I had the same problem on a 76 merc. A metric 8mm(I think) was just slightly
bigger than the 5/16. I tapped it in and it held to get the allen loose.
Good luck.

wrote in message
...
On Wed, 18 Oct 2006 13:34:49 -0400, let wrote:

Trying to replace trim tab on mercruiser outdrive.
5/16 bolt stripped by last owner. Any ideas how
to get it out.

Thanks
Called local marina. They say that they saw thru the bolts
from the side!! Little bit of housing gone NO big deal.
Sounds scary to me.
Thanks for all the tips & sugestions you guys have given me.
I'll be thinking about this one for awhile.I'll eventually let you
know how it turns out. Thanks again