I have spent most of my time over on the yahoo forum... so I hate to duplicate questions but I am in need of assistance... At this point in my restoration I have the QCGB off and getting ready to replace a broken gear, I replaced the pulley on the motor and changed out the belts. The only thing currently attached and running on the machine is the headstock. I am now chasing a vibration / thump issue in the headstock that has me scratching my head. So the runout on the spindle is about .0003 give or take a hair. But when running the machine between 600 and 900 rpm I get vibration that shakes the machine (leveled and double checked on solid concrete foundation). I also get a thump that I can feel but the thump only happens at an interval of about 1 -1.5 seconds. The gears are in good shape, no dings, no trash, no rough spots. I can't feel it below 600. I know I can just run it below 600, but I have come this far! I would assume if it was the headstock spindle bearing I would see that in the runout test, so for now I am holding my breath that those bearings are fine. Could one of the other bearings in the headstock cause this? If so, I would assume the drive set to be the most susceptible. If I am correct the other issue is can that bearing be replaced without removing the spindle. How does it come out? If I do have to remove the spindle, is there any danger in making anything worse? Thanks. I am also in the process of making the input gear for the QCGB its a phenolic presses onto a steel hub. If anyone in interested I can get you the specifics. BTW the gears are 20degree with a 12 diametrical pitch.