The O-320-H2AD is the engine of the 1977 to 1980 Cessna 172N, and the O-360-E1A6D and its relatives are on the early Piper Seminole and the Turbo Seminole. An owner of one of these airplanes whose engine came from Lycoming new, rebuilt or overhauled since June 2017, and has run fewer than 500 hours, has a rocker arm inspection to do at the next oil change. Lycoming revised the bulletin that asks for it on October 8, 2026, adding the hydraulic lifters to the first inspection and a separate visual check for the ones after it.
Which engines
Mandatory Service Bulletin No. 639A covers rocker arm part number LW-15014 in five engine models: the O-320-H2AD, and the O-360-E1A6D, LO-360-E1A6D, TO-360-E1A6D and LTO-360-E1A6D. It applies to those engines shipped from Lycoming new, rebuilt or overhauled since June 2017 that have fewer than 500 hours of operation, and to any of them with fewer than 500 hours since a rocker arm was replaced.
The bulletin lists engines, not airplanes, so it follows the engine into whatever airframe it is in. The FAA's type certificate data sheet 3A12 gives the O-320-H2AD for the Model 172N, serial numbers covering the 1977 to 1980 model years. The PA-44 data sheet lists the O-360-E1A6D and LO-360-E1A6D for the PA-44-180 Seminole from serial 44-7995001 through 44-8195026 (later Seminoles have the O-360-A1H6, which the bulletin does not name), and the TO-360-E1A6D and LTO-360-E1A6D for the PA-44-180T Turbo Seminole.
Lycoming first issued SB 639 on January 17, 2020, after field reports of damaged rocker arms found while investigating reports of engine roughness. Lycoming says it is still investigating, that the inspections are a precaution until the root cause and a corrective action are known, and that it will revise the bulletin again with a terminating action when they are. The bulletin is Lycoming's; it is not an FAA airworthiness directive.
When
The first inspection is due at the next scheduled oil change. After that the visual inspection repeats every 50 hours until the engine reaches 500 hours since new, rebuilt or overhaul, or 500 hours since its rocker arms were replaced.
What the first inspection involves
Revision A renames the first inspection "Disassembly, Rocker Arm Inspection, and Reassembly." The rocker arms, pushrods, shroud tubes and hydraulic lifters come out, following Lycoming's overhaul manual for the "76" series engines (60294-9). Each rocker arm is cleaned and looked at under a 10-power magnifier, on the pushrod socket side and on the outside, for cracks around the oil squirt hole.
New in this revision, each hydraulic lifter is taken apart whenever the valve gear is disassembled, with Lycoming's extractor tool ST-474 and a new circlip, and the lifter body and plunger are inspected for wear and chipping. The bulletin gives the replacement lifter where either is found. Parts go back in the same place they came from, the shroud tubes get new seals, the rocker arms new self-locking nuts, and the dry tappet clearance is checked and set.
The repeat inspections
Also new is a separate visual procedure for the 50-hour checks, so the rocker arms do not have to come out again. The rocker box cover comes off, the rocker arm is cleaned, and the area around the oil squirt hole is inspected under the 10-power magnifier. The cover goes back on with a new gasket.
If a crack is found
A cracked rocker arm is replaced and returned to Lycoming, and the finding is reported to Lycoming's technical support on the form attached to the bulletin. The 50-hour inspections then start again and run until 500 hours after the replacement.
Where to read it
The bulletin, its figures and its inspection form are Lycoming's and are not reproduced here. An owner or mechanic should work from SB 639A itself, with the overhaul manual it calls for. Lycoming says the bulletin's engineering aspects are FAA approved and that it provides Instructions for Continued Airworthiness. It gives no labor estimate.
Sources
- Lycoming Engines, Service Bulletin No. 639A, Inspection of Rocker Arm, Part Number LW-15014, October 8, 2026, supersedes SB 639 of January 17, 2020, and the bulletin itself (5 pages). Read October 29, 2026. Lycoming's publication, not reproduced.
- FAA, Type Certificate Data Sheet 3A12, Revision 84, Section VIII, Model 172N (copy hosted by Univair; the current revision is on the FAA's Dynamic Regulatory System). Read October 29, 2026.
- EASA, Type-Certificate Data Sheet IM.A.232, Piper PA-44, Issue 03, February 10, 2017, sections A.III.5 and B.III.5 (the FAA is the state of design). Read October 29, 2026.
- Photo: O-320-H2AD cutaway by KOMRADE DIMITRI, Wikimedia Commons, CC BY-SA 4.0.