There's another method of mounting them without drilling a 4" hole in the center, it's time consuming but does work.
i just did it in my 18.
you can get the old ones out and the nuts fall off the bottom, take a drill and slightly hog out the exiting holes, get new bolt drill a small hole in the side of the bolt and one from the top, thread a piece of picture wire through the bolt then fish a wire trough hole tape to picture wire carefully pull wire and bolt up from the bottom double nut when thightening the nut from above, yes pita but does work, whichever method good luck.
2006 CLASSIC 18 WHIPPLED MX 6.2, BRAVO 1, STAINLESS MARINE EXHAUST, LATHAM STEERING, IMCO 2" SHORTY, LABBED MIRAGE PLUS
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