......ThomD wrote:new clamp for the negative terminal. It was cracked
ThomD wrote:not a problem that would account for the past performance
Actually, that is exactly the type of problem that WILL keep the tractor from starting. 6v systems MUST have good paths for the high current they demand for starting. That includes ALL contacts to be clean ("and shiny"!), paint removed from between assemblies like battery box-to-chassis (or wherever the ground cable is connected), and the starter-to-block surface where they are bolted together. The gauge battery cable is critical and the terminals and their mates MUST be solid and clean.
It takes a lot less time to go through all and clean everything (wire brush, sandpaper, etc.) than to call the mechanic every time you need to start it.
if your tractor doesn't start after cranking it, turn it off and start at the battery touching all the contacts. When you find a hot area, either wire or connector, that is where you have a problem. It doesn't take much resistance to keep the tractor from starting. 12v systems have the advantage because with the higher voltage, they require lower current and are tolerant of more resistance in the "system".
Good luck!
