Frequently Asked Questions (FAQ)
Dirt is just about the worst thing that can happen to your model engine. Dirt causes a lot of damage because it's an abrasive. It will erode all of the clearances between the moving parts of your engine. Once the parts have been eroded enough, your engine will become difficult, if not impossible, to run.
Model car engines tend to be more susceptible to dirt damage just because they are run in dirty conditions far more often than are airplane or boat engines. Heli engines run a close second because helis spend most of their time in hovering mode, which stirs up everything on the ground beneath them. Even if you're running on or flying off grass, you'd be surprised at how much dirt can get sucked into your engine.
An air cleaner is necessary to prevent or reduce dirt damage. R/C car air cleaners are rather large affairs that use breathable foam to capture any dirt particles that can get sucked into the engine. A bit of oil applied to the foam makes them more effective because the dirt will stick to the filter. An airplane, boat, or helicopter can get away with a simple screen-type of air filter because their environment tends to be a bit cleaner.
In the long run, it's impossible to prevent eventual dirt damage in R/C car engines, because a lot of the dirt that gets into the engine is extremely fine and can pass through the air cleaner. An air cleaner doesn't prevent damage, but just postpones it. Regular maintenance of the air cleaner will allow you to get a very good life out of your engine. R/C car racers will use a fresh air cleaner for each race. A sport modeler can use a fresh air cleaner for each day's running, unless conditions are exceptionally dusty. Heli fliers should clean their air filters before each day's flying.
When an engine has been damaged by dirt, the first indication will be running problems, with the engine tending to overheat, and the idle and acceleration becoming bad. That's because the first place to be affected is the piston/cylinder fit. That erodes and the engine loses compression. The engine will tend to overheat because the modeler will tend to set the engine leaner to compensate, and there will be exhaust gasses blowing between the piston and the liner into the lower-end of the engine. This hampers running and cooling.
If the piston/cylinder fit has been damaged by dirt, you can easily see it on the piston. A normal piston will have a bright, clean light gray metallic finish. If it's been damaged by dirt, it will become a dark gray color, and sometimes almost black. That's because aluminum has been ground off of the side of the piston by the dirt, and aluminum powder looks black. You may also see deep lines gouged in the sides of the piston running up and down, parallel to the piston's direction of travel.
With non-ringed engines, dirt damage is also found when you disassemble the engine and can push the piston to the top of the cylinder liner. A piston/cylinder assembly with a good fit will become tight long before the piston gets to the top of the liner. If the engine's been run for a bit, you can even see a mark on the side of the liner where the piston normally stops its upward travel. If you can push the piston past that area, it's worn out and you need a new set.
Another indication of dirt damage is fuel being blown out through the front bearing. Even on engines with "sealed" bearings, the front bearing doesn't actually seal the crankcase. The "seal" you see is actually a shield to help keep dirt out of the bearing. The actual seal is caused by an oil film between the crankshaft and the crankcase just behind the front bearing. The fit between the case and crank is very close, and if it's eroded by dirt, the oil film can't make the seal, and raw fuel will be forced out the front of the bearing.
With all engines, though, some oil MUST be driven through the front bearing. That's how it's lubricated. On model cars and boats, you should see a stripe of fuel on the chassis just below the drive washer or flywheel. You may also see a fuel stripe on the body or hatch cover just above the drive washer or flywheel. A thin stripe is normal. If the model is becoming drenched with raw fuel, the engine's seal is work.
Model airplane engines, with their exposed propellers, will usually give you no evidence of the normal amount of fuel coming out of the front bearing. It's thrown into the air stream by the propeller. If the seal is eroded, then the nose of the model will become covered with raw fuel.
You can see dirt damage on the crankshaft under the carburetor. Score marks will be easily evident if you remove the carb. If you remove the crankshaft, you'll see scoring in the crankcase under the carb area.
Many times, we've had engines in the repair shop that have had so much dirt run through them that we can actually carve dirt out of the gas passage in the center of the crankshaft. When it's that bad, the best thing you can do is toss the engine and replace it, because virtually every part of the engine will need to be replaced.
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