Originally Posted by Will75
Galvanic action.....stainless bolts, aluminium casting, sea water.
"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. The same galvanic reaction is exploited in primary batteries to generate a voltage."
The best solution would be to electrically connect the SS bolts and the engine using a SS strap between the head of the bolt and some high up part of the OBM....but its a bit of an overkill....just replace the bolts.
If it was galvanic corrosion then the aluminium would erode, not the stainless. Stainless is noble to aluminium.
This is typical crevice corrosion in a situation where all the classic ingredients are present. The oversize holes in the saddle with a washer over the top provide the perfect spot for seawater to get in and sit there stagnating.
All you can do, as has been said, pump plenty of 291 in there to keep the water out. A good bit on the transom round the bolt before the outboard goes up against the transom, then fill the hole in the saddle round the bolt.
Most outboards are drilled for 1/2" bolts and us Brits usually end up using 12mm so there's a bit of space round the bolt which I've never liked much. But that's a whole different subject that's already been done on here!