Spark Plugs

[petersen_1955]

I am looking to buy plugs for a tune-up in the Spring. I have a 1975 Cobalt Tri-hull(18') with the Mercruiser 233 (Ford 351) engine. When I go to websites to look for plugs, I get two options. They are 14mm and 18mm Hds. Any idea on what that means? My boat is in storage for the winter, so pulling a plug isn't an option for a couple of months.

 

[javsam]

Re: Spark Plugs

 

[Don S]

Re: Spark Plugs

What plugs are in it now. <br />The 14mm is the diameter of the threads. 14mm being around 9/16" 18mm is almost 3/4".

 

[Bondo]

Re: Spark Plugs

The 14mm Sparkplugs use a 5/8" socket,....<br /><br />The 18mm Sparkplugs use a 13/16" socket.......<br /><br />Do you remember Which Wrench you used last time,..???...<br /><br />I don't do Fords,.... I have no idea when they changed.......

 

[Dunaruna]

Re: Spark Plugs

In the auto game (I presume marine is the same)18mm up until about 1972, then 14mm.

 

[paulie0735]

Re: Spark Plugs

Pete, your engine could have been fitted with 2 different type of heads, (commonly called, small or large plug heads) The only way to tell is to look, as stated above the main difference is in the size of the hole they screw into. Be very careful when you go to remove the old ones. Ensure you use a quality spark plug socket and bar and make sure you get a straight pull on the socket bar. They have a nasty habit of breaking off in the head, especially in Marine engines!!! Put a little smear of grease on the threads of the new ones and don't over tighten them. Good luck!

 

[paulie0735]

Re: Spark Plugs

Hay Dunaruna! Do you have an email address I can contact you on?

 

[Dunaruna]

Re: Spark Plugs

Yep, sure.<br /><br />aldek@(removeforspam)alphalink.com.au

 

[petersen_1955]

Re: Spark Plugs

Thanks for all of your replies!