In a recent thread, the subject came up whether the spark from a FMJ4B magneto reverses direction every second fire. That is to say the spark jumps from the spark plug ground strap to the center electrode on two cylinders. Knowing that a magnet has two opposing poles, it is tempting to conclude this. However, this did not seem like a sound design characteristic. So, I dug through my info on mags and found a different theory which I confirmed with observation. This is what I found.
Following is an excerpt that shows how magnets are actually oriented in a FM rotor.
For anyone who wants to read the tech manual from which this was extracted, click [b:479be72c27]Here[/b:479be72c27], and you will find it in the second section. Those familiar with the FMJ4B will recognize that its rotor is the one pictured in Figure 61, so it has two Block Magnets. Using Figure 62 for comparison, it can be seen that the orientation of the magnets is such that the magnetic field is oriented at right angles to the rotor axis, rather than parallel to it, as some might be inclined to suppose. This means that the coil does [b:479be72c27]not[/b:479be72c27] see a reversal in field direction.
Therefore, all spark plugs fire in the expected direction from center electrode to ground strap.
I did a few tests with a compass to demonstrate the constant direction of the magnetic field. The compass is suspended in the same position as the coil normally would be. You can follow the black dot on the face of the rotor to see that as it progresses clockwise in rotation, the needle of the compass points in the same direction.
The final pic shows that the field outside of the aluminum housing also is oriented at right angles to the rotor axis.
In the following two pics, the magnetic rod shows that either pole will stick to the laminations of the rotor. However, neither end will stick to the magnetic die cast segments.
Early FMJ mags came with a tartaped coil that was prone to failure when hot.
It was composed of a laminated iron core around which was wound a primary coil of heavy wire and a secondary coil of thousands of wraps of fine wire.
Here is a view of the mag housing showing the holes where the set screws fasten the coil core to the housing. The screws in this housing were already damaged and in removing them, I damaged the holes themselves.
This pic shows an original slot setscrew which is prone to damage opposite an Allen head substitution, which is a big improvement.
Here are additional views of the improved epoxy coil that reduced the overheating failures. The first pic shows the high voltage contact as well as the lead that goes to the points.
[b:479be72c27]Other components [/b:479be72c27]of the FMJ4B mag:
Drive coupling on rotor.
We made a puller to remove the coupling from the rotor.
Looks like Dad provided a little pursuasion with the ball peen as well.
Impulse coupling and driven end of rotor.
Rotor Pinion, pin and snap ring.
Breaker point set and ground tang.
Empty mag housing.
FMJ4B Rotor.
Endcap components.
Distributor cap & components.
Following is an excerpt that shows how magnets are actually oriented in a FM rotor.
For anyone who wants to read the tech manual from which this was extracted, click [b:479be72c27]Here[/b:479be72c27], and you will find it in the second section. Those familiar with the FMJ4B will recognize that its rotor is the one pictured in Figure 61, so it has two Block Magnets. Using Figure 62 for comparison, it can be seen that the orientation of the magnets is such that the magnetic field is oriented at right angles to the rotor axis, rather than parallel to it, as some might be inclined to suppose. This means that the coil does [b:479be72c27]not[/b:479be72c27] see a reversal in field direction.
Therefore, all spark plugs fire in the expected direction from center electrode to ground strap.
I did a few tests with a compass to demonstrate the constant direction of the magnetic field. The compass is suspended in the same position as the coil normally would be. You can follow the black dot on the face of the rotor to see that as it progresses clockwise in rotation, the needle of the compass points in the same direction.
The final pic shows that the field outside of the aluminum housing also is oriented at right angles to the rotor axis.
In the following two pics, the magnetic rod shows that either pole will stick to the laminations of the rotor. However, neither end will stick to the magnetic die cast segments.
Early FMJ mags came with a tartaped coil that was prone to failure when hot.
It was composed of a laminated iron core around which was wound a primary coil of heavy wire and a secondary coil of thousands of wraps of fine wire.
Here is a view of the mag housing showing the holes where the set screws fasten the coil core to the housing. The screws in this housing were already damaged and in removing them, I damaged the holes themselves.
This pic shows an original slot setscrew which is prone to damage opposite an Allen head substitution, which is a big improvement.
Here are additional views of the improved epoxy coil that reduced the overheating failures. The first pic shows the high voltage contact as well as the lead that goes to the points.
[b:479be72c27]Other components [/b:479be72c27]of the FMJ4B mag:
Drive coupling on rotor.
We made a puller to remove the coupling from the rotor.
Looks like Dad provided a little pursuasion with the ball peen as well.
Impulse coupling and driven end of rotor.
Rotor Pinion, pin and snap ring.
Breaker point set and ground tang.
Empty mag housing.
FMJ4B Rotor.
Endcap components.
Distributor cap & components.
