The real early Johnsons had the mag plate turned to fit the flywheel and both items had corresponding numbers stamped on them to identify them as a matched set. One of the service manuals even gives the dimensions for building an arbor to be used for turning different model mag plates but I find a large flat file works just as good.
I start by marking the lams with a Sharpie or something which will show up and then rotating the flywheel to see where the high spots are. After filing it down some, mark the coil again and check the fit. I usually stop once everything is clear but I believe some guys use masking tape on the outside of the lams to set the gap a bit more accurately.
Here’s a poor fitting aftermarket coil being filed down to fit. Same idea.
Attachments:
