I did some magneto research at the Blakesburg fly-in. Thanks to the generosity of Tom Wottreng ( A&P/AI) and his magneto repair shop ( Aircraft Systems in Rockford, IL), I can pass on some info that will hopefully make antique flying safer and cost less in the long run. There is a broadly held opinion that replacement parts for old mags ( coils, condensors) are new parts that were made long ago with marginal materials available at the time. There has been opinion offered in print that this true, but I don't have the details of the test conditions applied. So use your own judgement and I hope I don't get any threatening calls from ACS legal staff !!
One benchtop stress test used in the industry is as follows:
The magneto is heated to 165F
The leads are connected to plugs with a gap of 8mm ( 5/16" ). This is to simulate the spark resistance created by piston compression of the air.
The mag is turned at 2000 RPM
A steady fire confirms a healthy condensor and coil. This is a short test because the mag shaft seal is not lubricated by engine oil on the bench.
The best advice I have gotten to date is from David Schober and TJ at Aircraft systems. That is to replace my old Bendix S4 mag (s) with a new Slick. The materials and processes used in manufacture are modern and superior to the old mags, and they do routinely pass the above stress test.That said, a new mag will die quickly if overheated. Have you put a thermocouple on your mag in flight on a hot day
If the temp is over 160F, you may be shortening the like of the mag and last time I checked, they are not cheap!