Vesa Helanti wrote:

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.

Two-stroke manufacturers started putting those "extra" holes in the
rear side of pistons in the 1960's to help the engine breathe better.
At some point, they allowed fresh fuel/air mixture to pass from the
crankcase into a 5th, or "boost" port...

I first saw those holes in the 1967 Yamaha YDS-5 and TD-1C 250cc
2-strokes. Prior Yamaha 250cc models had two transfer ports on each
side, but the 5-port Yamahas had a 5th transfer port in the rear of the
cylinder. It was fed by a hole in the rear of the piston...

The 5th port blew fresh mixture across the top of the combustion
chamber, assisting to clear the cylinder of exhaust products...

Yamaha was learning from the hot-rodders here in the USA. There was no
room for a 5th transfer port in older model Yamahas, so the racers had
been grinding a sort of "ditch" up the rear of the cylinder wall to
allow the extra boost port mixture into the combustion chamber...

If your piston hole truly never lines up with any of the transfer
ports, and only has 4 ports, not 5, it is likely that somebody couldn't
find a Tohatsu piston and used a Yamaha piston instead...

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.

The arrow on top of the piston always points toward the exhaust port.
You would never want to have hot exhaust gasses intentionally pass into
the crankcase, you might get a crankcase explosion, or you might scorch
the thin film of lubricating oil off the tiny needle bearings in the
rods...

Also notice that the piston will have two wire pins in the ring
grooves. The pins keep the piston rings from rotating around so the
ends of the piston rings can never snag in the exhaust port and
break. If you turn the piston around and it snags a ring during
operation, you might wind up buying a new motor...

(I remember a guy who thought he knew more than the mechanics at the
Greeves factory. He looked at the L-shaped Dykes-pattern piston ring,
and he told me that the "idiots" at the factory had installed the
piston ring upside down. So he turned the ring over and tried to slide
the cylinder down over the piston ring. It wouldn't go back together
that way, the ring stuck out too far...)

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

Instead of raising and lowering the intake, exhaust and transfer ports,
speed tuners would commonly cut the rear skirts of the pistons off and
file the crown of the pistons to effectively open the exhaust and
transfer ports early...

I read a funny story once about some famous American flat track racers
who were being considered by Yamaha International to be members of
their factory roadracing team, at least for American Motorcycle
Association races. They knew that they could get more power out of the
TD-1 racers that they were loaned for the performance tests, so two
racers were working on one piston with a hacksaw. One was sawing, the
other was holding the piston down on the bench. The guy with the
hacksaw was sawing furiously and cut the other guy's thumb...

Yamaha was so happy with the flat track racers' riding, they hired all
of them for the team...

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.

Consider the possibility that I offered above: it's a *Yamaha* piston.
In the early 1960's, a young rider once proudly told me that he rode a
Tohatsu motorbike and how wonderful it was, better than my Honda. But I
have never even *seen* a Tohatsu motorbike, I have only read about
them...

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.

I been studying 2-stroke engines for 40 years now, and have never seen
a motorcycle 2-stroke that had a hole in the *front* of the piston,
only in the rear of the piston...