When I bought the Wossner for my 500 from Sandblaster, he had me drill and countersink two holes in the bottom of the piston for oiling. Here is his guide I used:
Both pistons are Wossners.
The one on the left has had several mods to it.
I use a 4mm drill and try to get it fairly well centered.
I drill the hole about 8-10MM from the inside edge.
It is not critical by any means.
Then I chamfer the hole with a low speed chamfer.
The exact size is not important.
Just make it look like the pic.
Next I feel into the wrist pin area and see if there are any burrs in the new hole.
If so you can take a coat hanger and bend the end at a 90 degree angle.
Then hold a piece of 220 sandpaper over the end of the wire and work it back and forth until the burr is gone.
Then I take 220 sandpaper and go around the outside edges of the piston (Not the top edge or ring glands) and lightly remove the sharp edges.
All you are trying to do it break the sharp edge.
What that does is prevents your new sharp edged piston from scraping the oil off the cylinder during break in.
After all the mods are done I wash the piston thoroughly... Pre lube and install...
