12) Place the black rubber Gasket [P2] on the four studs on the Crankcase Assembly.
13) Place the Piston Assembly [P1] on top of the Gasket. It will not fit exactly: there will be bulges and wrinkles. This is normal.
14) Remove the protective covering from the Large Brass Washer [D3].
15) Put the Washer on top of the Piston Assembly.
16) Thread four of the Brass Nuts [P4] onto the studs a few turns. The nuts should be loose enough that the “rubber” Piston Assembly material slides freely between the Gasket and the Washer, while staying securely on the studs.
17) Adjusting the Piston Assembly is important. When the Piston Assembly is adjusted properly, the center portion of the piston (the red washer) should travel up and down slightly more than 1/8 inch (3.2 mm). The Piston Assembly must travel through its entire range without stretching the diaphragm. If the diaphragm stretches at all, the engine will run very poorly, or not at all.
Note that Piston stretching can also occur due to misadjustment of the connectors: see the discussion near the end of the Assembly Instructions.
Gently pull the Piston Rod up, until the “rubber” Piston material has as few wrinkles as possible (stretching the material slightly). The red washer should be above the flats of the brass nuts. Hold the Piston Assembly in this position and tighten the Brass Nuts finger tight; then (after releasing the Piston) tighten the Nuts 1/4 turn further with a wrench. The Piston Assembly must make an air tight seal with the Crankcase.
Be very careful not to puncture or tear the “rubber” part of the Piston Assembly. The engine will not run if there are any cuts or tears in the “rubber”. Replacement Piston Assemblies are available from the manufacturer.