My 1986 H28.5
with the OEM white nylon stuffing box uses 1/4 inch packing material. That said, it is best to measure the space between your shaft and the inside of the gland lock nut to be sure you have the right size depending on the type of gland you have. Packing materials are available at most boat stores and range from ordinary flax to the more expensive and long lasting teflon impregnated material. Since the Law of Unintended Consequences can and often does arise when you least expect it, I suggest that you NOT try to repack the gland with the boat in the water THE FIRST TIME. Do it on the hard first and gain the confidence and experience that all the parts will come apart as planned, and more importantly, go back to together as you would wish. I have never heard of a boat sinking while on the hard, but I know of at least one case where one nearly sank in its slip when the gland broke after being over-torqued by an owner repacking his gland....Lots of stuff in the archives on this.