Despite all the science, the composition and temper of rivets varies slightly
from batch to batch. With smaller rivets the deviations from the norm are
usually not significant but for larger sizes you must fine-tune your equipment
to ensure an acceptable shop-head.

The numbers give us a handy place to start, as does experience, but if all you
have to go on is numbers then it would be wise to lay the books aside and use
data from that particular batch of rivets. Which can get a bit tiresome.

I think most will agree that one-off fabrication of small airframes remains as
much a manual art as a science.

-R.S.Hoover