Have an OS 40 LA (natural color) engine and took it apart for its yearly routine cleaning. Noticed while having the muffler removed and looking into the muffler port of the engine, viewing the piston, there is a thin slit starting from the top edge of the piston and extending downward on the side of the piston for about 1/2 inch. The slit seems to be very, very shallow, but concerned me nontheless. The engine seems to have good compression and always ran well. The engine is about 3 years old and never ran lean that I know. My fuel was always 10% nitro and castor based. Is this repairable or not worth any further attention.

Thanks,

Glowfuel

Sounds like a scratch in the piston. Over time, and after ingesting bits of foreign
matter...an engine will exhibit these. If you fly in a dirty, gritty location...expect to see
more of these. A bru-line air filter is a cheap means of limiting the amount of garbage your
motor is able to breathe in.

As long as you say the compression is good, and the motor is running fine, I wouldn't
lose any sleep over it.

Since there really isn't much in an LA motor to corrode...you might consider merely
keeping the motor tidy in your daily post-flight clean up, and minimize your disassembly of it.

Oil it up nice and it's good to go.

'Race