Selection of the Proper Class of Fit
As a general rule, the lowest class of fit compatible with pressure requirements should always be used. This will result in the lowest possible installed cost of the Lee Plug, since the lower the class of fit, the wider the tolerance band on the hole. The looser tolerances obviously reduce machining time and the time required to inspect the reamed hole.
Note that the selection of the lowest class of fit does not ensure the lightest interference fit in the reamed hole. It merely means that the interference can be anywhere from light to heavy. On the other hand, the highest class always ensures the heaviest interference fit. In cases where less interference is desirable, you should dimension your reamed hole using the maximum diameter from the lowest class and the maximum diameter from the next class up. In this way you will have a total tolerance of .001 inch and the lowest stresses in your material. Under these conditions the proof pressures would be the same as for the lowest class fits.