Engine reviews at Sport Aviator are a little different from most other model publications. While construction details are important, and extensively covered, an engine’s airborne performance is also considered a major topic. As much as possible, we will always try to include airborne information about the engines reviewed in Sport Aviator.

With that in mind, we would like to take a close look at the new O.S. Max 46 AX. Now, O.S. has been making quality engines since the mid 1960’s. In fact, my second engine in 1971 was O.S.’s first 60-size engine, the Gold Head 60. That engine flew for 20 years on many airplanes before it was donated to an engine collector.

O.S. developed the first mass produced four-stroke model engine featuring open, uncovered rocker arms. You can see this engine in Sport Aviator’s Test Pilot Report on the SIG Kadet Senior. Four Stroke model engines have come a long way since then and O.S. has been a major company leading the way. Many of today’s two and four-stroke designs produced by other manufacturers are based on O.S. Max technology.

Photo 1

The 46 AX represents another technological change for O.S. Max engines. The major change involves the coating inside the cylinder sleeve. The sleeve is the hollow round part to the right of the piston/connecting rod assembly in photo 2. The piston travels inside this sleeve which is positioned inside the main crankcase, the only part that still looks like an engine in photo 3.

 
Photo 2     Photo 3

O.S. calls the new liner material ABL, for Advanced Bi-Metallic Liner. Two alloys, unspecified, are combined to make a hard, durable coating that should last more than 500 flights. This engine is of the “ABC” type, for more details on what that means, check the “Engines 101” article in Sport Aviator’s Pri-Fly Section.

Basically this design features a chromed, brass cylinder nearly the same diameter as the piston. This would normally spell trouble as excessive friction heat would damage the engine. But ABC engines protect the cylinder with the hard chrome coating while the cylinder itself expands from the heat more than does the piston once the engine is running.

These two factors allow for a very effective “seal” between the piston and the liner, increasing power. Since ABC engines have no piston ring, drag is also reduced. ABC engines are the most powerful for their size, up to about .80 cu. in. displacement. Larger engines have rings since ABC construction does not work as well once the displacement increases.

The 46 AX still features brass cylinder construction but replaces the usual “chrome coating” with the new ABL plating. The ABL is designed to reduce friction and offer extended liner “life”. The engine shown in photo 3 had 74 flights on it at the time the photo was taken. No wear was evident at all. This liner should last in excess of 500 flights.

The 46 AX has other advancements: Some of the new features are:

Ø     The needle valve has a more gradual taper with a sharper “point”. This permits finer fuel flow control to make transitions, when the engine accelerates from idle through mid-range to high speed, more precise and stumble free.

Ø     The inlet cylinder ports are ball-milled for smoother, more precise edges. The smoother edges allow more consistent fuel flow and power output. (Ed. Note: This doesn’t increase total power output but does make it easier for the engine to reach the same top end consistently)

Ø     The new high-speed needle bracket allows the needle valve to be set horizontally or vertically (photo 5). The needle valve locking mechanism has been improved to ensure the valve stays where the pilot set it throughout the flight.

  (Photo courtesy of Great Planes website)

Ø     The propeller shaft is now longer. While this may make the engine safer to use since it allows more threads of the locknut to engage, it also widens the pilot’s spinner choices. Backplate thickness is no longer a problem when choosing spinners.

  (Photo courtesy of Great Planes website)

Ø     The muffler is a totally new design that offers quieter operation without robbing power. But the muffler is still held together with a long center bolt like most mufflers today. In all my modeling experience, this feature remains the only application for “red” thread locking compound. The red compound is heat proof up to 600 degrees, while the muffler bolt should not exceed 400 degrees. Position the outlet in the desired direction and then lock the nut in place with the compound. Otherwise, that nut will eventually loosen causing the muffler parts to separate.

Photo 3

Photo 3 is repeated here to illustrate the various engine parts. The crankshaft is supported by over-sized ball bearings front and rear. The rear bearing can be seen inside the crankcase. We don’t like to remove and then re-install the same bearings from  an engine after it has had 74 flights so they were left in place. For details on bearing removal and installation, see the Sport Aviator article, “Getting Your Bearings”.