For me, the definitive authority on fasteners is all on a couple of pages of Mark's Engineering Handbook, a weighty tome of about 450 pages.
There is a chart for all the UNC/UNF bolts and thread pitches, a chart for the machine screws, and a chart for the metrics. When it comes to Metric, there are the preferred series of thread pitches, with coarse only up until somewhere around 10 mm, where there is then coarse and fine. It also lists the alternate thread pitches in the order of preference with second and third levels to them. The Iltis actually used some of those lesser preferred thread pitches, making local ordering of replacement nuts and bolts a "dealer only" choice in some cases.
The spaenur catalogue is also a good source of all the "normal' thread pitches, in both the metric and standard, They also list the DIN numbers on most of their charts for the metric fasteners.
We have a TSC here in town that also sells the bulk bolts, nuts and hardware. I wait for the sales (usually around $2.50 a pound or thereabouts) and also when they decide to rationalize their stock, they often have huge batches of their overstock at $1 a pound.
NAPA here in town has an even better selection of bulk which includes the nylon locknuts in bulk. Their hardware is only $2.79 a pound, and goes up to almost 1" sizes. Unfortunately neither NAPA or TSC carry the fine threads in bulk.
For me, it is a 60km round trip in to town, so it makes a lot of sense to buy in bulk and have them organized in their maxi bins on the wall, ready for use.
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