Even if there are a few minor grooves left at the .090" oversize point, as long as most of the drum surface is smooth and straight, go ahead and re-use the drums. The major reason to turn a drum is to get a smooth, straight surface, with minimum out-of-round. A few small grooves don't really cause a problem, as the lining will wear in to a perfect contact within a few miles.
The owner of a small trucking company where I managed the shop insisted that a grooved drum added area to the brake, and he wouldn't let me get drums machined. The "golden rule" applies here- - - -the guy with the gold makes the rules! I coudn't convince him he was wrong, so I just did it his way!
Jerry
