ok,

i have two assembled pistons in my hand (from two different manufacturers).

one piston is assembled with the piston pin hole (opening) facing the exhaust side

the other piston is assembled with the piston pin hole (opening) facing the carb side

is there a right / wrong way?

the manufacturer exploded diagrams do not put any real emphasis on it, and there is no detail or mention of it.

some say to face the opening of of the piston pin towards the carb so it carrys the oil better

some say to face it towards to exhaust port to eliminate build up.

what are you thoughts?

if it makes no difference, why is one end of the piston pin opened, and the other closed?

I tried both ways but never notice any different.... I even remember one of my engine, sorry don't remember which one, have opening on both side of the piston pin (a tube). Maybe someone can show some insight regarding this.....

my point exactly.

i have never taken notice assembling them in the past and put them in randomly.

but why do the two unassembled pistons i am holding from the factory have them assembled in opposite directions?

if it doesn't matter, why is one end opened, and one end closed?

It doesn't matter.

Though there is a theory about which way would be better. Theoretically, having the open side of the pin facing the intake would slightly increase the crankcase volume, reducing crankcase compression. So, theoretically it should go the other way, open side facing the exhaust. In practice, it really doesn't seem to matter.

I have sometimes thought about another option. If the pin had a small, perpendicular oil hole and the open side faced the intake, that might improve lubrication to the upper rod bearing. Then again, maybe not. The pressure waves in there can do weird things

The pin is hollow to reduce weight.

One end is closed off so you don't have a clear path from the intake port straight out of the exhaust. That would really kill performance.

The closed end must face the exhaust, Through the open end only cool gasses will flow giving some cooling on the smallend.

I was told long ago that the closed end of the wrist pin always points towards the exhaust in order to prevent the heat from the exhaust from being able to enter the open pin and cause issues for the top end of the rod due to overheating.

Lee Muse

Does not really matter, in the past, almost all engines I ever saw came with the hole facing outward, but I've always put the hole to the exhaust for two reasons. One, like The Wingman said, it theoretically highers crank case compression and, more importantly, two the covered side is the heavy side of the pin, its good to put it in the shorter, lighter side of the piston to help that offset balance that the rod is always having to handle in the upper bearing/section.

X

Every one I have ever seen, the hole is facing the intake not the exhaust. I am talking about pistons that I have looked at straight from the manufacturers.

Picco.
But it is closed in the middle, if it really was open, you would loose pressure built up in the crankcase.

On all brand new Novarossi engines which I have owned, the hole was facing the exhaust. I will stick to this orientation

No, the pin was completely open. I remember it as I had a hard time try to remove it as I don't want to damage the hole in the piston or the rod bushing.... But it maybe a side exhaust engine for my bike or helicopter.....

Yeah, has to be a side exhaust engine then.

I do think one of my old Piccos had the pin that was blocked in the middle though.

Now I remember I also used to have an OS max side exhaust in my PB International (long time ago). The liner in this engine was rotated by some degrees, in a way that the piston pin could move in between the ports of the liner, so it even did not have piston pin clips!....

Yes, the piston pin was filled with teflon endpoints so it did not harm the sleeve.