The 100 hour Radial Engine Annual Inspection Series – Part 1

It’s annual inspection season again. I guess it like death and taxes…sure to come around. Lots of folks have asked “what should I have done to my engine during the annual?” The engine maintenance manual, part number 118611 contains the Periodic Inspection Tables listing the Requirements for the “A” (50 hr), “B” (100 hour inspection), “C” (200 hour) and “D” (Midway to Overhaul) inspections on R1340 and R985 engines. Of the requirements listed I’d like to offer some recommendations on seven of the “heavyweights”.

  1. Oil Change with filter/screen & sump checks.
  2. Valve adjustment – Positive or compression.
  3. Ignition timing check – Spark plug servicing.
  4. Compression check – differential.
  5. Air filter and carb – heat system check.
  6. Fuel System Screens.
  7. Cylinder Head Checks.

Since this is a bit lengthy let’s focus on one topic per article:

Oil changes: The periodic table indicates the time between oil changes should be determined by the type, and condition, of engine operation. As a general rule we recommend they be done on 25-50 hr. intervals, or at 90 day intervals. If you only fly 25-30 hours a year, a once a year oil change wouldn’t be frequent enough. You may cause engine damage due to acids and moisture accumulation in the oil. It is absolutely essential to drain both oil sump plugs and check the engine filter or screen at every oil change. If you have a spin-on or sock type filter, it should be cut open and checked for metal contamination.

It’s interesting that the engine’s oil “screen” is a pressure side screen filtering the oil after it passes through the pressure pump but before it flows throughout the engine. The after-market filters are all scavenge oil filters. They filter the scavenge oil just before it flows into the cooler. This is good because it helps keep contamination out of the oil cooler and oil tank keeping the entire oil system cleaner. However, because it is an outlet filter, it will effectively “mask” the engine oil screen.  The screen is the historic location to check for metal contamination. If you don’t cut the spin-on filter open and inspect the element you could well miss discovering metal shavings being generated by some form of distress in the engine!

The Scavenge oil system has a “finger screen” in the sump that prevents solid material from being pulled into the scavenge pump impeller gears. The screen is located inside the sump and can be accessed by loosening the attach bolts on the lower end of the larger of the two metal scavenge tubes attached to the sump. This screen is designed to prevent large pieces of metal from being sucked into the oil pump, resulting in pump and pump drive failure. The periodic inspection tables calls for removal and cleaning of this screen at 50 and 100 hours. It is fortunate that the sides of this screen can be viewed through the rear oil sump drain plug using a strong light (and after the oil stops dripping!).

985 Thrust Bearing Slinger and Spacer
R985 Thrust Bearing Slinger and Spacer

The oil sump is located between cylinders number 5 and 6 and is equipped with two oil drain plugs. The front drain plug accesses the rocker-box scavenge oil sump while the rear drain plug accesses the engine main case scavenge oil sump. The front sump typically holds around 1 ½ pints of oil or less while the rear sump contains about 3 ½ quarts engine oil. Quantities may vary depending on length of time in storage and engine idling / scavenging time prior to shut-down and what position the propeller (counterweight type) was placed in at shut-down.

Here’s a tip learned the hard way: Don’t take your eyes off of the 5 gallon bucket you’re draining that cold 60 weight oil into! It’s amazing how large a puddle a gallon of black 60 weight oil will make on a hangar floor!

Replacement gaskets for the sump drain plugs, new engine oil screens and check-valve O-rings as well as spin-on filter canisters for the Skytractor Supply and Airwolf filter kits can be purchased at our radial division stock room, speak to Donnie: 918-756-8320,

This series of articles is continued with a segment on Valve Adjustment, Positive Method or Compression.

Fly Safe!

Ron Hollis

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