I don't remember for sure--is there a drain plug on the bottom of a steering box? If there is a drain plug on the bottom, adapt a grease zerk to fit the drain hole and (using a grease gun filled with corn head grease) fill the gearbox from the bottom up. Take out the fill plug on the top so you can see the grease oozing it's way to the top as you fill the steering box.
This is how we fill the row unit gearboxes on corn heads and it is the ONLY correct way to fill these gearboxes.
If there is no drain plug, get piece of small diameter hose, like 3/16" or 1/8" and get the proper fittings to adapt a zerk to the hose. Work the hose around and get it down inside the steering gearbox as far toward the bottom as you can. Pump the grease inside the gearbox until it comes out of the fill plug.
