The International Cessna^® 170 Association

If i ever knew the answer I have forgotten


Both mags on 0-300 are S6LN-21 .. I understand that one mag fires the Upper Plugs and the Other MAG fires the lower plugs.

My question is simply can an A&P SWAP the mags from left to right? Am I to assume since the same model mags S6LN-21 are installed it makes no difference which side of engine they are fitted to?

That is correct, they are interchangeable.

gahorn wrote:That is correct, they are interchangeable.

In this case they are the same. But that is not always the case. For example on some aircraft only one mag has impulse couplings and that mag needs to go on a specific side.

Strangely enough, I have never run across an O-300 Continenal powered C-170 or C-172 with only one impulse coupling. It is, however, quite common on Lycoming engines.

Hmmmmmnnn.... if only ONE mag were to have an impulse coupling....(meaning that mag is used for starting)... then the starting procedure MUST SPECIFY that only the impulse-coupled magneto be switched ON during start. Otherwise, the non-impulse-equipped magneto would cause premature spark advance and possibly injure the starter and the engine.

SOME aircraft have ONE magneto retarded (for starting) and the engine only starts on that mag, then after start, BOTH mags are active. This can be done either manually with separate mag switches, or with a special mag switch which GROUNDS the advanced-spark magneto during start. (Most "shower of sparks" mags work similarly with two sets of points, one set being retarded and only used during start, with the second set of points adjusted for the running spark-advance.)

The reason the LEFT magneto is timed 2-degrees ahead of the right magneto is because it fires the LOWER plugs, which are more prone to fouling than the upper plugs and by which they are better kept clean. (This is an anecdotal statement provided by several trusted old-timers who've passed that along. I've never seen it in print from any technical source.)

gahorn wrote:Hmmmmmnnn.... if only ONE mag were to have an impulse coupling....(meaning that mag is used for starting)... then the starting procedure MUST SPECIFY that only the impulse-coupled magneto be switched ON during start. Otherwise, the non-impulse-equipped magneto would cause premature spark advance and possibly injure the starter and the engine.

SOME aircraft have ONE magneto retarded (for starting) and the engine only starts on that mag, then after start, BOTH mags are active. This can be done either manually with separate mag switches, or with a special mag switch which GROUNDS the advanced-spark magneto during start. (Most "shower of sparks" mags work similarly with two sets of points, one set being retarded and only used during start, with the second set of points adjusted for the running spark-advance.)

My Lycoming only has an impulse coupling on the left mag. It's also set up with separate magneto switch and starter pushbutton. Set switch to "both", then push the button. It starts quite easily, but I guess I should be turning the key to "START" while pushing the starter button. Thanks for the reminder.

It's possible your "start" switch simultaneously grounds-out (kills) the other magneto when pressed.... or, it's possible your starting check list or operating manual should instruct to start only on the impulse coupled mag when starting then switch to BOTH for flight...

It's always a good thing to occasionally re-read the OM.

gahorn wrote:It's possible your "start" switch simultaneously grounds-out (kills) the other magneto when pressed.... or, it's possible your starting check list or operating manual should instruct to start only on the impulse coupled mag when starting then switch to BOTH for flight...

It's always a good thing to occasionally re-read the OM.

Ha, I wish! This is an Avcon. There ain't no stinkin' OM!

The START position can be wired to ground either or neither magneto. I'm not sure how it's currently wired, because it's not used.

gahorn wrote:It's possible your "start" switch simultaneously grounds-out (kills) the other magneto when pressed

Ah, I misunderstood you the first time. I read it to mean the "start" position on the key switch. Yes, it's possible they wired the pushbutton to do this. I'll check.