The International Cessna^® 170 Association

I've got a friend with a 1960 172 with the O-300C. He has had a broken
starter drive three times - the latest one is the worst, as the pinion gear split, with a big piece jamming between the left mag drive gear and crank gear, punching a hole in the accessory case. He hand propped it after the starter gave out, bad idea! (I found him a Drake overhauled replacement online.)

His mechanic, who also happens to be my IA, advised him to start it on the left mag, since that's the one with the impulse coupler...Huh???
I thought all C-145/O-300s had impulse couplers on BOTH mags. In any event, that still hasn't stopped him from breaking two more starter drives
as the engine kicked back. My hunch is the impulse coupler is bad on the left one, regardless of whether it has one or two installed. The mags (Bendix) are off the engine, so it should be easy to determine what the set-up really is.

It's common for Lycomings to have this arrangement; the Aztec, Apache,
Commanche ect I've flown required starting on the left mag. I've never heard of this on a Continental powered Cessna. Can anyone shed any light on this? Russ Farris

OH MY! Your mechanic needs help. I wonder if he timed it correctly and if the Impulse coupling's are the correct ones. That is how the lag is attained.

Actually Tim, the mechanic in question I think is a very competent guy.
He re-built the engine in question, which has performed beautifully for 300
hours (except for the starter kick-back problem.) He just did my annual,
and found my timing was dead on - pulled the left mag (Slick) and did the 500 hour inspection. I've flown the 172 in question, and the mag check was good and the RPM on T.O. was in spec, so I think the basic timing is correct.

The possibilty exists of an incorrect model of mag, that's the next thing to check out. I know they were replaced some time ago. Russ Farris

check the P/N on the impulse.

P/N 10-400314 double check that P/N

Engines that start on only one magneto usually have only that magneto retarded and the other magneto advanced. (Or that magneto has a "shower of sparks" or "retard breaker" system on that one starting magneto.)
If the wrong magneto is selected for start then the timing may be too far advanced and may cause the gears to chip/break.
Our engines have two impulse couplings. If only one magneto has an impulse coupling, but the engine start is attempted on both mags, then the advanced mag may cause backrun or hard-starting (and possibly chipped/broken gears.)

Exactly George...I think Bob's memory is wrong on this one. He works on many airplanes, and frankly unless someone goofed, this O-300
would have impluse couplings on both mags.

He owns and flies a Cub with a C-85, which I believe only has one on the left mag. I'm trying to be diplomatic with him about this issue, since he is an experienced IA who has built two RV-4s (one for himself) and my last owner assisted annual with him cost 400 dollars, including filing a 337 on the C-mods fuel selector valve quick drain! Russ Farris

I'm not a certified mechanic, but from what I read, there is something seriously wrong here! To break a starter drive 3 times tells me to STOP, and ask "what the heck is going on here!"
You need to take some accurate measurements of all the mechanical items involved. Timing is a possibility, but I'd look a little deeper.
Was your engine case repaired at one time? Do you have true alignment? I would have to ask when did this start happening, and what type of work was done before this problem???

GOOD LUCK!

Joe

If the mechanic installed just 1 mag impulse coupling, that is what is wrong. because in order to do that he must time the other mag at 27-28 degrees advanced. and it will not have anyability to retard during start.

The mag without the impulse coupling can and will fire a spark at 28 degrees advanced, and will kick back on the starter drive as the engine is cranked past 28 degrees.

If there have been any chips run thu the gear train in the accessory case, tear the engine down. the case will be cracked.

If that engine is really equipped with only one magneto with an impulse coupler, the IA is also too remiss to be an IA. The airplane is not airworthy without some justification in writing in the records.