Nasty little problem here and my back's against the wall in terms of options... I'm trying to incorporate an o-ring seal in an inconvenient area - over a split between two aluminum parts that are assembled together. Obviously the solution below is completely unacceptable because there would be ingress into the split. Below the best solution I've got so far: Small rubber inserts (shown in red) inset into a recess that bridges the split line, thus preventing ingress. If it's done correctly the o-ring should seal against both the rubber and aluminium. Not ideal I know, but I've seen it done before effectively. Any thoughts on this? Anyone got any better ideas?