I want to seal the holes thru the Transom .. 4 u bolts ( 2@5/8" and 2@1/2") .. and 4 bolts (5/16").. they all attach silent thunder platform. I saw an article where you overdrill the holes, fill with epoxy and then redrill for the bolt size. This prevents water from rotting the transom ..

How much oversize would you need ? A 1" hole for the 5/8 would yeld a 3/16 wall thickness for the epoxy ...that enough ?

Also to be able to drill the oversize hole ...fill the existing hole with epoxy so a speed bit would have something to center on ??