In the past I’ve written about AN bolt, nut, and washer deployment at length. Here I’m simply drawing everyone’s attention to the not-so-consistent sizing of AN bolts. This may explain why the bolt the plans call for is too short or too long for your project, or why you need two washers when the documentation calls for one.
Grip Length Suffix
The nominal grip length for any grip length suffix (often called the “dash number,” as in AN3-31) is inconsistent across the various diameters. An AN6-31 bolt has a nominal grip length of 2 9/16 inches. An AN3-31 bolt has a nominal grip length of 2 3/4 inches. The reason for this is unknown, but gives people like me something to write about.
Manufacturing Tolerances
There is a manufacturing tolerance that can cause the grip lengths of any handful of “identical” length bolts to vary by +/- 1/64 inch. That’s not much for a 2-inch-long bolt, but it means any handful of AN3-3 bolts, which have a nominal grip length of 1/16 inch, can have actual grip lengths ranging from 3/64 inch to 5/64 inch. In other words, -3 length bolts can have actual grip lengths that vary by 50% from shortest to longest. It is for this reason a bolt length called for by a designer can be too long or too short for the project in your garage.
In the end, bolt length dash numbers, manufacturing tolerances, and bolt length callouts in plans don’t really matter. A proper bolt installation can be achieved simply by making sure a bolt’s transition extends fully, if only marginally, through all the parts being secured, as seen in the middle of the photo above. The left bolt is too short and the right bolt is too long.
Very well done and thank you, Kerry. I learned under a good IA and he alerted me to this inconsistency. You’ve made clear the reasons why.
Other than the extra weight, what is wrong with ‘washering up’ the slighly long bolt? Particularly if you dont have the correct length handy.
Good quick read, thanks.
I always learn something from you Kerry. I appreciate that