That's how I put a piston in a Mitsubishi it worked. wait we don't have oil pan's that we can take off. That's the prob.
A single cylinder four stroke has NOTHING in common with a multi cylinder engine when it comes to rod design. The rod on a single is solid, and the pin is pressed into the crank halves with considerable pressure.
Everyone appreciates help with a problem, but if you don't have knowledge about a specific problem, don't muddy the waters with an incorrect answer, kay?
JB
One of the reasons you may be having trouble getting the piston/rings to install from the bottom is that this is not a wear area. If you measured the cylinder at the top, then ordered piston and rings to fit that, the bottom of cylinder may be considerably smaller diameter (no wear). If you haven't checked the cylinder taper with a bore gaugue, you really should do that.
Also, another thing that can cause the top of the bore to be bigger than the bottom is ring flutter (from engines which are reved lots). There's an actual grove worn in at TDC where the rings sit at that point in the cycle. This can cause a cylinder to be up to .005 - .008" larger at the top than the bottom (or more in a non-chrome bore).
Try installing the rings without the piston at the bottom of the cylinder. If they fit, check end gap to be sure they're not too tight. On a four stroke, excessive end gap is really not a problem, but too little end gap can be disatrous!
Also, like Rich said, make sure the corrugated center part of the oil ring is not overlapped. Be sure the ends are butted.
If the rings will install without the piston, and have the correct end gap, then they should install with it!