Mag Breaker Points

NealRomeoGolf

NealRomeoGolf

Final Approach
PoA Supporter
Joined
Apr 12, 2016
Messages
5,123
Location
Illinois
Display Name
Display name:
NRG
I am what you call "magneto stupid." Anything to do with electricity and I don't get it. My A&P replaced my left mag's breaker points today after my partner had a bad drop on run-up. Mags are probably 300 hours since last 500 overhaul (don't have my logs handy....just estimating). I did some google research on what breaker points are. Anyone care to educate me on how these break?
 
Corrosion, wear, pitting, maybe a bad set to begin with.
 
Mags produce a pretty weak spark. As Paulie pointed out, corrosion or pitting on them can further weaken things. If you lived in the days of the old pre-electronic car ignitions you'd see this happening to some extent (though the coil/battery in the car was a stronger spark and could blow through the crud a bit easier). This is the same reason aircraft plugs foul more than cars do (either before or after electronic ignitions).
 
NealRomeoGolf said:
I am what you call "magneto stupid." Anything to do with electricity and I don't get it. My A&P replaced my left mag's breaker points today after my partner had a bad drop on run-up. Mags are probably 300 hours since last 500 overhaul (don't have my logs handy....just estimating). I did some google research on what breaker points are. Anyone care to educate me on how these break?
Click to expand...

They don’t break, they wear, burn, and erode.
 
The opening and closing of the points in a mag, control the dwell of the coil in the mag.
As the cam follower wears the points will open late and close early shorting the dwell time in the coil, this causes a smaller electronic build up in the coil, and thus produce a weaker spark.
 
Hmmm, I think you have that backwards. Points opening late and closing early will increase dwell time. Dwell time is when the points are closed, completing the circuit and allowing the magnet to charge the coil.
 
Then of course there's the issue of the primary and secondary coils. As the points open and collapse the primary coil, the field in the secondary collapses and fires the plug. The condition of the points, as well as the "dwell time", or the time the points are closed to allow the primary coil to receive current, is a key factor in creating the thousands ov volts necessary to firing a spark plug.
If the points were really burned and pitted, you might have asked your A&P to replace the condenser as well.
If you're really interested, download Bendix's service manual. Will tell you everything you need to know. And since you're life depends of them functioning properly, it's good info to have in your back pocket.
 
NealRomeoGolf said:
I am what you call "magneto stupid." Anything to do with electricity and I don't get it. My A&P replaced my left mag's breaker points today after my partner had a bad drop on run-up. Mags are probably 300 hours since last 500 overhaul (don't have my logs handy....just estimating). I did some google research on what breaker points are. Anyone care to educate me on how these break?
Click to expand...
They "break" when the rotating cam inside the mag (driven by a gear on the engine) push them open.
They "break" when they corrode, pit, wear, erode, or fail mechanically.

Which meaning of "break" did you mean?

Jim
 
Chip Sylverne said:
Then of course there's the issue of the primary and secondary coils. As the points open and collapse the primary coil, the field in the secondary collapses and fires the plug.
Click to expand...

Slight correction. As the field in the primary collapses due to the primary coil excitation current going to zero, the secondary field "goes bananas" in accordance with the L di/dt equation trying to maintain the field current by increasing the secondary voltage to thousands of volts, firing the plug. THEN the secondary field collapses from lack of energy taken by the plug arc.

Jim
 
flyingbrit said:
Hmmm, I think you have that backwards. Points opening late and closing early will increase dwell time. Dwell time is when the points are closed, completing the circuit and allowing the magnet to charge the coil.
Click to expand...
Correct, the points will be open a shorter time, when they open late the timing changes.

But remember the field in the coil will diminish when the points don't open at peak magnetic influence, thus a weaker spark.
 
The magneto's points are set to open at the specified E-gap, which is a few degrees after the neutral point of the magnet's rotation. That's where the maximum collapse of the magnetic field occurs. Dwell is a term used in old automotive ignitions, and is not considered in a magneto. If the E-gap is set, the dwell is OK.

06-piston-eng-ignition-29-638.jpg


Magneto+flux+curves.jpg
 
Dan Thomas said:
The magneto's points are set to open at the specified E-gap, which is a few degrees after the neutral point of the magnet's rotation. That's where the maximum collapse of the magnetic field occurs. Dwell is a term used in old automotive ignitions, and is not considered in a magneto. If the E-gap is set, the dwell is OK.

06-piston-eng-ignition-29-638.jpg


Magneto+flux+curves.jpg
Click to expand...
Go back and see post #1, and ask yourself If that person would understand anything in your post?
 
Tom-D said:
Go back and see post #1, and ask yourself If that person would understand anything in your post?
Click to expand...

If he bothers to Google a bit, he will. Furthermore, we should talk about the terms and settings that Slick and Bendix talk about in their magneto service manuals.
 
Dan Thomas said:
If he bothers to Google a bit, he will. Furthermore, we should talk about the terms and settings that Slick and Bendix talk about in their magneto service manuals.
Click to expand...
99% of all owners will never do any mag work, I even send them out.
I've found out if they need points, they need a lot of other parts too.
Plus, you change the points in your customer's mag, only to find out it needs another set 50 hours later because the condenser was bad too. now you have a POed customer, and the mag is off to the rebuild shop anyway.
 
Mags are bone simple devices. Other than an inside bearing puller and an internal timing scale, there are no special tools required. Really no different from tractor ignitions. I don't know why such a big mystery has developed around them. Rocket science, they are not.if a part is bad, you replace it. It's not like there are that many of them.
 
Chip Sylverne said:
Mags are bone simple devices. Other than an inside bearing puller and an internal timing scale, there are no special tools required. Really no different from tractor ignitions. I don't know why such a big mystery has developed around them. Rocket science, they are not.if a part is bad, you replace it. It's not like there are that many of them.
Click to expand...
Do you as an A&P want the liability for them?
 
Chip Sylverne said:
Mags are bone simple devices. Other than an inside bearing puller and an internal timing scale, there are no special tools required. Really no different from tractor ignitions. I don't know why such a big mystery has developed around them. Rocket science, they are not.if a part is bad, you replace it. It's not like there are that many of them.
Click to expand...

+1

Because we live in a solid-state age where we "throw it out" when it breaks.
When was the last time anybody here had an older cell phone repaired. o_O
When my 3 year old refrigerator failed and I hauled it off to the "repair shop" the tech told me the cost to fix it (it needed a new compressor) would be almost as much as just buying a new fridge with a warranty.

So anything that involves taking something apart and actually fixing it now has this mystique about it.

When it comes to airplane magnetos I've found even something that simple gets screwed up by people that are not familiar with doing that work. I finally found a grizzled old veteran that knows his stuff to maintain mine, and that has proved the most economical and reliable outcome for me.
 
Let'sgoflying! said:
My dad actually had a dwell meter for cars back in the 70s when they all had points and coils.
Click to expand...
Yeah, those were good for 8cyl GMs which had a window on the distributor cap that you could slide up and insert a tool and adjust the points while the engine was running. Kinda useless for anything else though; if you have the cap off you might as well use the feeler gauge.
 
Tom-D said:
Do you as an A&P want the liability for them?
Click to expand...

This is an argument I don't understand. You are willing to accept the liability for an engine OH, which has many parts, and if it fails in flight no redundancy. But yet, a magneto with few parts and complete redundancy you want no part of? What am I missing?
 
You can get away without much of that fancy stuff. Slick even gives away some of the calibrating tools at various shows and symposiums. I did a lot of mag inspections, and most of the time the points would go 1000 hours (two 500-hour inspections) and the condenser almost never needed replacing. Many digital multimeters--even cheaper units--have a capacitance test function, and if it's within the manual's spec range, it's fine. The most expensive part of the whole thing is the manual, but if you're looking after a number of airplanes that fly frequently enough, it pays for itself. It's not rocket science.

An awful lot of mags don't fly much, and corrosion sets in and destroys it. Ground-running the engine just "to circulate the oil" really aggravates that. If that impulse spring gets corroded you have a real threat. If it breaks, the timing goes to TDC or even after, and power drops to almost nothing.
 
Dan Thomas said:
and the condenser almost never needed replacing
Click to expand...
I just got mine back from aircraftmagnetorepair.com I think it is...in Missoula....and they told me the Slick condensor rarely needs changing and in the past it was standard to replace every time.....which was so counter-Mike Busch it has now been rescinded.
 
Dan Thomas said:
I did a lot of mag inspections, and most of the time the points would go 1000 hours (two 500-hour inspections) and the condenser almost never needed replacing.
Click to expand...

So are you saying at 500 hour overhauls these components don't get replaced?
 
NealRomeoGolf said:
So are you saying at 500 hour overhauls these components don't get replaced?
Click to expand...
Simply know the 500 thing is simply compliance with a service bulletin.

When you would like to know what an overhaul is, read FAR 43.2
 
NealRomeoGolf said:
So are you saying at 500 hour overhauls these components don't get replaced?
Click to expand...

A 500-hour inspection is not an overhaul.

If it was someone else's airplane, the mag would get new points. They're not terribly expensive, and you don't know how long the owner will push that thing before he gets it inspected again. In the fleet world where I worked, we monitored these things all the time and with experience we found that the points would last 1000 hours and replacing them if they didn't need it was a waste of time. We cleaned things up and made any point adjustments to bring the E-gap back into spec. If the gap (internal timing) had drifted much, or the mag's timing to the engine had required much adjustment between inspections, we'd take it off and find out why. If points erode quickly or the plastic cam or rubbing block wears too fast, the timing will drift a lot. Slick has had some QA issues with both points and cams. If the mag builder puts too much grease on the cam, it will get into the points and foul them. If he doesn't get that coil tab exactly right it will cause wear and failure of the rotor brush. All of that stuff and much more is in the manuals, but I suspect that many shops don't have the manuals. The stuff I'd find in airplanes proved it. Badly misrigged controls and lots of other stuff.

There's a good education on magneto theory in Slick's manual. That condenser is responsible for a lot more than preventing arc erosion of the points, for instance. Just try running any points-ignition system without it.
 
Let'sgoflying! said:
My dad actually had a dwell meter for cars back in the 70s when they all had points and coils.
Click to expand...
I still have one, some where. I also have the Allen wrench on a flex cable for adjusting the chevy points thru the little window.
 
I've got one top. Dwell/Tach/voltmeter. Used it it "set 'em cranking". Pull the cap and rotor, turn the engine over and set dwell by averaging the needle swing. Of course using the thickness of a matchbook cover was probably just as accurate. We still have a guy around here that will curve a distributor. Now that's old school stuff.
(But still more advanced than an aircraft mag, pun intended.)
 
Last edited:
flyingbrit said:
Yeah, those were good for 8cyl GMs which had a window on the distributor cap that you could slide up and insert a tool and adjust the points while the engine was running. Kinda useless for anything else though; if you have the cap off you might as well use the feeler gauge.
Click to expand...

A dwell meter was the first "complex" tool I owned as a twelve year old budding mechanic, bought it at Sears. Almost fifty years later, I still have it. I don't think I still own one of the flexible shaft Allen head points adjusters used in conjunction with the meter.

I still have a vehicle with points ignition, my 1966 F-100 has a 352 FE.

Now get off my lawn. :D
 
You must log in or register to reply here.
Back
Top