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What is difference between IO360-A3B6D and IO360-A3B6?


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5 minutes ago, 201er said:

Separate mags

So the A3B6 has separate mags whereas the A3B6D did not? I was under the impression that both had dual mags but the A3B6D had a single drive whereas the A3B6 has dual drive and dual magnetos.

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35 minutes ago, bcwiseguy said:

So the A3B6 has separate mags whereas the A3B6D did not? I was under the impression that both had dual mags but the A3B6D had a single drive whereas the A3B6 has dual drive and dual magnetos.

Dual mag is a single drive mag with 2 inside the same case. The A3B6 has two independent mags. Newer A3B6 come with roller tappets from Lycoming. I don't think you can get that on an overhauled A3B6D.

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A single, dual mag versus two separate magnetos.

If replacing an A3B6D with an A3B6 there are a few details that have to be changed, like the bracket for the prop governor cable, some baffling, etc.   There are threads here that cover the details.

 

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There are significant internal parts differences. You can download the illustrated parts catalog from the Lycoming website if you are interested in the details. The A3B6D was not as widely used and Lycoming has offered incentives to retire them.

The main things you notice externally are the different mags, different prop governor and different oil filter.

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What I'd like to know is the 'genius' reasoning behind this lame idea in the first place!

Really, how much money were they going to save by going to a single-drive, "dual" mag???  They had to not only design and tool for the mag itself, but for the accessory case, the new bracket, documentation costs with different BOMS (to call out the different prop governor, oil filter, other stuff?).  How many engines built before they paid back the R&D/NRE costs and were going to 'make a killing'?

No reliability history, either (but, they got that, didn't they?). Now they'd love to get them out of the field! No kidding!!

Who ever pitched, and SOLD, this ridiculous idea to Lycoming management deserves the marketeer of the decade award!

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When it came time for our overhaul three years ago my partners and I solicited opinions on whether to go from the A3B6D to A3B6 and the consensus was pretty much down the middle. The deciding one was our A&P, who deals almost exclusively with Mooney's, when he said that he's never had to deal with a magneto failure on the "D". He told us to save our money.

But I must agree, who was the bonehead who decided that a single shaft drive was the way to go.

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47 minutes ago, MikeOH said:

What I'd like to know is the 'genius' reasoning behind this lame idea in the first place!

Really, how much money were they going to save by going to a single-drive, "dual" mag???  They had to not only design and tool for the mag itself, but for the accessory case, the new bracket, documentation costs with different BOMS (to call out the different prop governor, oil filter, other stuff?).  How many engines built before they paid back the R&D/NRE costs and were going to 'make a killing'?

No reliability history, either (but, they got that, didn't they?). Now they'd love to get them out of the field! No kidding!!

Who ever pitched, and SOLD, this ridiculous idea to Lycoming management deserves the marketeer of the decade award!

Lycoming didn't invent it.   Dual mags were used even in WWII on things like the Rolls-Royce Griffon engine (very similar to the Merlin), so there was a lot of established history even on combat aircraft.  It's not a bad system, and starting spark is improved significantly since the impulse coupler drives both sides instead of just one.

On GA engines there is competition on the accessory case for space, so the dual mag helped to solve a number of engineering problems back in the day.   For the most part the only components that lose the redundancy of the separate mags are the actual gear drive and the magnet.   The magnet doesn't really ever fail, so that's not an issue, and the drive failure rate is quite low unless the unit is not installed properly.   So really the main thing was that the manufacturers were moving some of the reliability equation to the maintainers, and that hasn't worked out all that well.   I think the move away from the dual mag is that the manufacturers take the hit in liability and reputation for maintenance mistakes.   Nowadays the need for vacuum pumps and such things is diminishing, so there's even less need for the extra space on the accessory cases.

Lycoming shipped many thousands of these things on both four- and six-cylinder engines in a number of different aircraft.   I don't think their bad reputation is deserved, other than the increased consequences of maintenance mistakes, and it certainly wasn't bad decision making at Lycoming other than perhaps underestimating the frequency and impact of maintenance mistakes.

 

 

 

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2 minutes ago, EricJ said:

Lycoming didn't invent it.   Dual mags were used even in WWII on things like the Rolls-Royce Griffon engine (very similar to the Merlin), so there was a lot of established history even on combat aircraft.  It's not a bad system, and starting spark is improved significantly since the impulse coupler drives both sides instead of just one.

On GA engines there is competition on the accessory case for space, so the dual mag helped to solve a number of engineering problems back in the day.   For the most part the only components that lose the redundancy of the separate mags are the actual gear drive and the magnet.   The magnet doesn't really ever fail, so that's not an issue, and the drive failure rate is quite low unless the unit is not installed properly.   So really the main thing was that the manufacturers were moving some of the reliability equation to the maintainers, and that hasn't worked out all that well.   I think the move away from the dual mag is that the manufacturers take the hit in liability and reputation for maintenance mistakes.   Nowadays the need for vacuum pumps and such things is diminishing, so there's even less need for the extra space on the accessory cases.

Lycoming shipped many thousands of these things on both four- and six-cylinder engines in a number of different aircraft.   I don't think their bad reputation is deserved, other than the increased consequences of maintenance mistakes, and it certainly wasn't bad decision making at Lycoming other than perhaps underestimating the impact of maintenance mistakes.

 

 

 

Hmm, thanks for the history lesson, but I'm staying with, 'This was a BAD idea for GA.'

Good engineering is often in the devil IS in the details, like considering after sale maintenance.  I don't know if you meant to imply installing the dual mag is more difficult than a 'normal' mag, but either way you've got a better chance with TWO mags (unless the same A&P butchers BOTH installs at the same time!).  And, having TWO sets of drive gears does provide an actual redundant level of protection.  I wasn't aware of such competition for 'pad space,' but what are these 'number of engineering problems' that the dual-mag solved?

The fact that Lycoming discontinued production, and wants them retired, is pretty good evidence that bad decision making occurred.  Yes, it's easy to say that in hind-sight, but that doesn't change the reality of it.

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4 hours ago, 201er said:

Dual mag is a single drive mag with 2 inside the same case. The A3B6 has two independent mags. Newer A3B6 come with roller tappets from Lycoming. I don't think you can get that on an overhauled A3B6D.

The serial # should end in "E" if Lycoming installed roller tappets at factory rebuild or overhaul.  I'm not sure if there are any field overhauled IO-360's out there converted to roller tappets, and whether they have to have their serial number changed, so I suppose it might be possible there are some IO-360's out there with roller tappets but without the "E", but it should be a rarity.

I think it's safe to say there are no A3B6D's with roller tappets.

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5 hours ago, bcwiseguy said:

So the A3B6 has separate mags whereas the A3B6D did not? I was under the impression that both had dual mags but the A3B6D had a single drive whereas the A3B6 has dual drive and dual magnetos.

Put it this way, if you lose a magneto drive on the A3B6 you fly on, although the engine may run a little hotter and rougher than usual, and you look for an airport where you can get the lost mag serviced. If you lose a drive on the 6D you put your off field landing skills into use, hopefully safely. It has happened a number of times. When I was thinking about buying back in about 2008 I did some searches on accidents. I remember one in particular where the mag drive gave up and the plane landed upside down in a Texas pond, unfortunately with the pilot still on board. There was another within a week or so of that one. They were probably back in the 1990's.

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1 hour ago, jaylw314 said:

The serial # should end in "E" if Lycoming installed roller tappets at factory rebuild or overhaul.  I'm not sure if there are any field overhauled IO-360's out there converted to roller tappets, and whether they have to have their serial number changed, so I suppose it might be possible there are some IO-360's out there with roller tappets but without the "E", but it should be a rarity.

I think it's safe to say there are no A3B6D's with roller tappets.

Mine came with roller tappets 

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Regardless of whether your engine has a D3000 magneto or 2 separate magnetos, at the root all of the gears in the engine are driven by a single gear bolted to the crankshaft with a single bolt.

I’d say far more pilots reck airplanes than maintainers.  A quick look at the salvage sites confirms my suspicions.

Clarence

 

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6 hours ago, 201er said:

Dual mag is a single drive mag with 2 inside the same case. The A3B6 has two independent mags. Newer A3B6 come with roller tappets from Lycoming. I don't think you can get that on an overhauled A3B6D.

You can get roller tappets in the A3B6D, at least I did.  All it took was a phone call to ask, there was no way to order it on the order form.  I heard Lycoming is converting the A3B6D engines to A3B6 on overhaul/reman but I'm not sure about that.

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3 hours ago, jaylw314 said:

The serial # should end in "E" if Lycoming installed roller tappets at factory rebuild or overhaul.  I'm not sure if there are any field overhauled IO-360's out there converted to roller tappets, and whether they have to have their serial number changed, so I suppose it might be possible there are some IO-360's out there with roller tappets but without the "E", but it should be a rarity.

I think it's safe to say there are no A3B6D's with roller tappets.

Ive got a 2006 Roller A3B6D in my shop right now.  Most engines shipped after 2005 are rollers.

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1 hour ago, wishboneash said:

Isn't the standard timing on the A3B6D 25 deg BTDC compared to 20 deg for the A3B6 variant?

IIRC the A3B6D only has 25 deg BTDC, the A3B6 can be either 25 or 20 deg.  I'm not sure, but I think it comes standard as 20 deg, and you have to change the nameplate if you change to 25 deg

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30 minutes ago, jaylw314 said:

IIRC the A3B6D only has 25 deg BTDC, the A3B6 can be either 25 or 20 deg.  I'm not sure, but I think it comes standard as 20 deg, and you have to change the nameplate if you change to 25 deg

That's what I recall. 20 deg keeps the CHTs lower I presume.

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1 hour ago, wishboneash said:

Isn't the standard timing on the A3B6D 25 deg BTDC compared to 20 deg for the A3B6 variant?

Yes.

45 years ago, Lycoming changed the standard timing on IO-360s from 25 degrees to 20 degrees. Reasons given were lower CHTs and increased detonation margin in extremely cold temperatures. The change requires a magneto with a 15 degree lag angle for starting and the dual magnetos were not available in that configuration so the dual mag engines were not included. If you have a A3B6, you can change it back to 25 degrees by installing the correct left magneto, retiming and marking the data plate with only a logbook entry since both timings are in the TCDS.

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SI1325 Timing Change for IO-360 Series Engines.pdf

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1 hour ago, PT20J said:

Yes.

45 years ago, Lycoming changed the standard timing on IO-360s from 25 degrees to 20 degrees. Reasons given were lower CHTs and increased detonation margin in extremely cold temperatures. The change requires a magneto with a 15 degree lag angle for starting and the dual magnetos were not available in that configuration so the dual mag engines were not included. If you have a A3B6, you can change it back to 25 degrees by installing the correct left magneto, retiming and marking the data plate with only a logbook entry since both timings are in the TCDS.

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SI1325 Timing Change for IO-360 Series Engines.pdf 470.94 kB · 3 downloads

Thanks for the info!

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38 minutes ago, larrynimmo said:

Was rebuilt in late august 2018...Lycomming factory

Wow, so Lycoming actually is still selling A3B6D rebuilds, I never realized that.  I thought they were in the business of selling only A3B6's so they wouldn't have to rebuild the TCM/Bendix dual mag.

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