Valves, Ignition Timing, Snap Rings, and More!
Before we get to our letters this month, I have good news for Magnum engine owners. As those with Magnum engines who have tried to get the engines serviced or buy parts have found out, they are no longer available due to Hobby People, which had imported the engines, having gone out of business. The good news is that John Greenshields and his son Mike are now importing the engines and are making parts and service available again.
In 1972, a fellow named Paul Bender founded Hobby Shack, later to become known as Hobby People, with John Greenshields as his first employee. As the company grew, John became general manager of operations and second in command, holding that position until his retirement in the 1990s. Mike joined the company in 1984 as a counter helper in the La Habra, California, store and worked his way up the ladder until he was vice president of marketing and development, and later as vice president of retail operations.
Mike’s oldest son, Tim, now handles website operations and content, and Mike’s youngest son, Chris, does the social media and bookkeeping, making the company a three-generation family operation with all four Greenshields active modelers and RC fliers. John has been around for a while, as I first met him back in the 1960s when he was a serious competitor in control-line Rat and Team Racing events. Over the years, I have reviewed four different Magnum engines and found all four to be above-average quality and performance. With most of the Chinese engine manufacturers now using CNC machining equipment, some really good engines are coming out of China where the Magnum engines are made—and the Magnums are no exception, running equal with the best from China. To order an engine or parts or to get additional information, visit the website at mikegoesflying.com. Now on to the letters.
HONED LINERS
I’ve heard that rings will not seat unless there is a “cross-hatch” pattern honed into the cylinder liner (the piston-to-liner interface). Is this true for chrome-plated liners?—Dirk Dedoes, Fountain Valley, California Answer: Dirk, cross-hatch honing helps seat the ring, especially if the ring is not dead round to begin with or gets distorted during installation. It also helps the cylinder wall retain lubrication. Hard chrome that is used for cylinder lining (for wear purposes) differs from display chrome that was used on automobile accessories and bumpers. Hard chrome during the plating process forms “teeth” that require honing to obtain a smooth surface. During the stroking process (i.e., at speed), the honing stone or part is moved back and forth. The cross hatch can be 90 to 60 degrees. Automotive engine builders have different ideas of which hatch is best. An exception to cross-hatch honing is if you have a blind cylinder (i.e., closed at the top), such as the Saito and YS cylinders. The part being honed has to have the end of the mandrel and honing stone extend about a third of the length of the part being honed. Otherwise, the part being honed has more material removed from the top of the cylinder, resulting in a taper. In this case, the cylinder is finished by internal grinding, which leaves no cross hatch.
As a side note, back in the 1960s, Ford used chrome-plated rings in its engines. In those days, automobiles required a slow break-in period of 500 to 1,000 miles. Some people would drive too slowly, keeping the rings from seating, resulting in high oil consumption. This was solved by the mechanics floor boarding the gas pedal with the car in low gear until near rpm red line and then decelerate, which loaded the cylinder with oil. Three or four times accelerating and decelerating would usually seat the rings and lower the oil consumption.
EXHAUST VALVE
I am a longtime model builder and reader of your column. I have always used 2-cycle engines until recently. I am now building a 1/3-scale Stearman biplane and decided to power it with a 4-cycle engine due to the more realistic sound. I purchased an O.S. 120 Surpass engine and, being a curious person, decided to take it apart to see how a 4-cycle engine works. I was surprised to see that the intake valve was considerably larger than the exhaust valve. I would think it should be the other way around (i.e., the exhaust valve larger for the hot exhaust gases to exit the engine. I have had considerable experience on automobile engines, and the intake and exhaust valves were always the same size. Can you explain why my O.S. 120 should be different? —Ai Johnson, Bakersfield, California
Answer: I sure can, Ai. The more air/fuel mixture you can get into the combustion chamber, the more power an engine will develop. Some higher-performance engines do use larger intake valves, multiple valves, and (many cars nowadays) exhaust-driven turbochargers. Racing and most larger-displacement aircraft engines use crankshaftdriven superchargers, which can develop higher manifold pressure than turbochargers. A nonturbo- or supercharged engine depends on atmospheric pressure to fill the combustion chamber (i.e., as the piston goes up, a vacuum or negative pressure is created). Atmospheric pressure then fills the combustion chamber with air/fuel mixture. The exhaust, on the other hand, is driven under pressure from the engine by the upstroke of the piston.
IGNITION TIMING
I have an SV 26cc gas engine that has never run well. It has never died in flight, but I have struggled with the midrange adjustment. It gets a slight gurgle, misses, and generally sounds bad, but only in the midrange. I have tried multiple carburetors, and I have replaced the intake manifold and the reed valve, and have adjusted the needles endlessly. The last thing I tried was replacing the ignition system. On CH Ignitions’ website, they recommend setting the pickup for their ignition at 30 degrees before top dead center (BTDC). I checked where mine is set from the factory using a degree wheel and found it to be at about 65 degrees BTDC. The sensor is bolted to the crankcase in predrilled holes with a limited range of adjustment, and the prop driver, holding the magnet, is keyed to the crankshaft, so there doesn’t seem to be an easy way to change it. Is there a general rule of where this should be set? Do different brands of ignition require different settings? What are the consequences of having it incorrect?—Jeff, via email
Answer: Jeff, if you determined the timing correctly, 85 degrees is far too much timing advance. It is hard to believe the engine would even run and have a bad habit of starting up backward, let alone have a vicious kickback tendency when starting. A more logical number is 30 degrees. Not having an SV engine, I cannot say how to go about changing the timing other than to make a new bracket for the pickup.
SNAP RINGS
I, like so many, have enjoyed your column for decades. For years, it has been the first article I read in MAN (and previously in RCM). I have also had the pleasure of purchasing a couple of engines from your kind self, and I enjoy them immensely. In my humble opinion, you should be declared a national treasure! Not to flatter, but all true. Now to the meat of why I am finally writing to you. Being recently retired, I have started buying and working more on my engines (glow) and sometimes have to remove the wrist pin from the piston to clean out the oil hole in the top of the connecting rod. This is where the trouble begins because when I go to reinstall the e-clip pin retainer, it often goes flying and disappears forever. As many of my engines are no longer in production, finding parts for them is a problem. Is there a source that you know of (since you work on so many engines) for these small wire clips? I could probably make them up myself but don’t think I would trust them not to come out during running and destroy an engine I already can’t get parts for. Any information you can give us (as I imagine I’m not the only one who has had this problem) would be greatly appreciated. I often wonder if engine manufacturers even take the time to make these clips themselves or just buy them from a supplier. An assortment of them would really be nice. Anyway, thanks for letting me bend your ear, and keep up the good work.—Chris McMillian, Henderson, Nevada
Answer: Chris, I’m afraid you are out of luck when it comes to finding a source for modelengine snap rings. I have run into this problem many times myself over the years. There are any number of companies that make them, but 1/4 inch is usually the smallest size. McMaster-Carr is one of the larger companies making snap rings, but again, 1/4 inch is the smallest size. Most engine manufacturers have to have them made specifically for their purpose. K&B quit using them some years back due to the tang having a tendency to break and do bad things to the engine’s internal parts. K&B later changed to Teflon end pads, as have many of the model-engine manufacturers. I do make this service available, but if anyone knows of a source for smaller than 1/4-inch wire snap rings, please let us know.
That does it for this month, gang.