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This question gets asked a LOT on forums and I finally decided to post a very simple "how to" for determining the proper packing size for your boat.
#1 Measure the ID of thread to thread of the stuffing nut with a set of calipers.
The nut on my old box measured 1.38 inches and I know my shaft is 1".
#2 Simply subtract the shaft diameter of 1" from the nuts ID of 1.38" as in:
1.38" - 1" = .38" inches
#3 Now divide this number (.38) by two as in :
.38 divided by 2 = .19 inches.
3/16 of an inch, in decimals, is .1875" or about as close to .19 as you're going to get!! So I know my packing is 3/16 not 1/4 because that would be .25"