M heads aren't that expensive to worry about trying this difficult repair. Yes,quills were lapped to fit their hole. I don't think it feasible to try doing that,either.
To grease M heads,a Bridgeport tech told me to remove the plug screw at the top. Screw in the threaded end of a tube of lithium grease,and squeeze out a teaspoon of grease. Then,screw the plug back in. This is valuable and hard to find information.