This is about a vintage air-cooled Tohatsu 5 hp outboard, from seventies, I
think.

I am currently putting back together pieces of my outboard. When inspecting
the piston I started to wonder an extra hole (about about halfways up the
piston side, a little below the rings.

Piston has an arrow on top, which has quite definitely previously been
pointed to the exhaust port. When assembled this way, the hole on the piston
side never opens to anything. If the piston is turned 180 degrees this hole
would open to the exhaust port, when the piston is at the TDC. That would
mean there is a momentary straight passage from the crank case to the
exhaust, maybe about 10 -20 degrees around the TDC.

Other cuts in the piston apron are fine both ways for the mixture to flow.

I'm used to think the manufacturer would not put a hole in the piston side
or any other details just for fun. And there is a quite high probability
someone has opened this engine before and possibly could have assembled the
piston incorrectly. And, of course, there is a faint possibility I have
messed with the disassembly and those holes have been aligned all the time.
So now I am not certain what to do.

The reason for this opening to exhaust would obviously be some kind of crank
case filling enhancement but I have not stumbled on this kind of
construction before. Have to admit I have had very little to do with engines
in practice.

Would you please give me your opinion, or preferably solid facts, if
anybody has them. Which way to put the piston?

--
Vesa Helanti