Posted by Greenie727 on July 09, 2010 at 08:32:48 from (24.61.144.125):
In Reply to: Magneto rebuild posted by Rusty 2 on July 08, 2010 at 18:27:30:
Rusty, I was a reciprocating aircraft engine mechanic in the Navy years ago and worked on dual magnetos for large Curtis-Wright R1820-82A 9 cylinder radial aircraft engines. (Nothing can duplicate the sound of one of these engines at a slow idle or field barometric Left and Right Magneto RPM drop check). The internal timing of a magneto as you thought and Bob also stated in a reply is very important in order to produce a hot spark from the magneto coil secondary output. This setting is called the E-Gap or efficiency gap on various aircraft magnetos. (Google efficiency gap and Lenz's law for additional information). This setting is defined as the number of degrees that the permanent magnet rotor is past the neutral position in the magneto body when the magnetic flux line stress is at its greatest value. It is at this point that the breaker points should just open. The opening of the breaker points at this exact time causes the current induced along with the associated magnetic field in the primary windings of the magneto coil to RAPIDLY collapse and cut across the secondary coil windings producing the high voltage spark output. The internal timing with the magnetic stress at its maximum value and the condensor aids this RAPID collapse of the primary current. This internal E-gap setting is very important and can be set on most aircraft magnetos. As Bob stated the setting can also be checked and on certain serial numbered H4 magnetos the cam can be turned on the rotor shaft to adjust this setting. The IH Blue Ribbon service manual describes this adustment on certain H4 magnetos. If the breaker point cam is keyed to the rotor shaft and the points are correctly set to specified amount of opening the internal timing of the magneto should be correct and not change. I am not sure how the J4 mag is set up but most likely the cam position on the rotor shaft is fixed and thus the internal mag timing OK. Make sure that the points are mirror smooth and clean and properly set to allow maximum current build up in the coil primary windings prior to points opening.
I am also relatively new to this site but the information and willigness to help by ALL members that post here is incredible. I am not a Farmall expert but the sound of a smooth running H reminds me of the old aircraft recip engines in days gone bye.
I am working on making a copy of the original IH number SE-912 service tool for pulling the impulse couplers on the H4 and other magnetos with the F type impulse coupling. I have made one and pulled a stubborn impulse coupler off an H4 mag with no problem. If anyone out there has an interest in one of these pullers let me know and if there is enough interest I will make as many as possible to help anyone that would like to pull the coupler from the mag without damage to the permanent magnet or coupler from heat or striking with a hammer and jaw style puller.
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