Arrow on crown is shown correctly...  not uncommon to see the type of "creative license" taken with the skirt windows in parts drawings...  those details could not be seen if part was drawn accurately and drawing protocol maintained.

(Yamaha parts drawing protocol generally shows front of bike to the left.)

Sometimes mistakes are made as well...  my guess is this was intentional...  in any case, good example of why we need to be careful following parts diagrams for assembly.  

A friend in Indonesia sent me the picture below when I was ordering parts for my Kawasaki Ninja 150RR...  The picture came from Brazil...  person there had a motorcycle shop install a new piston in his 150RR...  mechanic insisted that arrow on piston always points to intake?!?!?!  Engine ran about a minute before ring ends caught in exhaust port.   Lucky that on these old Yamaha's they don't catch rings that easily as I've see a couple of running engines with piston in backwards.

(Every two stroke I can remember had arrow on piston crown pointing to exhaust port...  but maybe in Brazil that's not the case?)

 

It’s unlikely that exhaust gases need a window to exit the combustion chamber.?. The explosion occurs at the top and blows out pretty much immediately. Now, I don’t completely understand why “a window piston at BDC would allow fuel atomization through the window below the piston dome” I assume DEET could explain this phenomenon to all of us. It would be thought that the fuel atomization would occur above the piston dome forced upward.?.

So the windows illustrated on the incorrect side of the piston isn’t really a wrong. It can’t be placed anywhere else in this actual diagram. It’s not three dimensional.

just learning how air / fuel / combustion moves is interesting - isn’t it??

see asco’s bikes run correctly.?.

Porting and air flow through engine is a bit of a "black art".... 

Early yamaha enduro engines were piston port where intake skirt acts as and basically controls intake valve timing.  When the piston travels up, the crankcase pressure drops, the piston skirt uncovers the intake port and the fresh fuel/air mixture is allowed to flow into the crankcase.  The challenge is that when the piston travels back down, the skirt needs to close the intake port to prevent backflow into the carb and to allow the air/fuel mixture in the crankcase to be forced up through the transfer ports.  It's not that simple though.  inertia and pressure waves affect the gas flow as much if not more than the timing of the piston skirt opening and closing the intake port. 

On later model the reed valve was introduced.  Reed valves provide a significant flow restriction but that is offset by the fact that the reeds open and close in response to the changing pressures/flows.  With a reed valve, you don't need to use the piston skirt to control the intake timing...  Some engines make this obvious with "case induction"...  reed valve opens right into the crankcase and piston skirt isn't used at all to control intake timing.  Reeds just open and close in reaction to changing pressures and inertia. 

The intake windows in the skirt are there to emulate having the reeds opening directly into the crankcase...  piston skirt never closes the intake port due to the holes.  Intake timing is controlled by reed valve. 

And then some reed valve engines still use the piston to control the intake port timing, there's all kinds of ways to do things.

The piston window evolution is interesting. They appeared with reed valves, they moved up and down. My KX125 is a case reed design and it still has a window in the piston. My 250 is a piston port design with boost ports into the crankcase for 360 degree timing but no windows in the piston.

Yamaha seemed to be splitting the difference with these early motors. They used windows to increase intake duration well beyond what they used on the piston port motors but they didn't quite go for 360 degree timing, the windows usually drop below the bottom of the intake port a few degrees before bottom dead center. Would probably still run with a damaged reed, just not very well.

Did the KX125 have the piston window up higher by the piston pin?  That was done on some case reed models to help cool and lubricate the small end bearing. 

Yes it does. The only time it comes into play is when it opens to the boost port near BDC.

Thanks.  That makes sense...  I think it was Jennings or maybe Bell (or both) that discussed that method of cooling the underside of the piston crown and cooling/lubricating the small end bearing. 

RT325 did a mod like that on one of his pistons...  it may have even been shown in magazines as it was popular to do for desert racing (or any racing where the throttle was held wide open for long periods).  Only works on engines with a rear boost port which usually means reed valve.  Though some modifiers (I think Klemm was one of them) were able to add rear boost port(s) on piston port engines by creating channel(s) in the cylinder wall that were fed from hole(s) in side of the piston

Cutting holes in the skirt of a cast piston (area below the pin) is generally a bad idea as often a cast piston will crack and possibly break a skirt of soon afterwards.  (Not talking about factory holes in the skirt...  talking about creating new ones or even making small holes bigger.)  Forged pistons are far more tolerant of that type of mod. 

Either type of piston is usually tolerant of creating a hole in the area above the pin if you avoid stress risers like sharp corners. 

piston backwards and arrow on it too

Haha, yes as you say---arrow on it too, i'd not noticed.