Are 6 volt batteries always positively grounded?

And are you suppose to use the same spark plugs for a 6 volt vs. a 12 volt system?

No.
Spark plugs don't care about voltage.

While many many many of the older 6 volt tractors were indeed Positive grounded (and then many switched to Neg at 12 volt except allllllll two cylinder Deeres 6 or 12 volt were Pos ground), theres no reason (if configured and polarized etc) they cant work at Negative ground. Way back before Neg became the standard Ford had a different idea then GM and Chrysler as I recall and there was some thought there was less corrosion where frame members were joined if Pos ground was used.

SPARK PLUGS they dont know or care if a 6 or 12 volt coil is what produces the several thousands of volts necessary to arc jump current across their gap. They fire at maybe 5000 to 15000 volts or so depending on the gap width and medium (fuel and compression) in which they operate NOT ANY 6 OR 12 VOLTS. They are designed to retain or dissipate x amount of heat to the relatively cooler head which is the plugs "heat range" and thats a mechanical and thermodynamic property and design cosideration NOT if a 6 or 12 volt coil is uded to produce the thousands of volts to arc jump current across their gap.

WHILE MANY OLDER 6 VOLT TRACTORS WERE 6 POS GROUND AND 12 WERE NEG, ITS NOT NECESSARY ANY 6 VOLT VEHICLE MUSTTTTTTTTT BE POS GROUNDED.......THE SPARK PLUGS ARE NOT NECESSARILY CONCERNED IF THE TRACTOR IS 6 OR 12 VOLT.........

John T

Some voltage regulators are designed for positive ground. If there is no notation on the regulator, it makes no difference. As noted the spark plugs don't care.

Like others have said, the spark plugs can care less about voltage or polarity of your battery.

With battery ignition, the the wires into and out of the coil do matter with a change in polarity. IIRC, it's in on the hot, out to the distributor on the ground. Otherwise the spark will be weak and the ignition contacts can bridge instead of pit.

I was also told that years ago voltage regulators were polarity sensitive and the material the contacts used differed from positive to negative ground regulators. The old 6V regulators I've taken off have been stamped "6V pos. grd." or something similar.

AG