Service Instruction No. 1325A 
(Supersedes Service Instruction No. 1325) 
Engineering Aspects are 
FAA (DEER) Approved 
DATE: May 14, 1976 
SUBJECT: 
MODELS AFFECTED: 
TIME OF COMPLIANCE: 
Timing Change for IO-360 Series Engines 
IO-360-A series (not -A1B6D), -C, -D series, HIO-360-C1A, -C1B, AEIO- 
360-A1B6 with serial numbers prior to 14435-51; IO-360-C1D6 with 
serial numbers prior to 14445-51, LIO-360-C1E6 with serial numbers 
prior to 1063-67 and AIO-360-A1A, -A1B, -B1B with serial numbers prior 
to 219-63; TIO-360-A1B with serial numbers prior to 145-64. See 
Service Bulletin No. 380 for IO-360-C1C and -C1F models. 
At overhaul or at owner's discretion. 
Certain advantages in engine operation have 
been found by changing engine timing from 25° 
BTC to 20 BTC and reducing the lag angle to 
15° on impulse coupling magnetos and 15 
retard angle on retard breaker magnetos whi-chever 
is applicable. Significant improve-ments 
in engine operating characteristics are 
achieved in above listed aircraft engine 
installations particularly in the areas of 
normal operation and cold weather starting. 
During cruise operation, cooler cylinder head 
temperatures can be expected resulting in 
improved service life of intake and exhaust 
valves. During full throttle operation the 
new spark setting will reduce possibility of 
detonation during extreme cold weather opera-tion. 
Consequently this modification has been 
incorporated in the production of new engines 
and may be incorporated in engines in service 
as follows: 
1. Remove the left magneto and determine if 
it has an impulse coupling or if it is a 
retard breaker magneto; if it is a retard 
breaker magneto it cannot be converted to 
the 15 retard angle. However, a Bendix 
blue magneto which incorporates a 15 re-tard 
angle is available through Avco Ly-coming 
distributors. See "Parts Data" be-low. 
Install the replacement magneto as 
described in step 3. 
2. If the left magneto has an impulse 
coupling, replace the coupling with a new 
one, LW-391429 on all models except reverse 
rotation engines; on these use impulse 
coupling LW-391427. 
3. Install the magneto on the engine and 
retime both magnetos using the 20 timing 
mark on the starter ring gear instead of 
the 25 mark. See Avco Lycoming Operator's 
Manual 60297-12 for ignition timing proce-dure. 
4. Replace the engine nameplate with a new 
one, Avco Lycoming P/N 61548 which indi-cates 
new timing angle. Also restamp the 
magneto nameplate to indicate the magneto 
is -9, -10 or -55 instead of -3, -6 or -45. 
PARTS DATA: 
15 Lag Angle 
Magneto 
LW-349365-9 
LW-349365-10 
LW-51360-55 
Replaces 
Magneto 
LW-349365-3 
Magneto 
Model Engine Application 
S4LN-1227 IO-360-A1B,-A1B6,-C1C,-C1F,-C1D 6, -C1E6, AIO-360-A1A, 
-A1B, -B1B, AEIO-360-A1B6 
LW-349365-6 S4RN-1227 LIO-360-C1E6 
LW-51360-45 S4LN-21 IO-360-A1D

PARTS DATA (CONT.): 
15° Retard Angle 
Magneto 
LW-349285-7 
LW-349285-8 
LW-163005-11 
LW-163005-12 
Replaces 
Magneto 
LW-349285-1 
LW-163005-2 
Magneto 
Model 
S4LN-1208 
S4RN-1208 
S4LN-200 
S4RN-200 
Engine Application 
IO-360-A1C, -C1C, -D1A; HIO-360-C1B; AIO-360- 
A1A; TIO-360-A1B 
LHIO-360 C1B 
IO-360-AlA, -A2A, -C1A; HIO-360-ClA; AEIO-360- 
A1A 
LHIO-360-C1A 
LW-391429 
LW-391427 
61548 
18484, 18485 - 
Impulse coupling, 15° lag angle (replaces LW-349359) 
Impulse coupling, 15 lag angle (replaces LW-349358 for magnetos on reverse ro-tation 
engines) 
Engine nameplate (replaces nameplate with 25° BTC timing specified). See Ser-vice 
Instruction No. 1304 for procurement procedure. 
These numbers for Avco Lycoming reference only.