It’s obviously also relevant to thus one.

EGSC

If it was a new magneto in 2010 / 500 hours ago, I would certainly do an inpection, not an overhaul.

Mainz (EDFZ), Germany

This also came up here

In your case, Lucius, you have the D3000 single shaft dual mag which I have, and that is definitely the weakest link on that engine. I would not take any risk whatever with maintenance on that.

I get mine overhauled every 500hrs but there is a point of view that a “repair” is as effective. The difference is that in the OH one does NDT on the casings and there is a mandatory replacement of a number of parts (e.g. the impulse coupling spring), and of course cost. I have just paid $1400 for an OH by QAA in Tulsa, and I recall their repair cost would have been under half that.

Send them to a good shop that re-magnetises them. If you do, the drop in rpm is almost imperceptible as you switch.

There should be a very obvious drop in rpm as one goes to half mag, but there should be no difference between one half and the other. Usually there is, and on the single shaft dual mags one cannot adjust it individually, but with the QAA-overhauled mags there has never been any difference.

Last Edited by Peter at 26 Apr 07:36
Administrator
Shoreham EGKA, United Kingdom

Here is another thing to throw in – although for a different type of magneto. It concerns a four-year life limit which has been at times incorrectly interpreted.

Administrator
Shoreham EGKA, United Kingdom

Thank you Peter! How does a Magneto typically fail? Would there be some initial indications, prior to a catastrophic failure? Would a regular inflight mag check, while running LOP help to identify upcoming problems and reduce the probability of an “unannounced” catastrophic failure?

I get mine overhauled every 500hrs but there is a point of view that a “repair” is as effective. The difference is that in the OH one does NDT on the casings and there is a mandatory replacement of a number of parts (e.g. the impulse coupling spring), and of course cost.

Peter, your information is incorrect. The impulse coupling spring is a mandatory replacement part when the coupling is taking apart. Taking the coupling apart is part of the 500 hrs inspection, so this spring gets renewed every 500 hrs. Just like the contact assy, cam screw, felt etc.

The only real difference betwee an overhaul is that during overhaul is the NDT / respray, and new hardware on the outside. Technically I would say it would be better to do 500 hours inspection only, and use an experianced shop for that. I would not use factory overhauled if I had the choice.

Send them to a good shop that re-magnetises them.

The magnetism can be measured, it is really rare that these “young” parts neet re-magnetises. This is different on old magneto’s for Cars, motorcycle’s, or aircraft (BTH for example). On Slick and Bendix this doesn’t happen.

JP-Avionics
EHMZ

I don’t know what ‘mandatory’ means in relation to impulse coupling spring replacement, as there is no AD on them that I’m aware of. However, based on experience I would advise replacing them on Slicks. I had one break on startup and it was an odd kind of failure: looked like fatigue but the part had relatively few hours. My guess was bad heat treatment but I have no proof. I got a story about corrosion pitting but little or none was visible.

Last Edited by Silvaire at 27 Apr 13:49

May be a little bit offtopic, but on first 200h form of my airplane after purchase (it was around 2000h on tach) techs examinated both magnetos and both (!!!) of them were almost destroyed inside. I belive, it’s not failure of previous owner, but some kind of factory defect. Otherwise how to describe, that both of magnetos could stop at any time. Ofcourse, I’ve replaced them by new, but it’s crucial at least to check them time to time :)

EVCA
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