I've got a bunch of cotter pins all mixed together... Where do you you measure from to get the correct length? Overall length? from the bottom of the "crown" to the longest leg? Or the length of the shortest leg?
I think thickness is more critical for best function. Use a cotter pin that fills up the hole and is long enough to be bent so it won't get out - and you can cut them with your pliers if they're too long. I would measure overall length if I'd bother measuring, anything else would be a bit overkill
Otherwise you need to get the KRW Cotter Pin Stretcher part number T-9999
You'll also need the 5-Z-9999 cotter pin gauge.
Odd, I thought this was October 31, Not April 1st!
Here is a table of standard sizes for cotter pins. Length can vary over a range for each diameter.
Many of the newer bulk packs of cotter pins I have bought are foreign made! very soft! and I doubt conform to any American scale. As others have recommended I find the largest diameter that will fit the hole and still have enough length to bend properly.
True Value Hardware has a nice variety pack of various sizes and lengths. The plastic case is easy to carry in your car and also is easy to open and remove the pins for use. They also make good door prizes for club meetings and other events.
Looking at Thomas' chart.... I didn't know there were so many different "ends" on a cotter pin. What is the official cotter pin design for the "T"?