Replacing 4373 Mag with 4372

My original mags coming with the engine got close to 500 hrs on it, time for 500 hour inspection. The mags were running fine. Left mag was 4373 (impulse coupler with 25 deg lag), right mag was 4370. However, I got a SUPER good deal on a pair mags, 4370 and 4372, with less than 100 hours from a friend. The 4372 mag has a lag angle of 15 deg. My engine is IO-360-M1B with timing on the data plate as 25 BTDC. If I time the mag to 20 BTDC (I know I will sacrifice a little power by doing so), can I use 4372 mag with 15deg lag? Has anyone done this? I am concerned about kick back.

I am also thinking about swapping the impulse coupler between 4373 and 4372 (after inspecting the coupler), which is less preferred at the moment. Is the coupler (or impulse coupler shell which determines the lag angle) the only difference between 4373 and 4372 mag?

I am pretty sure the lag angle is controlled by the position of the trip pin on the mag case, so changing the impulse unit will not change the lag angle.

you might be right, i have been reading the how the impulse coupling works, the pin on the case and the pawl kick off on the coupler shell determine the lag. it make more sense it is the pin since the dataplate is on the case. now the question will be can 15deg lag ON coupler work on a timing of 20 BTDC?

15 degree lag

seattleworm said:

you might be right, i have been reading the how the impulse coupling works, the pin on the case and the pawl kick off on the coupler shell determine the lag. it make more sense it is the pin since the dataplate is on the case. now the question will be can 15deg lag ON coupler work on a timing of 20 BTDC?

Click to expand...

The 15 degree lag impulse mag is what lycoming supplies on the 20 degree BTDC IO-360 angle valve engines.

Skylor

Impulse Coupling

If you take a look at the parts list in the slick overhaul manual, the 4372 and 4373 mags use different impulse couplings but have the same frame part numbers. This tells me that the impulse lag is angle is a function of the impulse coupling itself and is not a function of the trip pin location since that location is drilled into the mag frame by the manufacturer. Also note that even the non-impulse frames have holes in them for trip pins. There are two hole locations, 1 for left-hand rotation and 1 for right hand rotation. I happen to have both model impulse couplings (for the 4373 and 4372) and they are different. Therefore it appears that you could put an impulse coupling from the 4373 mag onto the 4372 to change it to 25 degree lag. I would recommend obtaining w impulse coupling rather than re-using your old one.

Skylor

Yes, when I went through the overhaul manual, I also noticed that the only difference between 4372 and 4373 is the impulse coupling assy based on part list. I took the old 4373 mag apart, and everything looked pretty good and clean, including the coupling assy. The overall manual calls to inspect coupling assy for corrosion and excessive worn out, the coupler assy looks still looks like new to me. I will reuse the coupling assy. But I have the right mag replaced with a low time mag so I guess the risk is low. The overhaul manual actually allow reuse many parts if they are still within spec, even the coil and condenser.