Soundguy, [quote] Phil.. 3 ohm is a standard primary value for a 12v coil. You can buy off-the-shelf round can coils like this.. the NAPA IC14SB[/quote]
The IC14sb is a low RPM coil used on old Jeep & Checker Motors, 4 & 6 cyl engines. 3 ohms is not a normal automotive 12 coil. The hi-performance coils will be 1 ohm or less.
The round coil is oil filled and robust but the front mount is not, although some believe that little elves are responsible in redistributing melted tar back to the top of the coil or that some other unknown phenomena will overcome gravity. The physical properties of overheated Copper are well known, brittle, discolored & resistive (in spots) This is responsible for most household fires when a new receptacle is installed, because of a loose overheated connection, without re-stripping the old wiring to good copper. (oxidation is not the same as discoloration)
The OEM 6 volt Front Mount normal 6 volt thermal dissipation is 9 watts, with Ballast. If the ballast is bypassed watts = 49 and this results in over heat damage.
A coil with 2.5 ohms & .5 resistor will need to dissipate a MAX of 57.6 watts (4.8 X 4.8)x 2.5
A coil with 2.5 ohms & OEM Ballast will only need to dissipate a Max of 23 watts (3 X 3)X 2.5
If the tractor "starts" & runs with the OEM ballast you have a ~ 3:1 thermal advantage.