Now it's time for the decorative pieces — I chose the pistons. Because aluminum can’t be welded to steel, we need to use bolts welded to the frame. Drill two holes in each piston in a place where you can access them to put nuts on them. Drilling into the bottom of the piston where the wrist pin rest works well, using two large holes on each side. I chose 1/4" -28 1/2" long bolts, so I drilled holes 17/64" — the next size up from bolt size. I use a larger hole than needed because after all the welding without a proper fixture, bolts move around a bit. This can save you some frustration.