This will give you a feel for simple things.
The max load on a motor mount tube is something like 3,000 to
4,000 pounds at six G's which only happens once in awhile.
The half inch bolt at almost 24,000 pounds is ludicrous.
Take half these loads and max stress and things are still 
usually way over designed. Double shear is twice the load.
Again, what usually happens is mechanics over torque them.

Paul Lamar
 
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Max stress is 120000 psi. Single shear or stright tension.
Bolt Dia      Load
 .1875         3313
 .25           5891
 .3125         9204
 .375         13254
 .4375        18040
 .5           23562

CLS
OPEN "bolt.dat" FOR OUTPUT AS #1

pi = 3.1416
maxstress = 120000
PRINT #1, "Max stress is 120000 psi. Single shear or stright tension."
PRINT #1, "Bolt Dia      Load"
FOR dia = .1875 TO .5 STEP .0625
PRINT #1, dia,
load = maxstress * ((dia / 2) ^ 2 * pi)
PRINT #1, USING "#####"; load
NEXT
CLOSE
SHELL "more < bolt.dat"
SYSTEM