The Cessna 120-140 Association Forum

Can anyone tell me if a Buzz Box will work on a Bendix S4LN-21 Magneto if it connected to the capacitor screw at the end of the P lead. I have read that you must have a P lead adapter made up to connect directly to the mag for the Buzz Box to work.

I'm not sure exactly what you're asking (a photo might help). If the P-lead screws into the mag housing then unscrewing it will short the mag internally and it will not fire, nor will the timing box work. The timing adapter defeats this safety feature.

To time the mags you must connect the Buzz Box to the P Lead of the magneto. With the - 21 mag this becomes a problem, when you disconnect the P lead you need an adapter to screw into the mag so you can connect your buzz box. I have read that instead of disconnecting the P lead from the mag you can fasten your Buzz Box to the capacitor at the end of the p lead and it will work. Will send a pic when at the hangar.

Try it! If the buzzer goes on and off the way it should, you're good.

I didn't think you could get access to the capacitor without taking the cover off the back of the mag. However, it's been a long time since I've had to work on a Bendix mag on a plane, so I may be thinking of a different model.

He is probably talking about a "coaxial" capacitor mounted in the P lead at the mag to prevent RFI ( radio frequency interference) noise. Obviously, one starts with top plugs out and mag switch "on" and impulse fired. I think the buzz box should work - the buzz box is an updated technology from the WW2 version that illuminated a pair of small neon bulbs that required AC or "chopped DC" which compensates for the direct dc windings in the mags -otherwise a DC meter would work _(impedance VS resistance circuit).
In 1961 while at Cape Canaveral, I built my own mag timers using a pair of Morse code oscillators kits that used a small transistor to do the switching and its accurate as can be - one can hear the frequency of the sound increase as the mag points just start to open - down side is if one fails to fire the impulse first, then the small NPN transistor in the oscillator would be fried!! I upgraded to the buzz box with the Eastern "dish pan" but now using the "smart level" with the buzz box. FWIW

P.S. Just remembered, I used the buzz box on my Bonanza with 1200 series Bendix mags that has the coaxial capacitor integrated into the points cover with no disassembly required.

Spruce sells adapters for Bendix Mays.

Yes, found the adapters in the Spruce catalog under P lead. Fasten the buzz box to this and you are in business. Thanks.

An old trick if you don't want to wait for shipping for those 'special adapters' is to find a couple bolts the size of the cigarettes on the Bendix mag p leads and fatten them up with tape, shove them in against the spring then clip the mag timer leads to the bolt heads. When you lose them for the next time you can make 2 more

I'm back - went off line when ATT DSL modem, my computer, and a router I was trying to reinstall stopped talking to each other!! ATT DSL repair got things talking after I spent many hours "stumbling" around!!

I have now located a S4LN-21 mag that I had in storage and now understand 6319 question relative to the "capacitor screw at the end of the P lead".
In order to get to this point, as John C said, one would have to remove the cover plate at the back of the mag which covers the compartment that contains the points and the capacitor and the pigtail lead from the internal coil. When the cover plate is removed, one can see the 'capacitor screw" which actually holds the points in place, and in addition one sees the terminal end of the P lead pressing on the safety spring.

So, the short answer to 6319 question is YES, if that is the "screw" - thus the cover plate has been removed and the P lead remains as installed and the cockpit mag switch is placed in the "on" position, then the buzz box will work when its lead is connected to that "screw" - but of course, not touching ground anywhere.

Just to emphasize for safety, any time one is timing an engine, a spark plug must be out of each cylinder since one is dealing with a HOT engine, in addition, any time the prop is moved, even if you know the mag switch is "off", treat the engine as HOT. Years ago, I would prime my engine with four revs of the prop with the switch "off" - I could hear the engine "suck" its prime thru the Stromberg carb. Imagine my shock when the engine started one day!!! The OEM mag switch had failed and was not grounding the mags!!!! I got a field approval and replaced the OEM one with one from ACS (price saving) , the Spruce related company.

Phil26 your suggestion is "right on" for the ones only checking the timing annually - but once the adapters are made keep them with the buzz box!!