Thanks again for all the information and encouragement.
I have rebuilt car several engines but always stayed away from manual transmissions and diffs. Something about shift forks, multiple shafts, and adjusting / setting backlash just scares the hell out of me. As Clint Eastwood said “a man has to know his limitations.” As it turns out I have quite a few
Visited busy bee yesterday and looked closely at their new version of this lathe - now called model CX708. Only real difference appears to be they now use sealed bearings, have done away with the lubrication point above the front spindle bearing for that reason, and have a cover over the lead screw to prevent swarf / chip from getting on / entangled in it.
Long story short, the problem is solved.
I tried the stethoscope trick as suggested by one of the posters above. Attempted this at all speed ranges. This gear head lathe is an incredibly noisy machine when using a dowel (i don’t own a stethoscope) all i could hear was a cacophony of gear related noises. This are definitely not a Swiss watch.
I took the lid off the gear box and manually rotated the chuck and could not replicate the noise. Tried to move shafts back and forth and up and down and no joy.
I did not want to run and observe it with the lid off due to the splash lubrication system.
I tested the runout using some tips i found online (piece of wood inserted into the the chuck and spindle and pry on it to try and get it to move up down side to side in order to determine excessive bearing wear / shaft wobble / excessive runout. Runout seemed uniform across all axis.
After examining the gear box and determining what steps I would have to take to begin the dreaded disassembly I decided to first check how tight all the bolts were that attach the covers that hold the various shafts in place. All front covers are Allen Key and everything at the chuck side of the lathe was snug.
At the rear of the spindle at the back / outboard side of the gear box where the drive belts are there are two very large nuts threaded onto the rear of the spindle that appear to put tension / pre-load on the spindle bearings. As an aside this has given me a good idea for machining a spider but that is another story.
No specs for adjustment and no way to put a torque wrench on them (they have several slots around their circumference and the reason there are two appears to be to lock them once the required tension is achieved), but through trial and error managed to tighten them, run the lathe, tighten them again, run the lathe, until the noise went away. It did not take much.
I have changed the oil in the gearbox several times before, and did so again. I use Rolair Systems 30W Air Compressor Oil. Before I attached the top cover to the gear box i did my best to pack some grease into bearings i could gain access to. Again it is very cramped in there so i did not get much in there.
Thankfully no filings in the gear box (used a telescoping antenna style magnet and dragged it through the oil) before changing it.
