From the Hansen website http://www.hansenproducts.co.nz/products/tank/SFTFT-install.htm RECOMMENDED HOLE SAW SIZES15mm = 22mm or 7/8" Hole Saw20mm = 27mm or 1 1/16" Hole Saw25mm = 34mm or 1 3/8" Hole Saw32mm = 42mm or 1 5/8" Hole Saw40mm = 48mm or 1 7/8" Hole Saw50mm = 60mm or 2 3/8" Hole Saw80mm = 95mm or 3 3/4" Hole Saw100mm = 121mm or 4 3/4" Hole Saw MINIMUM RECOMMENDED HOLE SIZESHole Size for 15mm = 22mmHole Size for 20mm = 27mmHole Size for 25mm = 34mmHole Size for 32mm = 42mmHole Size for 40mm = 48mmHole Size for 50mm = 60mmHole Size for 80mm = 95mmHole Size for 100mm = 121mm Remove the Back Nut and the Low Density Poly Washer (the firmer of the 2 washers) from theTank Fitting Body. Slide the Tank Fitting Bodythrough the hole in the tank. The Rubber Washermust be between the Fitting Body Flange and the tank wall (ensure the ribs on the Rubber Washerare sealing against the tank wall). On the inside of the tank wall, slide the Low Density Poly Washer back onto the Tank Fitting Body and then wind on the Back Nut. Tighten the Back Nut so that the face of both washers make full contact with the tank wall to obtain a water tight seal. Ensure that you comply with the tank manufacturers specifications When adding additional fittings it is recommended that either a thread sealing paste or thread sealing tape be used on all threaded connections. We recommend Loctite 5331