Mercruiser Drive Impeller | Sierra 18-3017 Aftermarket direct replacement impeller fits Mercruiser #1, MR, R, and Alpha 1 marine engines thru serial number D494568 as well as many Mercury outboards. Sierra Mercury impeller kit Fits Mercruiser #1, R, MR, and Alpha 1 units thru s/n D494568 2 11/32 in. diameter x 1 19/64 in. width x 7/8 in. keyed shaft Replaces Mercury 47-89984, 47-89984T4, 47-30221, and 47-65960 Replaces Chrysler/Force 47-803631T, 47-F694065, and 47-89984T4 Replaces OMC/Johnson/Evinrude 775460 Replaces Mallory 9-4530