I believe the 20 volts is AC coming from the stator before being converted to DC by the rectifier. Which by the way is a brand new Harley unit.
OK but that would likely be an off load voltage. To get the rectified and regulated 14.4 volts expected from a fully working charge circuit and a good battery, I would expect only a volt or two more of AC from the stator on each of its three phase windings.
You suggested your guy was looking for 20 volts AC direct from stator but what voltage did he find? If either of the three phase outputs was near zero then it looks like stator is u/s, which as you feared is an engine out job.
Just to repeat if your guy said the charge circuit is "only putting out 12 volts" then by definition it's putting out NO charge current at all and all he is measuring is the directly connected battery voltage. Sorry but this comment, if directly from him, made me suspicious.
There still could be a broken wire into the rectifier/regulator or even the new HD unit faulty. Easy enough to check.
Good luck Dutch.
