Solving engine vibration

RPM REAL PERFORMANCE MEASUREMENT

This month's column answers questions that concern engine vibration and crankshaft balancing, engine disassembly and how to set up an O.S. .61 SF-P engine without instructions. If you want your questions about engines and/or their related systems answered here, please write to me care of Model Airplane News at 100 East Ridge, Ridgefield, CT 068774606 USA, or email man@airage.com.

BALANCING SINGLE-CYLINDER ENGINES

Patrick W. Moore of Spokane, WA, writes, "I'm looking for information on how to balance a single-cylinder engine. I believe this involves weighing the big end/small end of the connecting rod [along with] the piston, wristpin, etc., and calculating how much and where to grind material from the crankshaft."

Patrick, numerous formulas have been developed for balancing the single-cylinder engine, but all contain a flaw: single-cylinder engines can't be perfectly balanced for all rpm ranges. In other words, if the engine runs smoothly at one speed, it will run roughly at another. Balancing is a compromise, at best.

The most commonly used formula suggests that all of the rotating (internal) engine parts and 50 percent of the reciprocating weight (back and forth motion) should be balanced by the crankshaft counterbalance. Because the conrod both rotates and reciprocates, weighing individual ends is a difficult and inaccurate process-especially if you don't have a very accurate scale. Over the years, various methods for weighing rod ends have been suggested, but my old friend Clarence Lee has found a way. "If you balance all of the conrod and wristpin weight and 1/4 to 1/3 of the piston weight, you will be pretty darn close to having an engine run as smoothly as is possible." From this point, Clarence suggests that you add or remove weight from the crankshaft counterbalance by trial and error to discover the smoothest run within a given rpm range.

There are many variables associated with balancing an engine: stroke/bore ratio, rod length/stroke ratio, piston weight and the engine's primary operating speed range. Engine designers know that an engine's piston/wristpin weight must be kept as low as is structurally possible to reduce the inertia loads that increase by the square of shaft speed. Stated another way, if you double the engine's shaft speed, the reciprocating inertia forces increase four times (2^sup 2^ = 4). In general, short-stroke engine designs run more smoothly than their long-stroke, low-rpm counterparts.

Balancing can be performed on the crankshaft as it is supported by a homemade razor-blade balancer (see illustration). Hang the conrod on its own crankpin balanced by the wristpin, and decide which percentage of piston weight (slide flat washers onto the wristpin) you will start with.

If you determine that the crank's counterbalance is too light, how can you add weight? If the crank's counterbalance hasn't been hardened, you can drill it and fill the hole with lead or solder. As I mentioned in my propeller balancing article in the July 2002 issue, you can also externally balance an under-balanced crank. With the piston at top dead center (TDC), position an unbalanced prop heavyend down, adding to the counterbalance weight. Another way to achieve the same end is to add weight to the inside of a spinner backplate and fasten a washer/machine-screw combination into a drilled and tapped hole through the backplate in line with the crank's counterbalance.

All of this seems unscientific, but with careful trial and error experimentation, you can coax most vibrating engines into smoother operation.

UNCOOPERATIVE FOX

Mike Fleck emails, "Can you tell me how to get the wristpin out of the connecting-rod assembly of a Fox .50? The piston doesn't have any wristpin retainers."

Mike, it sounds as if you have an early Fox .50 that has a meehanite (fine-grain, cast-iron) compression ring installed on an aluminum piston that runs inside a steel cylinder sleeve. Fox fastened the wristpin to the piston in a unique way; he drilled a hole through the bottom of the piston's wristpin boss and the wristpin. Fox then pressed a roll pin into place. If you have one of these engines, it's probably impossible to disassemble. If you clean the internal parts, leave this subassembly together. If the assembly needs repair, return it to the factory. Newer versions of the Fox .50 secure the wristpin to the piston with the more conventional E-clip.

Many modelers become confused when they remove the pistonrod/wristpin subassembly from an engine that has a one-piece crank-case (no removable front housing or upper and lower segments). After the cylinder head and backplate (rear cover) have been taken off, you must first remove the cylinder sleeve and slip the conrod off the crankpin. To dislodge the sleeve, place a soft copper glow-plug washer on top of the piston so that it protrudes less than the thickness of the sleeve into the exhaust port (Photo 1). With a prop securely fastened to the crankshaft, turn the engine over until the washer engages the top of the port; if you lightly turn the prop, the sleeve should lift from the case. If it doesn't, don't force the issue! Instead, apply a few drops of 3-In-One oil (or a similar product) to the piston crown, and a little even heat from a propane torch to the upper crankcase (Photo 2). When the oil begins to smoke, stop heating and try again to turn the prop. In most cases, the sleeve will now slide right out (Photo 3). If it doesn't, send the engine back to the manufacturer for servicing before you damage it. Speaking of damage, never use any tool that's made from a harder material than the engine component you are working on. For example, the copper glow-plug washer is softer than the piston or sleeve material, so if you need to push on the bottom of the sleeve to remove it, a wooden ice-cream stick would be a great tool to use.