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Magneto Timing

NunavutPA-12

FRIEND
67.8N,115.1W CYCO Canada
Home-built modified PA-12 clone. AME not easily accessible - I'm in Kugluktuk, nearest is in Yellowknife, 375-miles distant. Otherwise I'd get a pro to do this.

I got my right mag back from a 500-hour inspection and repair. Now I have to time it to the engine. I have the tools but this will be the first time I've attempted this particular job. I think I have figured out how to find TDC on the compression stroke, etc. thanks to YouTube (!).

Obviously, the mag itself has to be in a certain position prior to installing it on the engine, and I'm not too clear on how this is done. So I need some advice.

It's a Bendix mag on an O-320B2B.

Thanks.
 
Also, make sure the Mag gear doesn't move when you install it. They make a little tool to keep the gear from moving
 
If you have dual impulse unhook all the plug wires, take a top and bottom wire from the same cylinder and install a plug in each wire. Lay plugs on top of cylinder grounding their body. You have one mag installed and timed already, turn your mag switch on and get the overhauled mag to spark at the same time as the installed one will get you real close.

Glenn
 
Hmmm - you probably want to "spark out" the mag before you align the little red gear. Get some friendly help, or a better instruction book, before you go nuts installing and re-installing. It is fairly straightforward.

Or, you can learn the hard way, like we did.
 
Hmmm - you probably want to "spark out" the mag before you align the little red gear. Get some friendly help, or a better instruction book, before you go nuts installing and re-installing. It is fairly straightforward.

Or, you can learn the hard way, like we did.

what is "spark out"?
 
All spark plug leads removed from all plugs.
Remove all four top plugs.
Place thumb on #1 cylinder top plug hole.
Rotate prop until air pressure pushes thumb off plug hole. #1 is now on compression stroke.
Turn prop to align timing marks. 25 BTC on aft side of ring gear to split in crankcase halves. OR 25 BTC on forward side of ring gear with dimple on starter housing.
Remove round screw in cap and point cover plate from mag.
Rotate mag until white mark on rotor gear shows in cap hole. This is the #1 firing position.
While looking at points, slightly rotate the mag gear in both directions feeling the magnetism. It will pull in two opposite directions. Leave it in between the two. The points will be open and the magnetism will be holding the rotor steady. The white timing mark should still be visible but does not need to be centered as the points are controlling.
Hold the magneto with your left hand with two fingers on the cam which moves the points. This will assure that you can feel if the mag rotor moves while you are installing the mag. If you feel the rotor move at all, start over. Make sure that the cam is magnetically holding the rotor steady.
While holding the mag as stated, place it in the engine aligning the gear teeth. You may need to do this a few times until you get the feel of it. You need to be certain that you do not let the rotor move and that the gear teeth engage smoothly. Otherwise the timing will be off by one gear tooth.
The slots which go over the mounting studs on the crankcase should be somewhat in the center. This will give adjustment space. A gear tooth could engage at one end of the slot. This is not a very good idea as it limits your ability to correctly time the mag to the engine.
Once you have the mag against the accessory case, it can move while you install the large washers, star washers and nuts finger loose.
If you have a timing light or buzzer use it at this time. Otherwise use a piece of paper placed between the points.
Rotate the mag opposite the direction of normal rotation until the points hold the paper.
Rotate the mag in direction of rotation until the points just release the paper with your slight pull on the paper.
Snug the two nuts.
Reinstall the paper to check by rotating the prop. Back up then pull in direction of rotation until paper moves in points.
Check timing marks. Are they 25 degrees?
Tightening the mag nuts can move the timing slightly. When you tighten them do each one a little at a time until both are tight.
Replace the screw in cap, the point cover and connect the "P" lead wires. Two wires unless the ground wire is on the other mag.
Reconnect the plugs and wires. You should be good to go.

This assumes that there is no impulse on this mag. If there is an impulse the procedure is the same except that you must be sure that the impulse cam is NOT engaged while you are setting the mag rotor in the correct position and that it does NOT inadvertently become engaged while you are installing the mag. It's more fussy than without the impulse. And when you are checking the timing after installation you must be certain that the impulse is not engaged. Don't turn the prop backwards too much which will engage it.

Don't forget the gasket between the mag and the case.
 
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Skywagon8a gives excellent and complete advice. My only suggestion is to get a mag. lock tool and a buzz box. It's true that magnetism will tend to keep the mag. from moving but it will likely drive you crazy getting the mag. in position with out it moving. Especially hard with a very short mount coupled with an impulse where there is hardly any room. A Pitts Special or my Tu-holer has zero extra room while my Acroduster too with 24" mount isn't too bad. I guess the SC is average.

The down side of the mag. lock tool is that you might forget to remove and move the prop while installed and damage the mag. gears, BAD IDEA! It is spring loaded and I confess I did move the prop once with out damage but I wouldn't count on it?

Buzz box makes job much easier and accurate but I've used a piece of paper and checked it later and it was within 1/2 degree.

Mag. lock tool and buzz box are relatively inexpensive.

Thanks for an excellent write-up.

Yea, the gasket. I've done the complete job and am very satisfied with myself until I count an extra gasket laying there and have to do the job all over.......chalk it up to practice...or ask yourself if your mind is going and you shouldn't be working on airplanes? (Of coarse we are being supervised by a real airplane mechanic if certified aircraft)

Jack
 
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If you use a mag lock tool don't forget to remove it before anyone bumps the prop or you will break a tooth off the gear-or worse. jrh
 
Put some type of grease on both sides of the round mag gasket. That way in a year when you go to retime the mag you don't tear the gasket and start an oil leak!! I did mine with out the pin, had to take a few shots at it but not hard at all.
DENNY
 
If you use a mag lock tool don't forget to remove it before anyone bumps the prop or you will break a tooth off the gear-or worse. jrh
That is the reason that the mag lock tool is not recommended. Once you understand the procedure as I've described it above you will not need the tool.

Put some type of grease on both sides of the round mag gasket. That way in a year when you go to retime the mag you don't tear the gasket and start an oil leak!! I did mine with out the pin, had to take a few shots at it but not hard at all.
DENNY
NunavutPA-12 has a Bendix mag. It does not use the Slick pin.
 
And stand under the nearest shade tree while using this technique


Sorry Glenn, I just could not help myself





If you have dual impulse unhook all the plug wires, take a top and bottom wire from the same cylinder and install a plug in each wire. Lay plugs on top of cylinder grounding their body. You have one mag installed and timed already, turn your mag switch on and get the overhauled mag to spark at the same time as the installed one will get you real close.

Glenn
 
Once again I find myself questioning aircraft maintenance principles. Just like the "static" differential compression test, static timing is great as a starting point but it assumes points will act the same while running. Having worked as a mechanic during the points era, there often were times while watching points fire with a timing light I would see the timing drift or intermittently change at higher RPM. This was usually caused by either a weak spring or rust between the pivot pin and the point rotor. How would you catch this problem with a buzz box?
 
Skywagon
When I first saw the post I said the same thing to myself (bendix don't use no stinking pin). But, a internet search shows a lock, Aircraft spruce and several other venders have them for sale. It goes in the window on the side and locks the gear. I have never seen one used but lots of things I have not seen in this world yet.
DENNY
 
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Skywagon
When I first saw the post I said the same thing to myself (bendix don't use no stinking pin). But, a internet search shows a lock, Aircraft spruce and several other venders have them for sale. It goes in the window on the side and locks the gear. I have never seen one used but lots of things I have not seen in this world yet.
DENNY
DENNY, Is this what you are referring to? http://www.aircraftspruce.com/catalog/fbpages/FBsafetymag.php?clickkey=7741 These locks only lock the rotor in one position. If one is timing the magneto internally to produce the best spark, the rotor will not necessarily be in this exact same position. In other words the breaking of the points will not always agree with the location of the rotor gear teeth. When you time a magneto to an engine at a specific number of degrees BTC, you want the points to break at that number of degrees. The use of a timing light or buzz box will give you the most accurate moment of points breaking. The method which I described above of using a piece of paper is more accurate than Spruce's tool but less accurate than the other two devices. Yet the paper will get you home when you have minimum tools and will be accurate. Practice makes perfect.
 
That is it, they make a metal one also but I think the rubber would be safer. I will most likely use your method next time I pull them. I only had to try a few times to get it in without the gear moving.:oops:
DENNY
 
I will say that the rotor lock would help by eliminating the need to hold it with two fingers in order to engage the gear teeth, then remove it and set the timing.
 
Once again I find myself questioning aircraft maintenance principles. Just like the "static" differential compression test, static timing is great as a starting point but it assumes points will act the same while running. Having worked as a mechanic during the points era, there often were times while watching points fire with a timing light I would see the timing drift or intermittently change at higher RPM. This was usually caused by either a weak spring or rust between the pivot pin and the point rotor. How would you catch this problem with a buzz box?

Pardon my ignorance on this topic, but I THOUGHT that the old-type automotive distributors used some sort of a "vacuum advance" to adjust the timing of the spark to the plug? If true, wouldn't that cause exactly what you observed - timing drifting and changing as the RPMs went higher?

I also understood that aviation magnetos have a "fixed" timing - nothing like the vacuum advance involved - and that the "fixed" timing was set so that the spark took place at the right time when operating at "max power" settings (usually redline RPM). I was told that this fixed timing is one of the reasons that airplane engines can be so much more "cantankerous" to start at times...

But I'm not a mechanic (much less an A&P), and I didn't even sleep at the Holiday Inn last night, so WTHDIK?
 
Usually when checking timing I disable vacuum advance.

You're right about fixed timing mags. A common misconception is that vacuum, mechanical, and electronic advance bring in more timing as more power is demanded. The opposite is true. Lower rpm, lower power settings generally need more advance to run optimally.

I tuned my LS1 V8 Trans Am a few years back. Ended up with 40 some odd degrees of timing in some places on my timing table. Wide open throttle started at 26 degrees at lower rpm and ended up at 28 degrees at higher (6k+) rpm.


Sent from my iPhone using Tapatalk
 
You're right about fixed timing mags. A common misconception is that vacuum, mechanical, and electronic advance bring in more timing as more power is demanded. The opposite is true. Lower rpm, lower power settings generally need more advance to run optimally.

I tuned my LS1 V8 Trans Am a few years back. Ended up with 40 some odd degrees of timing in some places on my timing table. Wide open throttle started at 26 degrees at lower rpm and ended up at 28 degrees at higher (6k+) rpm.
Cam, You have contradicted yourself. Lower rpm, lower power settings generally need less advance to run optimally. The timing of the spark is related to the length of time that it takes the fuel mixture to reach it's maximum pressure after ignition. The faster the crank turns the more advance is needed.
 
My comments about the timing drifting or sometimes firing intermittently retarded were with magneto fixed timing small Honda motorcycles that did not have any advance mechanism. The timing light was used to confirm the timing was correct "running" after using what we called static point timing to set the timing. The timing light would show a defective set of points that static timed just fine. I was just making the observation. Maybe aircraft points never go intermittent running.
 
Sky is right. Any type of advance mechanism brings in more advance as the RPM increases.

Edit: Actually I am just talking old school. All bets are off with some of the modern engines (variable cam timing, electronic this and that, so I don't know how the timing rules apply when you start screwing around with the cam timing.
 
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Cam, You have contradicted yourself. Lower rpm, lower power settings generally need less advance to run optimally. The timing of the spark is related to the length of time that it takes the fuel mixture to reach it's maximum pressure after ignition. The faster the crank turns the more advance is needed.
I don't think he did. He said lower power settings, not lower RPM. I think he's exactly right. Lower power settings include lower cylinder pressures, which result in lower flame front speed. Hence more spark advance is appropriate. Or did I read things wrong?
 
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