Bendix SN621 Bearing Failure

Post by **GAHorn** » Thu Dec 14, 2006 8:53 pm

If you ever observe a well-equipped overhaul facility, that is exactly how they test overhauled magnetos. They connect it to a "spark board" which is nothing but a board with thumb-screw adjustable open-spark electrodes and they run it and let it fire away in the open. True, there's a spcification for that spark distance, but it's pretty big, and I believe part of the test involves seeing that it sends that spark out to the board at higher resistance/spark-distance to confirm the magneto does not short internally.
It would be likely to short internally if the mag were run totally disconnected from a harness, but it's unlikely to have any ill effect if it's connected to other plugs or any type of harness, even a defective one, because the harness itself is grounded, and offers the path of least resistance.
This becomes an issue in magnetos in high-altitude service and is why turbocharged engines also provide pressurization to the magneto.

lowNslow · Post by **lowNslow** » Thu Dec 14, 2006 10:58 pm

"Which Magneto is the Best" according to "The Magneto Ignition System" by John Schwaner (Sacromento Sky Ranch). John also used to have a mag overhaul shop.

It varies by which model, but for the mags used on the O-300/C-145:

Ease of maintenance- Slick

High altitude capability- Slick

Durability-Bendix

Starting performance-Bendix (however the models used on the O-300/C-145 were basically tied at the bottom of the list)

Weight-Slick

EMI supression-Slick

blueldr · Post by **blueldr** » Fri Dec 15, 2006 4:30 am

Bendix was builbing Scintilla Magnetos under license when the slick brothers were in grade school. They didn't get involved in magnatos until after their freight airline, using WWII surplus Curtis C-46 airplanes, went down the tube and they bought out the old Eiseman ,or some other, magneto manufacturer.

lowNslow · Post by **lowNslow** » Fri Dec 15, 2006 4:40 am

Slicks are still good mags. They got kind of a bad rap when they tried the "disposable" mags a while back which weren't that good. Not only are there fewer ADs on the Slicks but also far fewer Service Bulletins.

Post by **Bruce Fenstermacher** » Fri Dec 15, 2006 3:02 pm

Slicks are better when you use Mogas.

Ok this is total BS but I just had to add it to the discussion.

Bill Hart · Post by **Bill Hart** » Fri Dec 15, 2006 3:52 pm

Well Bendix mags work better with MMO!

Post by **GAHorn** » Fri Dec 15, 2006 10:20 pm

Actually, the best mags for mogas and MMO are Eisemann. (on a tractor)

dacker · Post by **dacker** » Fri Dec 15, 2006 11:21 pm

Getting back to the more serious discussion of which mags are better...
I replaced my Bendix with Slicks 150 hours ago, mainly because I was fed up with getting my Bendix mags worked on and trying to figure out whether the previous owners actually tried to meet the AD requirements or just pencil whipped them (you guessed it, pencil lead is cheaper than maintenance). The Slicks so far have been great, and they cost only a little more than properly overhauled Bendix mags. Are they better? I will tell you when I replace them someday depending on how long they last. Now as far as MMO....

David

Post by **GAHorn** » Mon Dec 18, 2006 5:20 pm

No matter which magnetor brand you have, it's regular inspection and repair is paramount.
In 1992, the NTSB recommended that the FAA require Mandatory Inspection and Maintenance on ALL brands of magnetos, due to he number of accidents caused by magneto neglect. (Info derived fromTCM SIL646.)
Magnetos are like exhaust systems. They are expensive. They are neglected. They can hurt you.
I have an old, retired mechanic in my area who misses the airport and needs something to do. He's scheduled to use my mags over the holidays as tools to teach me how to better inspect/maintain my magnetos. I'll bet some of you guys know someone like that who can pass along some tips, and you'll kil,...err... save two birds with one stone(-age mechanic.)

Post by **GAHorn** » Mon Dec 18, 2006 5:20 pm

No matter which magneto brand you have, it's regular inspection and repair is important.
In 1992, the NTSB recommended that the FAA require Mandatory Inspection and Maintenance on ALL brands of magnetos, due to he number of accidents caused by magneto neglect. (Info derived fromTCM SIL646.)
Magnetos are like exhaust systems. They are expensive. They are usually neglected. They can hurt you.
I have an old, retired mechanic in my area who misses the airport and needs something to do. He's scheduled to use my mags over the holidays as tools to teach me how to better inspect/maintain my magnetos. I'll bet some of you guys know someone like that who can pass along some tips, and you'll kil,...err... save two birds with one stone(-age mechanic.)