Duel Magneto Retrofit to Two Sperate Magnetos

[scottt175]

The Lycoming IO-360-A3B6D engine in my M20J has a duel magneto setup (two magnetos in one housing). I would like to change this to a two separate magneto setup for safety, ease of maintenance, and better redundancy reasons. Does anyone know if this change is approved through STC or the type certificate for a M20J? Thanks for any help on this. 

[EricJ]

17 minutes ago, scottt175 said:

The Lycoming IO-360-A3B6D engine in my M20J has a duel magneto setup (two magnetos in one housing). I would like to change this to a two separate magneto setup for safety, ease of maintenance, and better redundancy reasons. Does anyone know if this change is approved through STC or the type certificate for a M20J? Thanks for any help on this. 

The TCDS is your friend.   Approved engines:

Engine Textron-Lycoming IO-360-A1B6D or IO-360-A3B6D or IO-360-A3B6. (Bendix fuel injector, Model RSA 5AD1, P/N 2524054) See Note 12 and Note 20.

The A3B6 has separate mags.   Lycoming also has an STC to put an IO-390 in, which also has separate mags.

Nothing wrong with the dual mag setup, though.   Some of us prefer it.

[toto]

There are many discussions on MooneySpace covering this, and several members who have done it. Essentially, you have to convert the A3B6D to an A3B6, which is not a small amount of work. I think the more common approach is to trade in a D case to Lycoming for a factory OH with separate mags at overhaul time. 
 

[toto]

1 minute ago, EricJ said:

Nothing wrong with the dual mag setup, though.   Some of us prefer it.

Yeah. This is one of those things that people get hyper-focused on from a safety perspective, but actually represents a vanishingly small percentage of aircraft incidents. As long as the shop follows the manual and is conscientious about torquing the install bolts correctly, there just aren’t many failures of the Bendix dual mags. 

I'm flying an A3B6D myself and not overly concerned about it.

[scottt175]

Thank you EricJ and Toto, very helpful.

[N201MKTurbo]

T convert it you basically keep the crank, cam and cylinders and replace everything else.

[OR75]

the IO-360-A3B6 and A3B6D are the same except what is called the "accessories panel" (the back of the engine that the mag, tach, oil filter, governor, ... attach to)

if you make that change, you will also need a few brackets related to some of the accessories (the governor for example) and also some fluids hoses will need to be changed 

you will also need to re-clock the prop governor to a different ratio

 

 

[PT20J]

33 minutes ago, OR75 said:

the IO-360-A3B6 and A3B6D are the same except what is called the "accessories panel" (the back of the engine that the mag, tach, oil filter, governor, ... attach to)

if you make that change, you will also need a few brackets related to some of the accessories (the governor for example) and also some fluids hoses will need to be changed 

you will also need to re-clock the prop governor to a different ratio

 

 

Although the accessory case is the most obvious external difference, there are many other differences. Even the crankshaft is different. Check the parts catalog. They are really different engines and since relatively few A3B6Ds were built, Lycoming would really like to retire them which is why they will take a A3B6D core in exchange for an A3B6 (Lycoming's usual core policy is that you have to exchange like for like). In fact, when I got my rebuilt engine in late 2018, Lycoming gave me a discount to order an A3B6 instead of an A3B6D.

So, bottom line is that the only reasonable way to change a A3B6D to an A3B6 is to use the A3B6D core when ordering a rebuilt A3B6 from Lycoming. Fortunately, both engines are listed on the TCDS for the M20J and the IPC lists all the parts necessary for the conversion. Another option is to convert by STC to the IO-390.

Skip

[N201MKTurbo]

5 hours ago, OR75 said:

the IO-360-A3B6 and A3B6D are the same except what is called the "accessories panel" (the back of the engine that the mag, tach, oil filter, governor, ... attach to)

if you make that change, you will also need a few brackets related to some of the accessories (the governor for example) and also some fluids hoses will need to be changed 

you will also need to re-clock the prop governor to a different ratio

 

 

You have to remember that every gear that mounts in the accessory case has a complementary boss in the crankcase. They are not interchangeable.

[Pinecone]

19 hours ago, toto said:

Yeah. This is one of those things that people get hyper-focused on from a safety perspective, but actually represents a vanishingly small percentage of aircraft incidents. As long as the shop follows the manual and is conscientious about torquing the install bolts correctly, there just aren’t many failures of the Bendix dual mags. 

I'm flying an A3B6D myself and not overly concerned about it.

 A friend of mine in the Cardinal world would disagree.

It seems there is another failure mode where the condenser fails and the arcing of the points over heats them and melts the follower block leading to the points not opening.

 

[N201MKTurbo]

33 minutes ago, Pinecone said:

 A friend of mine in the Cardinal world would disagree.

It seems there is another failure mode where the condenser fails and the arcing of the points over heats them and melts the follower block leading to the points not opening.

 

That can happen on any mag.

[Skyland]

37 minutes ago, Pinecone said:

 A friend of mine in the Cardinal world would disagree.

It seems there is another failure mode where the condenser fails and the arcing of the points over heats them and melts the follower block leading to the points not opening.

 

Maybe you just offered an explanation for my dual mag failure last year.  The follower blocks on both points wore or melted down simultaneously causing both point gaps to go to zero.  Turns out the A&P who installed the mag unknowingly had a condenser wire touching the cam or shaft.

 

[N201MKTurbo]

If that wire gets grounded, the mag will stop firing. Same as turning off the key.

[Pinecone]

22 hours ago, N201MKTurbo said:

That can happen on any mag.

But only one mag.  The other one should still be working?

[N201MKTurbo]

1 hour ago, Pinecone said:

But only one mag.  The other one should still be working?

Same on a dual mag. if you overheat one set of points, it won't affect the other set.

[N201MKTurbo]

The real shortcoming of the dual mag is the point cam. They aren't machined perfect. The syncing between the two mags is controlled by the cam. The cam has two lobes on it so it can fire both mags at the same time. The points are 180 degrees apart on the cam. You can fudge the point gaps to get them to fire at exactly the same time on cylinder 1, but when it gets to cylinder 3, which is both mags firing on the opposite cam lobe, they can be off by a degree or two. There is nothing you can do about that other than split the difference. This can take a long time to do with a lot of trial and error. Most mechanics don't know how to do this or just don't want to.  

[Pinecone]

On 12/22/2023 at 11:43 AM, N201MKTurbo said:

Same on a dual mag. if you overheat one set of points, it won't affect the other set.

Does it have two condensers?

[EricJ]

10 minutes ago, Pinecone said:

Does it have two condensers?

The only things that are common to both sides in a dual mag are the magnet and the case.   Everything else is redundant, just like separate mags.

[N201MKTurbo]

1 hour ago, Pinecone said:

Does it have two condensers?

The condenser is in the cap. It is where the P leads connect. Magnetos require their own condenser. The dual mag is just what it says. Two mags in one assembly.

http://www.aeroelectric.com/Mfgr_Data/Magnetos/Continental_Motors/X42003_D-2000_D3000_SM.pdf

 

[Flash]

On 12/20/2023 at 1:51 PM, PT20J said:

Although the accessory case is the most obvious external difference, there are many other differences. Even the crankshaft is different. Check the parts catalog. They are really different engines and since relatively few A3B6Ds were built, Lycoming would really like to retire them which is why they will take a A3B6D core in exchange for an A3B6 (Lycoming's usual core policy is that you have to exchange like for like). In fact, when I got my rebuilt engine in late 2018, Lycoming gave me a discount to order an A3B6 instead of an A3B6D.

So, bottom line is that the only reasonable way to change a A3B6D to an A3B6 is to use the A3B6D core when ordering a rebuilt A3B6 from Lycoming. Fortunately, both engines are listed on the TCDS for the M20J and the IPC lists all the parts necessary for the conversion. Another option is to convert by STC to the IO-390.

Skip

This is what I did in 2014. I think it was $4000 cheaper to get an A3B6 than an A3B6D in exchange for my A3B6D core. One of the differences is the oil filter; my A3B6 uses an oil filter with a male attachment; the A3B6D used a female.