What size zerk fitting?

[Dr. D]

On most of the thru hulls on my boat there is a bolt on the side (see below). I believe those can be removed and a zerk fitting put in. What size (or threading) fits? After eight years I guess it is time to grease the thru hulls. At least I work them every few months.

 

[Don S/V ILLusion]

Looks like you are highlighting the drain plug.

 

[Dr. D]

Could be. If so, then where do you inject grease?

 

[jssailem]

Not all through-hull valves are designed to have grease inserted. I might check with the valve manufacture to see if the valve should be greased on a regular basis.
That said, you have a couple of options.

1. If out of the water, on the hard: you could inject grease up through the through-hull to the valve. Then operate the valve to spread it around.
2. Alternately you could remove the attached hose and inject the grease from inside the hull into the valve, then open and close the valve a few time to spread the grease about. I think I would prefer to use “Super Lube” rather than grease, especially if the intake is a raw water source that may be used in a water maker.
3. With the boat on the hard you could remove the drain plug and squirt some grease into the opening, then exercise the valve, also seems to be a way of greasing the valve.

Looking up Groco valves, I found this resource which may be of help.