M20J A3B6D -> A3B6 conversion considerations, Slick magneto maintenance + MT props

[Martin S.]

Gents,

does anybody know the overhaul and maintenance intervals for the A3B6 slick 4370/4372. The bendix dual mag (on my A3B6D) needs overhauls every four years. Do the slicks need less maintenance? (Trying to evaluate whether the conversion will lower my magneto maintenance bills in the long term)

As far as I understand there should be no issues running an MT three blade prop on the A3B6 after conversion. Am I overlooking something?

Many thanks,

Martin

[EricJ]

In the US there is generally no difference between the Slick and Bendix or Bendix dual mags for maintenance intervals.   Broadly speaking, for Part 91 (our private general aviation use), there are no maintenance intervals other than "on condition".    Many/most people observe the 500-hour and overhaul intervals just because they're generally a good idea with magnetos.    I don't know of any four-year time interval requirements or even guidance for any magneto.

 

[KSMooniac]

The dual mag needs inspections every 500 hours, not overhauls every 4 years. Big difference.

Slicks are generally throw-away after 500 hours, if they last that long.

You might investigate electronic ignitions instead, although I doubt many have been installed for a full 2000 hour run thus far.

MT props are STC'd for our engines and I've been very happy with mine.

Sent from my motorola edge plus 2023 using Tapatalk

[Martin S.]

18 minutes ago, KSMooniac said:

The dual mag needs inspections every 500 hours, not overhauls every 4 years. Big difference.

Slicks are generally throw-away after 500 hours, if they last that long.

You might investigate electronic ignitions instead, although I doubt many have been installed for a full 2000 hour run thus far.

MT props are STC'd for our engines and I've been very happy with mine.

Sent from my motorola edge plus 2023 using Tapatalk
 

As far as I understand the overhaul requirement originates from SB643B.

[KSMooniac]

Nothing in that SB indicates you must overhaul it every 4 years, only when the engine is overhauled.  There are periodic INSPECTIONS, but not overhauls.  Big difference.

[EricJ]

13 hours ago, Martin S. said:

As far as I understand the overhaul requirement originates from SB643B.

Also, Service Bulletins are not required to be followed in the US for Part 91 unless they're part of an Airworthiness Directive.

[philiplane]

Compliance with all manufacturer's instructions, including SB's, is required by many authorities outside the US.

From SB643B:

1. In addition to the requirements listed above, magnetos must be overhauled or replaced at the expiration of five years since the date of original manufacture or last overhaul, or four years since the date the magneto was placed in service, whichever occurs first, without regard to accumulated operating hours.

Edited March 6 by philiplane

[Guillaume]

1/ SB643B is obsolete and has been replaced by SB643C many years ago (D2000 / D3000 mags are no longer included) 

2/ For European aircraft maintained under Part-M light, aircraft owners may choose to deviate from TCH recommendations (including SB). 

[Martin S.]

Thanks for clarification everybody!

[PeterRus]

On 3/5/2025 at 6:06 PM, KSMooniac said:

MT props are STC'd for our engines and I've been very happy with mine.

Read your posts, just to confirm the high-level points:
1. The prop is lighter and in addition to the increase in UL the prop moves the CG backward a little, which is beneficial

2. Performance is marginally better and the prop is quieter

3. No issues with paint

tia

 

[KSMooniac]

6 hours ago, PeterRus said:

Read your posts, just to confirm the high-level points:
1. The prop is lighter and in addition to the increase in UL the prop moves the CG backward a little, which is beneficial

2. Performance is marginally better and the prop is quieter

3. No issues with paint

tia

 

Correct, mostly except #3.  I've written a bunch over the years here and perhaps you found some of my posts.  I did have paint shedding in rain immediately, followed by a repaint at my local MT shop.  It failed again, and then I got some new blades from Germany and those have held up very well.  I have stainless leading edges, and it is hard to get anything to stick to stainless... I think they use nickel now and it may be better.  My main gripe would be MT wants the paint to cover as much of the blade as possible, where I would rather have no paint on the stainless or nickel... I don't mind that look and it would be more durable, but they're German and thus difficult  and inflexible.  

[201Mooniac]

My MT prop has no paint on the nickel leading edge strip, it came from the factory that way.

[Pinecone]

On 3/14/2025 at 4:25 PM, KSMooniac said:

Cbut they're German and thus difficult  and inflexible.  

Ve know what you want better than YOU know what you want.  

 

[rturbett]

I've been very impressed by the electronic magneto installed on mine. The surefly has almost zero drop in rpm when its tested in runup.

[EricJ]

2 hours ago, rturbett said:

I've been very impressed by the electronic magneto installed on mine. The surefly has almost zero drop in rpm when its tested in runup.

That's a little suspicious because it may mean that the other mag isn't turning off.   Regardless of how good they are, there should be an rpm drop when either side of the ignition is shut off.

[Pinecone]

The hotter spark on an electronic mag does a better job igniting the fuel-air charge, so you see little mag drop.  Also, most report being able to run deeper LOP.

[KSMooniac]

5 hours ago, Pinecone said:

The hotter spark on an electronic mag does a better job igniting the fuel-air charge, so you see little mag drop.  Also, most report being able to run deeper LOP.

True.  My observation with dual ElectroAir now is essentially no drop on runup at 2000 RPM when turning off either side.  The only indication is the LED status light going out.   Running LOP at cruise power I will get a drop, though.

[A64Pilot]

6 hours ago, Pinecone said:

The hotter spark on an electronic mag does a better job igniting the fuel-air charge, so you see little mag drop.  Also, most report being able to run deeper LOP.

How deep do you want to run a Lyc IO-360? Mine will continue to run smoothly well below the point where it loses a LOT of power, (D single drive mag) at least 75C LOP, although a hotter spark will allow larger electrode gap and that will ignite lean mixtures better.

Oh, and the throw away Slicks were decades ago, back then they were cheap, so much so that a new one wasn’t much more than an overhaul, but they ain’t cheap anymore. You overhaul them now I’m told.

Nickel is much better than SS

Edited March 18 by A64Pilot

[cliffy]

@Martin S. 

Can you clarify just what is required in Germany as far as compliance with Service Bulletins issued?

Are you required to accomplish every SB issued or are they optional in some way?

I've found Slick mags to be just as reliable as Bendix when the 500 hr INSPECTIONS are completed.

An OVERHAUL is not required each 500 hrs BUT if you send your mag in to someone it must be delineated

that you want a 500 hr INSPECTION AND REPAIR (IRAN) and not an OVERHAUL (If the shop will do just a 500 hr Inspection) 

Neglect of the mags is the #1 reason many have "issues" with either mag system.

I've seen airplanes with mags untouched for 1000 hrs.  and more.

I never had mags issues even with dual mags as long as the Inspections were completed.

I did dual mag 500 hr IRANs on my hangar bench when I was running a Navajo. 

In fact Bendix even put out a very well done color chart/poster delineating exactly how to 

do a mag service, with detailed pictures. Wish I still had the chart. 

It ain't rocket science.