I have successfully turned GEM brake drums on a small lathe. The lathe will need to have a swing over the bed of at least 8", preferably 9". The cast iron is easy to cut and no oil or coolant is needed. A rubber silencer such as Ammco # 906920 (actually made for rotors, but fits the smaller GEM drum well) is helpful. Anyone who has turned automotive drums will be familiar with these. If you are good with a four jaw chuck, the drum will mount right to it. Otherwise, a faceplate or chuck back plate mounted to the headstock spindle can be faced down 1/8" to a 80MM (3.145") shoulder so the drum fits snug on the shoulder. Then, using a 1/2" transfer punch mark two opposite wheel lug holes to secure the drum to the faceplate. Drill these holes to size âFâ (0.257") and tap to 5/16" X 18. The drum can then very effectively be secured by using two flat head socket cap screws size 5/16"X18X3/4". Mcmaster-Carr number is 91263A573 for a box of 25. These fit perfectly because their taper closely matches that of the inside of the wheel bolt hole. The depth of the screw taper is 0.198" (5MM), which is just less than the depth of the drum taper of 0.218". The outside diameter of the screw head (0.656") is only a few thousands more that the large end of the drum taper. So the screw just barely protrudes and is no interference with turning the drum. I have contacted the factory and was told that GEM does not specify a maximum inside diameter of the drum. The technician also told me that GEM drums should not be turned. He could not explain why this is so. Over the years, I have turned Cushman, EZ-GO, Yamaha, Columbia, and Honda ATV drums. Never once had a problem.
Daniel