What dictates alternator amperage?

[Detour]

My 67' K20 327 has the factory style alternator that is stamped for 37A. I found a guy locally selling a NOS 55A for $10. I grabbed it and didn't look in the old greasy box until I got home. It is definitely a heavily used alternator that was remanned in the past. (Heavy aluminum color overspray in all the vents). I pulled it apart and it was full of crud and dust from the brushes.

Thing is everything looks identical between the old 37A and the recently purchased 55A. Housing, diode/electrical components in base, stator and rotor even appear to have the same # of windings. What components should I swap into my OE alternator housing to up the amps? Thanks

[Dead Parrot]

At this point, I wouldn't trust the 37/55A stamping as being accurate. I doubt the rebuilding places paid attention when reassembling units. When I was a bit younger and playing the alt parts swap game to save money on a 68 Impala, there was a noticeable difference between the 37 and 55A stators. Don't remember if there was a noticeable difference in the rotors.

Unless you are wanting to keep your truck original, I would swap to a newer style 12SI with the internal regulator. Should be a simple physical swap with some rewire needed. Plenty of threads on this forum on doing that. You could drop a 70ish Amp unit in and it could loaf along powering your truck. Bonus is it will look very close style wise to your older unit.

[Detour]

I follow what you mean on the reman of units and I have briefly considered the 12SI alternators. Is there a way to test the components of the two alternators I already have and pick the highest output items to assemble one?

[franken]

"Use up the amps" is nonsensical since current (measured in amps) is drawn by the load. If the load is less than what the alt can deliver, all is well. If over, the voltage supplied will start to drop.

Everyone should spend a few minutes and learn Ohm's Law. It isn't difficult.