Greetings Dave H!

This engine didn't have a factory marking on the piston - it was a marking from the previous owner. The previous owner said he either marks the piston for the exhaust port or carburetor. In this case, it was the carburetor/front of engine.

The engine ran no differently with the piston installed in either direction. It's fairly worn - the piston seal isn't that great, so it won't idle super low. 2,800 is the brink of quitting. It doesn't hand start very well either. But it holds a needle setting and peaks at 11,200rpm on a 10x6 and 16,000rpm on a 8x6 (APC) using 10% nitro. Ran it with a Davis head and my own diesel mix and got 11,200 on the same 10x6 I ran on glow. Compression screw buried - contra not flush with chamber yet so it might give a little more power with a longer contra screw. Would not hand start on diesel either. I think it could use a new piston/liner (iron/steel).