Up for sale is a set of exhaust riser gaskets. These will fit Nicson, Hardin, Harman and Glenwood Marine aluminum exhaust manifolds. The outside diameter is 4 7/8 inches, the inside diameter is 3 inches and the holes are 5/16 inches. These will not fit Harman Marine manifolds that have 3/8 studs. We have these custom made for us from HTX-900 material and when installed properly they will not leak.