The International Cessna^® 170 Association

Are the correct sparkplugs for our Cont'l O-300 (B in my case) the 5/8" 24 thread or 3/4" 20 thread? I know, I should know this, but I want to be absolutely sure before I order new ones

The easy way for you to know the answer to this is does it take a 3/4 inch wrench or a 7/8 wrench to undo the B-nuts on the end of the plug.
3/4 wrench = 5/8" 24
7/8 wrench = 3/4" 20 thread.
It could actually be either one if someone has changed out the harness to install the 7/8 wrench = 3/4" 20 thread large format (high altitude) sealed ignition leads.
Jim

Thanks Jim.

Pokey, I'm fairly confident your greenie does not have the "high altitude" anything installed.

So George I guess I don't need to get pressurized mags