What’s the proper mag timing for my engine?

Q: Hi Paul, you helped me out a few years back regarding a question on an O-290 D2B and I really value your advice and opinions, so now I’ll ask for your help once again.

I’m new to a beautiful 1974 Piper Arrow, which still has the original IO-360 C1C installed. Total aircraft time is 3,395 hours and TSMOH is 1,309 hours.

My question is regarding proper mag timing on this engine. When going over the aircraft during annual, the timing was checked and found to be at 20° BTC. Looking at the engine data plate it appears someone in the past had re-stamped the data plate to 20° degrees.

Thinking this was a bit unusual, we consulted the Lycoming Operators Manual for this engine and found that this engine should be timed at 25° BTC.

It should be stated there is a special note ** for these models regarding timing that is serial number specific (with note exceptions for the -C1C) and refers the reader to the “nameplate” before timing magnetos.

I can see where this could lead to confusion on proper timing, but we concluded these instructions/specifications state our IO360-C1C sn L-12928-51A should really be timed at 25° BTC.

That said, our mechanic suggested we leave the timing “as is” since the engine performance and mag drop were all in the desired range.

While my years in aviation have generally supported “if it ain’t broke don’t fix it, don’t go looking for trouble” thinking, I’m also a big fan of making sure everything is maintained “to spec. and by the book.”

What is the correct magneto timing for this engine?

ROBY DENMAN

A: It’s great to learn that I was able to help you once, so let’s see if I can do it again.

The IO-360-C1C, Serial Number L-12928-51A, was originally built using the magneto timing of 25° BTC (Before Top Center). However, Lycoming introduced Service Bulletin 380A on Nov. 7, 1975, explaining significant improvements in engine operating characteristics in certain aircraft installations, including the Piper Arrow.

My recommendation is to review the engine logbook looking for any reference as to whether compliance with Lycoming Service Bulletin 380 or 380A was ever accomplished.

What may have occurred with your aircraft is that the SB was complied with, but the engine nameplate may never have been replaced per the SB and the magneto nameplate may never have been re-stamped per the SB.

If these two items were overlooked at the time of compliance, anyone checking the timing on the engine who looked at the engine nameplate would use the 25° BTC setting since that was the original timing for this engine when new.

My suggestion is that you review a copy of Lycoming Service Bulletin 380A in conjunction with a close review of the engine logbook and I am certain you’ll learn exactly what you have or what you’ll need to do.

I hope this keeps my track record with you on the positive side!