This is an interesting thread for discussion,
Yes I can remember hearing the stories not only from my dad but many visitors to shows who remembered working with tractors of the 30's and 40's era of "how they took the magneto's off their tractor to warm them up in the side oven of the house cooking range" to make them start better, I know here in UK we have a very damp climate and I am sure this has a lot to do with magneto failures.
As John says the common fault is the points and a "greasy like green film" can build up on them over the winter or period of time whilst they are not used, when you come to start them a simple clean of the points and you have a good blue spark again. I often wonder if it is the type of material which the points are made of?? as I have tractors here particularly the styled Red 25 and Pacemaker, one has the Fairbanks Morse and the other is a Bosch flange mounted magneto, both these magneto's have never been touched since 1983 and 1987 respectively, I have never had the covers off them at all and can go to them and they will always start, other Bosch U4 magneto's which on "U" frame tractors have been rebuilt by professional magneto specialists in the last five years or so will give troubles if not used regularly, why is this??
I often think about Larry Weber and his Dome Valley Museum in Arizona, I know he had to haul his treasures many thousands of miles to their home, but when there they were kept in a superb warm / dry environment, I remember when our containers arrived here after the auction I soon had the tractors running which had not been run for a long time there, the magneto's were hot, only a few weeks later when John got his Orchard GP to Wales it would not start and the magneto was full of Arizona dust which in our climate had gone damp and so another magneto to the repair specialist, and so the saga continues.
There seems to be no answer to this and so many variables, on what should be such a simple thing, I know our rebuilders here are using modern technology for coil winding and replacement etc, but surely there is a simple answer to this problem, we have tried covering them for the winter period, to keep the damp off but still we ask why some and not others just go off.
Other than heated temperature controlled storage buildings which we cna't justify here it looks like we have not progressed from our previous generations of "putting the magneto in the side oven to bake"
I look forward to your stories on this subject from across the pond , how is it down under in Australia / New Zealand??