ENGINE MAINTENANCE

Model Airplane News, Aug 2005 by Gierke, Dave

HOW TO TAKE CARE OF YOUR MODEL'S POWERPLANT

True or false: technical equipment requires regular maintenance. No, this isn't a trick question, but it's an important one nonetheless. The car gets a regular oil change; the lawn tractor is serviced at least once a year; the gutters are cleaned seasonally; furnace filters are changed; the cat's litter box?-I hope so! Why, then, do many modelers neglect their engines? Taking care of your engines isn't a waste of time. If you forget to perform routine tasks or haven't been shown how or what to do, read on!

HOW'D THAT GET IN THERE?

If you fly from a grass field, as I do, there's always a chance of a nose-over, during which dirt can find a way onto-and into-your engine. Landing a model outside the mown grass strip subjects it to a host of other pollutants, while an infinite variety of weeds can deposit all sorts of minute organic matter on everything, including the engine.

A shallow swale runs diagonally across our site, and models regularly track through it on landing without harm. After a rainstorm, however, poor drainage turns it into a long, shallow pond. During the rainy season, at least one pilot misjudges a landing and-to the entertainment of everyone present-splashes his airplane into the swale; it has been a tradition for more than 40 years! Generally, damage is minimal, but mud, grass, bugs and water usually have to be dealt with. In all cases, a cleanup is required before flying can be continued.

FLYING-FIELD CLEANUPS

Once upon a time, there were two areas through which dirt could enter the engine: the carburetor and the exhaust. Because most engines now operate with either mufflers or tuned exhaust systems, contamination through the exhaust doesn't pose the problem it once did. Dirt can still enter through the pipe's or muffler's exhaust outlet, but since it faces rearward, the chances of this happening are minimal-that is, unless you submerge the entire engine in our swale!

* Carburetor venturi When your engine gets dirty because your model has landed in the weeds, look first at its air intake. During the engine's last microseconds of operation, this trouble spot serves as a miniature vacuum cleaner, ingesting anything that happens to be in the vicinity. This is where your syringe or fuel bulb comes in handy. Find an out-of-the-way location (not in the pits), tip the model up on its nose, and squirt a good dose or two of raw fuel into the carburetor venturi. Avoid rotating the crankshaft before doing this; if dirt is sitting on the crankshaft journal, you don't want it to fall into the induction port. If you're lucky, the induction port was closed, and nothing got into the crankcase. If you're the suspicious type, you may want to remove the engine from the model for partial disassembly. You'll certainly want to do this if you know that dirt has entered the crankcase-especially after a crash.

If dirt remains in the crankshaft's front housing (below the carburetor), it can wear away the seal area, increasing clearances and producing a loss of crankcase compression. This degrades the engine's idle performance while allowing a messy fuel leak from the engine's front bearing. When dirt penetrates the piston/cylinder-sleeve interface, it often produces damaging scratches (called "scores") in the softer piston. The bearing surfaces in connecting rods and piston wristpin holes are also candidates for excessive wear.

* Front crankshaft bearing If dirt is deposited behind the engine's thrust washer (sometimes called a "drive washer"), it may have entered the front ball bearing that supports the crankshaft. Some of these bearings are shielded to prevent this from happening, but you should clean the area with raw fuel squirted from a syringe, anyway. On engines with bushed crankshafts, dirt behind the drive washer can cause rapid wear of the engine's front housing; that leads to excessive shaft endplay and backplate wear. The best way to clean off this dirt requires removal of the prop nut, washer, propeller and thrust washer to allow direct access to the contaminated area. Unfortunately, many modern engines use a drive washer that locks onto the crankshaft with a collet (that is, a split-cone retainer). These sometimes fall rather easily off the shaft but may also require a "puller" to accomplish the task. O.S. sells a special puller for its engines; a battery-terminal puller works well for almost all engines that I've worked with over the years.

If you're at the flying field, and you find that dirt is packed between the rear of the drive washer and the front of the engine housing, and you don't have a puller, don't try to remove the drive washer. Instead, clear the dirt by squirting raw fuel through the gap and then forget about it. Whatever you do, don't pry the drive washer off with two screwdrivers! You will butcher the engine's front housing and probably the drive washer, as well.

* General engine cleanup Plug the carburetor venturi and the exhaust outlet of the muffler with small wads of paper towel. Stand the model on its nose, and squirt the engine with your plane-cleaning fluid. I use a plastic spray-pump bottle filled with 10-percent dishwashing detergent to 90-percent water. Pay particular attention to the cylinder and cylinder-head cooling fins, where dirt can be trapped. If necessary, use a toothpick or another soft instrument to clean them out. Carefully wipe away any excess cleaning fluid with a clean rag or paper towel. Reassemble the engine's components, and you're ready to fly again. Don't forget to remove the paper towel from the exhaust outlet. In the future, stay out of the ditches and weeds!