I chuck mine into the drill press (on slowest setting) and use something like 400/600/800 grit wet/dry paper on the commutator till it nice ans shinny. Blow any copper dust from out between of the gaps of the commutator pads and fire it up!!........If it's just dirty with stuck on old grease, carbon and such, the wife's acetone cleans um good.....GM might use other methods tho.
If the crank batt. only runs the main motor i.e., the FF's and TM all use/share one common ground, not sure adding the crank batt. ground in the mix will do anything for the noise .....However, doing it, then having one common ground between the entire lot isn't wrong to do and won't hurt anything. (most larger systems are this way and FWIW, mine is. I don't have a floating/ISO'd ground in the 12VDC bunch!!)
Additionally then running a non-current carrying ground wire is basically just then "bonding" that floating metal TM heads/pipe and isn't wrong to do either.
Are you sure the TM head is floating right now? i.e., You continuity measured between the TM's head/pipe to the TM/ FF's battery neg post? Its open?
Overall though, seems a guy could just temporally run/climp on alligator jumper/battey cable to the purposed grounding spots and test before hand. Might save you some heart ache or lead you down on the path of righteousness
Side note........My Honda OB, from the factory, was floating. I proceeded to bond it to system ground.
