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For a 2 stroke model, a 500 to 650 cca, 12 volt starting battery will be sufficient. The recommended battery for a four stroke model is 1000 cca,
Mercury 1984 6 cylinder L6 what type & size of starting battery is required
550 cca
The 75 hp Mercury outboard will need a starting battery in the 650 CCA range, although, up to 1000 CCA would work just fine.
Any place that sells batteries can tell you the correct size to buy.
For a 2005, Mercury 3.3 hp outboard, a NGK BPR6HS.
Mercury wants you to install a battery with at least 525 marine cranking amps if you boat in the summer, and 1000 marine cranking amps for winter boaters.
1 gauge cable should do the job
OD281588
An 8 hp Mercury outboard uses the following plugs, depending on the model. 1986-2000, 2 stroke, NGK BP8H-N-10 (0.40 gap) 2001-2006, 2 stroke, NGK BPZ8H-N-10 (0.40 gap) 2006, 4 stroke, NGK DCPR6E (.035 gap)
size and gap of plugs
1-1/2x16