"1. The inside of the back plate is scratched in a circular pattern. Looks like the connecting rod is rubbing against it. This could not possible be normal??
2. There seem to be a lot of play in the crankshaft when the piston is at TDC. i.e. the piston is not moving at all, but I can rotate the propeller ca. 35-40 degrees. Is this normal?"


The circular pattern on the backplate IS in fact a sign that either the rod big end, or the crank pin is rubbing. It's sometimes the result of using an electric starter and forcing the crank assembly rearward while starting the motor. This really shouldn't happen in a ball-bearing engine, but it's possible. If there was excessive fore-aft play in the crank itself however, the assembly would be pulled forward in flight and contact with the backplate is then unlikely.

The "play" you note at TDC is a characteristic of any ABC type motor. It's not a sign of a defect, but rather a designed-in tolerance
to allow the crank to keep moving even though the piston actually comes to a complete stop for an instant at TDC while it's travel reverses. The "play" also allows for adequate lubrication of the rod.

Black oil after 5 gallons isn't usually a sign of well-being. Hot, lean running, loose mufflers, or even a off-grade of castor can cause a dark colored, or black oil to emanate from the exhaust. If it's truly a "black" oil...that usually signifies aluminum is grinding someplace. Make sure your muffler and any hardware on it are absolutely tight.

I'm shooting in the dark here a bit however. If the motor has run 5 gals of fuel while exhibiting these issues...it's probably going to keep on running. If this is something that is the result of disassembly of the motor, or a crash or some other undisclosed issue...then all bets are off.

I hope I've been able to clarify at least a few issues....

'Race