Threads...

hello
you are partly correct on the aluminium threads, although although there are odd occasions when British cycle threads are used. Lighter steel parts use British cycle (26tpi) and heavy steel and cast iron use another cycle thread 20tpi. Presently I use UNC for these parts as they are almost identical.For electrical components tend to be BA threads.
Hope this helps
regards
Andrew

Get yourself a couple of thread gauges. Early machines (pre AMC) depended on BSCy threads into castings etc. ( often 5/16" BSC), a good fine thread but is easily confused with BSF if you are not aware.  If other thread bolts have been used you need to be careful changing back to original as much of the material will have been destroyed.  For internal threads it is wise to produce some wooden inserts that you can wind into the holes to leave a witness mark on the wood and measure that with your gauge. Thread form and thread angle also differ between  same pitch threads so get yourself a Zeus notebook or similar to understand the differences. Steer away from stainless, it is hard, sharp and does not hold when subjected to vibration as a steel bolt would.  Electrical items often use BA thread forms. All will be seen in the Zeus ( or similar) engineers note book.

Hello again
Must have had a senior moment in my first reply, I said UNC, should have been UNF, also buying a thread gauge will be money well spent.
regards
Andrew