Piston in Tohatsu

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?

 

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...

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...)

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...

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...

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...

 

Most, if not all "small" 2 strokers with an arrow on the piston top
will need the arrow to point toward the flywheel end of the crankshaft.

BTW, I lived in Tokyo in 1962-1963 and worked with a Tohatsu factory
rider, I was invited to visit the Tohatsu R&D lab and saw a 125cc 2 stroke
turn 27K RPM and produce 47HP on a dyno , in 63 that was an unheard of
output

Aubrey in big D
96 Virago 535
78 Suzuki GS750E

 

crankshaft.

The arrows on top of every Yamaha and Suzuki piston I ever saw pointed
forward, toward the exhaust port. The OP's engine could not possibly be
assembled with the arrow pointing *sideways* because the
connecting rod would also have to be twisted sideways...