Protect the holy seal

"Hey Chris; my engine starts up and runs fine for a few seconds, and then it quits. What's up with that? Please help."

"Hey, Chris; my engine used to run great. Now it is so temperamental."

"Hey, Chris; lately the needle setting on my engine has been very finicky. When I finally do get it running OK, I set it down on the track thinking I'm ready to rock 'n' roll, and it quits within the first lap or two. It never used to be like this. If you don't help, I'm going to smash the thing with a sledgehammer. "

It's the piston/sleeve seal, guys!

I know, right now, some of you are thinking, "Oh, no-not that boring subject again. Forget that, Chris. Just tell me how to go faster!" It's true, I have touched on this subject before, but judging from the volume of email and snail mail I get on the subject, it's obvious that some of you clowns weren't listening! I can only assume that the subject needs a more lengthy treatment.

Before you can go faster, you first have to go. You have to get the car running and then keep it running without ruining it. So pay attention this time, 'cause I'm trying to help you.

One of the most annoying things about 2-stroke glow engines is that after the piston and sleeve seal-or "fit"- has worn out, they still run-well, sort of run. It would be so much simpler if the engines just refused to run at all with a worn-out seal. But NOOOOOO! The tenacious little things have to keep on trying to run-albeit very badly-- and giving us devoted R/C'ers a glimmer of hope. With the brand of blind faith only modelers have, we forge onward in search of the magic adjustment that has, thus far, eluded us.

But, of course, there is no magic to any of this. Probably the most sadistic trick these little gems pull on us is that they will often run perfectly for a few moments on a cold start only to sputter and quit once they have reached running temperature. And there you have the answer: the secret lies in the hot and cold of it all.

If, at this very moment, you're thinking, "OK, he's being a lunatic this month. What the hell is he talking about!" please stay with me on this; it's so important.

Our engines incorporate an "ABC" piston and sleeve assembly. That simply means they have an aluminum piston running in a brass sleeve that is chrome plated. In some cases, nickel plating is used in place of chrome-- but that's another "Piston Power." The piston is made of high-silicon aluminum while the sleeve is of brass (simply because chrome adheres to it well). Some R/C racing airplanes use engines with chrome-- plated aluminum sleeves to save weight, but sticking chrome directly to aluminum is a complicated process and, again, that's another "Piston Power."

When an engine is brand-new, the piston fit is tight-sometimes very tight-toward the top of the sleeve; the sleeve is tapered to make it so. The reason for this is easy to understand. Brass and aluminum have different expansion properties; the brass sleeve expands more than the aluminum piston. So, for the pair to have the correct fit at running temperature, they must be machined in such a way that the sleeve has a tight fit around the piston when the metal is cold. If the piston and sleeve were machined for an easy push-- though fit when cold, the fit would become way too loose once the engine had reached normal operating temperature.

Let's test your engine's piston/sleeve fit. Take the glow plug out and rotate the crankshaft. Even if the engine has been well broken in, there should be at least a little resistance as the piston approaches top dead center (TDC-the highest point reached by the piston) during rotation. If it goes through the stroke with no resistance at all, you're probably due for a new piston and sleeve assembly. Here's the final test: put a couple of drops of Pacer's* After Run Oil into the plug hole, and replace the plug. Turn the engine until you feel the piston squeeze the air in the cylinder, then hold it there. It should hold compression. If it doesn't, your piston and sleeve have definitely gone south. If you can turn the crank so that the piston goes to TDC while the compression leaks out so quickly it offers little or no resistance, we can call you "the baker" because your engine is "well done." The engine may start, but it will never run properly again. Why? Because the piston function is not only for power on the downstroke, but it also acts as a suction plunger on the upstroke. This suction, or "negative crankcase pressure," as it's called, is your engine's fuel pump. When the piston and sleeve seal is compromised, the piston doesn't produce enough negative crankcase pressure (suction) to draw fuel past the carburetor's spray-- bar at sufficient velocity and into the engine at all rpm to meet the engine's needs at all rpm levels. The engine may run, but it will do so very erratically.

So how do we protect that holy seal?

1. GOOD FUEL

Use only fuel containing 16 to 18 percent oil (lubricant). How can you be sure of the mix? Buy fuel whose manufacturer gives the oil percentage on the label.