If it is the old style plugs it is a 13/16s socket size.

bowgus wrote:

Dunno but my spark plug socket (auto) fits exactly.

"David" wrote in message
news:caxhd.31973$G15.6994@fed1read03...
Hello,

For the 04 Mercruiser 3.0L Alpha I I/O engine, what size of prop wrench I
should buy. Thanks.

David