These sheets are free to use, modify, and distribute, but not to sell. Also please mention where you got the files.
Repairing Your Outdoor Power Equipment, Jay Webster
A well-written guide to understanding and maintaining small engines and equipment.
Used Farm Equipment: Assessing Quality, Safety, and Economics (NRAES)
Guide to assessing and buying used equipment. NRAES publications are now offered through the Cornell Extension.
Farm Machinery and Equipment, Harris Pearson Smith
A guide to farm machinery from mid-century, which is still somewhat relevant to small-scale farming. Contains information on setting up and operating moldboard plows, maintaining small combines, and general information on cultivators and harrows. Widely available at libraries, often for sale used.
Steel in the Field: A Farmer’s Guide to Weed Management Tools, Greg Bowman
Not exhaustive, but the only in-print guide to small-scale cultivation tools and equipment that I’m aware of, and thus a valuable introduction. Goes well with a video series produced by UVM Extension.
Two of my first stops for information-gathering. Wikipedia has plenty of articles which can explain the workings of engines and mechanical parts, as well as, say, choice of lubricants, etc. YouTube has loads of hit-or-miss instructional videos on mechanical topics such as mounting ball-bearings in pillow blocks, or cleaning a carburetor.
If you’re in need of some generic mechanical part, but not sure of what to call it, give this catalog a look. McMaster carries just about everything industrial, and their catalog provides background information on each item to assist in ordering. Why choose an oil-impregnated bronze bushing, rather than plain steel? McMaster is also an excellent place to look when brainstorming custom projects. I’m not endorsing their catalog as a place to purchase parts, necessarily, since they can be a little over-priced sometimes, but they’re the best look-book around. I would compare their prices with your local suppliers, as well as with internet shops like Grainger and Fastenal.
An enterprising group of farmers who’d love to put the means of designing and manufacturing in your hands. Part of the National Young Farmers Coalition, their project is to gather, disseminate, and promote DIY solutions to mechanical problems. Doing great things.
Factor E Farm / Open Source Ecology
These folks might be a little far out at times, but they’re also interested in developing ways to make the farm more mechanically self-sufficient. Project plans tend to require skill and access to a machine shop. You’ll have to form your own opinions of their project…
Roeters Farm Equipment
Based out of Michigan, Roeters offers used equipment and parts from the many different epochs of mechanized ag. Their website is never up-to-date, so give them a call if you need something specific.
If you need hydraulic cylinders or valves or whatever, check Bailey. They have the widest selection, with information on applications of different products. Whether you buy from them or not, their catalog will tell you what's available.
One day I'd like to make a visual glossary of common mechanical terms, as well as add more informational links, a list of vendors, and my own reference charts and tables.