Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

Engine — Mechanical

ATA Chapter 71 — N720AK Systems Reference

Overview

N720AK is powered by a Lycoming IO-540 series engine, fuel-injected and normally aspirated, rated at 260 HP at 2700 RPM. This page covers the mechanical aspects of the engine — oil system, cooling, baffling, exhaust, and mounts. For the electronic engine management (ignition and fuel injection), see EFII System32.

Components

ComponentPart NumberSupplierNotes
EngineYIO-540-D4A5LycomingS/N EL-36315-48E, 260 HP at 2700 RPM
Engine mountSteel tube
Oil drain plugMagnetic Oil Drain Super PlugAircraft Spruce (05-12373)1/2“ NPT, Alnico magnet (1-1/32“ long), CAD plated, safety wire drilled. Experimental — verify sump/pickup clearance before install
Oil quick drainP5000Saf-AirQuick drain valve on engine oil sump
Oil filter
Oil cooler
Oil separatorASAAntisplat AeroInstalled with install kit — manual, installation guide
Crankcase vacuum valveComplete kitAntisplat AeroCrankcase vacuum system — manual
Valve coversBilletSDS/LycomingO-ring #160 Viton 75 duro, torque 75–85 in-lbs — specs
Filtered air boxFAB-540Van’s AircraftVFR only, 1/4-20 mounts tab-locked or safety wired — installation guide. Modified alt air door — see Alt Air Door below
Engine mount coversAerosport ProductsCarbon fiber covers, installed
Preheat — engineReiff XPReiff Preheat SystemsCylinder band heaters on all 6 cylinders
Preheat — oil sumpReiff HotStripReiff Preheat SystemsOil sump heater; plug is inside the oil door
Spark plug adaptersSPA-6flyEFII18mm→14mm thread adapters, naval brass, 6 installed. Allows automotive spark plugs with EFII ignition
Spark plugsNGKAutomotive 14mm plugs in EFII SPA-6 adapters

Engine: Lycoming YIO-540-D4A5, S/N EL-36315-48E, firing order 1-4-5-2-3-6, spark plug gap 0.016“–0.022“.

How It Works

Oil System

Oil separator evacuation tube: check every oil change or 50 hrs for coking/buildup. Hose: 3/4“ (6-cyl). Pop-off valve 0.5 PSI. Oil return #4 AN. See Anti-Splat installation guide.

Cooling

Exhaust

Ball joint hardware: AN3 bolt, AN960C10 washer, MS21042-3 nut (3 places). Springs face forward toward prop. Install nut with 2–3 threads showing; do not bottom spring on bolt. See Goldberg ball joint diagram.

Exhaust hanging kit: cable assembly, anti-sway assembly, support strap bends (30° up LH, 20° down RH). See exhaust hanging kit sketch.

Alt Air Door

N720AK uses a modified alternate air door on the FAB-540 filtered air box, based on this design. Unlike the stock Van’s design (which only allows opening), this modification uses an aluminum sliding gate mechanism actuated by cable from inside the cabin, allowing the pilot to both open and close the alternate air door in flight.

Preheat System

N720AK has a Reiff XP preheat system with cylinder band heaters on all 6 cylinders, plus a Reiff HotStrip oil sump heater. The power plug is located inside the oil access door.

Engine Mount

Steel tube engine mount.

Inspection & Maintenance

Lycoming SB 634 — Cylinder Retirement

Lycoming Mandatory Service Bulletin 634 (October 2018) requires retirement of certain parallel valve cylinder and head assemblies shipped between Sep 2013 and Apr 2015, due to compression loss from head casting leakage. N720AK cylinder serial numbers were checked against Table 1 — none are listed, aircraft is not affected.

Airworthiness Directives and this engine

No FAA Airworthiness Directive reaches this engine, and the reason is not that the airframe is experimental. Lycoming Form 2784 states that non-certified engines are “not type certificated in accordance with Title 14, Part 33” and “not made under a Federal Aviation Administration production limitation record.” A Part 39 AD attaches to a type-certificated product, and a YIO-540-D4A5 is not one — which is why it shipped on a Certificate of Conformance (Form 2700) rather than an 8130-3, and why every Lycoming AD model list reads IO-540-D4A5 and never YIO-540-D4A5.

Do not over-generalize this. Per AC 39-7D, an AD written against a type-certificated engine, propeller or appliance does reach that item when it is installed in an amateur-built aircraft. This aircraft’s operating limitations (8130-7, 27 conditions) never mention ADs either way. So each installed item is judged on its own certification status, not the airframe’s.

Mandatory Service Bulletins are a separate question, and they do bite. Lycoming writes bulletins against ship dates and serial numbers regardless of certification status — MSB 632B’s Table 1 contains 119 experimental EL- serials, proving the point. One currently applies:

BulletinAppliesTrigger
MSB 630B — connecting rod bushing inspectionYES, by ship date (window 2009-01-30 → 2021-09-09; this engine shipped 2015-07-06)At every cylinder removal, before reinstalling. Not calendar or hour driven.

The parallel AD (2026-04-11) would additionally require a repetitive bronze-particulate check of the filter and screens at every oil change. That requirement exists only in the AD and does not bind this engine — but it is cheap and aimed squarely at the failure mode, so it is worth doing voluntarily.

Full determinations, including the negative ones, live in ad-sb-compliance.tsv in GDrive Private/Maintenance/.

Billet Valve Covers

Replacement O-rings for billet covers: COTS Viton O-ring #160, 75 durometer. Torque: 75–85 in-lbs. Lubricate O-ring and groove before install. See SDS billet valve cover specs.

Lycoming Accessory Case Oil System

Reference: VAF thread — Lycoming accessory case oil schematic by Dan H

References