I have an old PICCO P60 rear exhaust bought about 10 years ago , I stored it disassembled , and now I don't remember wich way to mount the piston and the rod , because I wonder if the piston has a side that must be in the exhaust position or not , and the rod has a small hole in one side (for lubrification I suppose ) , it must be mounted facing the ball bearing or not ??

Bert,

The only clue I can give you is the piston.

If you have not "brilloed" the piston and it still has carbon and varnish on it, you can usually trace, which side was facing the exhaust port(s).

That side of the piston ran hotter than the side that faced the intake bypass ports. it will generally have more varnish and carbon on it. The port lineout may be traceable.

If the rod is still attached to the piston, that problem would be solved as well. If not, the oil hole should face the pickup side, as you trace the route the bottom end goes through as the engine turns.

I am not familiar enough with this engine to know if the rod has any built-in offset.

Sending a clear picture of the rod may help (don't disassemble it if it is assembled).

Thank you DarZeelon , unfortunately the piston is NEW ( no signs on it ) and so this d'ont help me , but I hope that any position is good because it is new , for the rod I will check the hole as you told , tahk you again .

Some rear exhaust engines will have the front of the piston skirt cut away to clear the rear bearing, and it will be straight on the rear side of the piston. If your piston isn't straight all around across the bottom, I'll bet the side that isn't straight will be towards the front of the engine.

This brings up an interesting point I'd like to share; Piston Pin direction on Rear Exhaust engines. One end of the pin is hollow and the other end is capped or solid.

I've seen them installed both ways on various models of Rossi, YS, and OS engines.

I always install them with the hollow end to the rear in a Rear Exhaust engine. BECAUSE the piston pin is lined up with the boost port at a time when you are trying to build crankcase pressure and any volume in the pin tends to reduce crankcase compression ratio.

I don't think it effects lubrication one way or the other.

My NIB P60 has the oil port facing foreward, also look for the chamfer on the rod journal, this must face the crankshaft. Most MFGs only chamfer one side, this is to clear the shoulder left at the base of the crankpin after finish grinding.