"When" do you think "maybe" he should be taken to a doctor?

 

5/8 sounds right, while the threaded portion of the spark plug is 14mm. I reckon the metric socket(16mm) will be fine too, albeit not as tight a fit on the spark plug, but at 18ft/lbs you won't be stripping anything.

 

It ain't 5/8 .... it is a metric size.

Edit: My Halfords socket says 'For 10mm'

 

5/8 and 16mm are virtually identical, something like 2 thousandths of an inch difference between the two, with the 5/8 socket fitting best.

 

No worries, but you really can use either a metric or imperial socket on the plugs, just make sure you thread the plugs in by hand first before you torque them down with the wrench - so as to avoid cross threading them.

 

I think mine says 10/16mm on it, just to confuse you. Spark plug sockets have their own weird sizing scheme.

 

Are the plugs not 10mm thread, but then 16mm across the flats to give the socket size? don't have one handy on the couch to check.

 

you need a Deep 16mm socket that will fit down the plug whole in the rocker cover, my deep snap on one fits like a glove

Job Done

 

You mean 1/2" drive? 1" drive might be a little bigger. Glad you got it sorted and the seems like a good un.