I was also going to ask about pistons for the S811, but wanted to get the Durango squared away first and see how much improvement the pistons make. The shocks are 16mm bore, with a 3mm center mounting hole. I'm currently running 8 hole pistons front and rear, all holes drilled out with a #55 drill bit. Serpent also sells 6 hole and 10 hole pistons. I'd have to measure the actual OD of a piston to be precise, and the thickness of the mounting step on the shock shaft. Or I could send you a shock shaft with a piston on it to measure from.