Arsobo Posted February 14 Report Posted February 14 Hi All, just bought a 64 M20C. Mags are a little on the old side so I'm prepping to get them overhauled. However, when I took photos of the data plates I realized I have Bendix S4LN-204 on BOTH the left and right magneto. The Lycoming parts catalogue calls for S4LN-200 on the left (Impulse coupling) with S4LN-204 on the right. Powerup Magnetos calls for the same in their compatibility chart. What is the correct arrangement for the M20C? I know we have a "starting vibrator" per the manual but I'm not sure what is supposed to be on the engine. Thanks, A
LANCECASPER Posted February 14 Report Posted February 14 1 hour ago, Arsobo said: Hi All, just bought a 64 M20C. Mags are a little on the old side so I'm prepping to get them overhauled. However, when I took photos of the data plates I realized I have Bendix S4LN-204 on BOTH the left and right magneto. The Lycoming parts catalogue calls for S4LN-200 on the left (Impulse coupling) with S4LN-204 on the right. Powerup Magnetos calls for the same in their compatibility chart. What is the correct arrangement for the M20C? I know we have a "starting vibrator" per the manual but I'm not sure what is supposed to be on the engine. Thanks, A In the logs when were they last inspected by shop?
PT20J Posted February 14 Report Posted February 14 The left magneto is used for starting and will either have an impulse coupling, or a retard breaker used for a shower of sparks (a.k.a. starting vibrator) system. According to the attached chart, the left mag should be a S4LN-200 for a shower of sparks system. (S4LN-21 for impulse coupled). 2
markgrue Posted February 14 Report Posted February 14 (edited) Look at the tcds for the engine. It uses a -200 and a -204. -200 should be retard breaker, not impulse coupling. Edited February 14 by markgrue 2
Arsobo Posted February 14 Author Report Posted February 14 (edited) Thanks all, I did a deep dive last night. SL4N-200 Left and SL4N Right is correct. PN: 10-600614-1 and 10-600644-1 for anyone that searches this in the future are the correct "new" magnetos due to superseding part numbers. How I have the wrong magneto installed is beyond me - it starts and runs fine. Maybe the Data plate is wrong? Hopefully I can find a replacement SL4N-200 without getting bent over the barrel. Odd question for anyone, on the parts manual the drive gear (LW-72495) for the S4LN-200 is different than the S4LN-204 (LW-73683). However, in another manual LW-72495 drive gear was superseded by LW-73683 and now matches the S4LN-204 drive gear. Does anyone know if these gears are fundamentally different? Going to be another kick in the shorts to find another gear if needed. Edited February 14 by Arsobo
PT20J Posted February 14 Report Posted February 14 2 hours ago, Arsobo said: Thanks all, I did a deep dive last night. SL4N-200 Left and SL4N Right is correct. PN: 10-600614-1 and 10-600644-1 for anyone that searches this in the future are the correct "new" magnetos due to superseding part numbers. How I have the wrong magneto installed is beyond me - it starts and runs fine. Maybe the Data plate is wrong? Hopefully I can find a replacement SL4N-200 without getting bent over the barrel. Odd question for anyone, on the parts manual the drive gear (LW-72495) for the S4LN-200 is different than the S4LN-204 (LW-73683). However, in another manual LW-72495 drive gear was superseded by LW-73683 and now matches the S4LN-204 drive gear. Does anyone know if these gears are fundamentally different? Going to be another kick in the shorts to find another gear if needed. Call Aircraft Magneto Service. They are very knowledgeable and may be able to take your -204 core in trade for a -200. Gears should be the same, but they will have the most current part numbers and you’ll get a magneto back good as new.
Jim Peace Posted February 15 Report Posted February 15 22 hours ago, Arsobo said: Hi All, just bought a 64 M20C. Pictures or your lying....LOL
McMooney Posted February 15 Report Posted February 15 man, my mag is due this annual, looking at the price i'm seriously thinking of replacing with a second surefly. pay a little more now and never have to do it again
Ragsf15e Posted February 15 Report Posted February 15 44 minutes ago, McMooney said: man, my mag is due this annual, looking at the price i'm seriously thinking of replacing with a second surefly. pay a little more now and never have to do it again Do you already have two batteries or dual chargers or whatever they required as backup? I really liked my sf, and I plan to get one on my 252, but I think I also like having a mag…
McMooney Posted February 15 Report Posted February 15 1 hour ago, Ragsf15e said: Do you already have two batteries or dual chargers or whatever they required as backup? I really liked my sf, and I plan to get one on my 252, but I think I also like having a mag… no, i'll have to install that at the same time. it's really not the mag that bothers me, it's the price, 2k right now, maybe 3k next time and if i'm lucky probably 4k the next inspection
Ragsf15e Posted February 15 Report Posted February 15 1 hour ago, McMooney said: no, i'll have to install that at the same time. it's really not the mag that bothers me, it's the price, 2k right now, maybe 3k next time and if i'm lucky probably 4k the next inspection Jee, are they really up to 2k just to oh a mag? That does seem excessive.
Z W Posted February 15 Report Posted February 15 In November of 2025 I was billed $225 in labor and $725 in parts per mag to overhaul. Probably some mark-up in there too from the shop that sent them out. I had tried to buy another mag to just swap one out and speed up the annual. Had one due for inspection but 200 hours or so left on the other. Couldn't find one for sale in stock anywhere, talked to QAA who said they'd build and sell me one for $1,800 or so, but after two months of waiting I canceled the order and just had the shop send both mags out for overhaul. 1
Arsobo Posted February 15 Author Report Posted February 15 Interestingly enough, I looked at my magnetos a little closer the other day, the left magneto indeed has shower of sparks (two wires to the magneto) vs just the P-lead for the right. So maybe the data plates are just wrong. Thanks for the advice, we will see what the overhaul shop says.
cliffy Posted Sunday at 01:46 AM Report Posted Sunday at 01:46 AM Mag drive gears- They are (can) be different If when you go to tim them to the engine and one has to go all the way to the end of the slot to just make timing - the 2 gars are on the wrong mags. Swap the gears and majically you can easily time them both in the middle of th adjustment slots!! 1
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