I'm in the process of sorting out a 1957 M50. As I received the bike it was hooked up as a Negative earth. The wiring diagram for this year shows it as a Positive earth. The wiring is not original and I do not know the history of who's had their fingers in it. My thoughts are to wire it back to the way the Norton print shows it. Bike also had a very badly frozen piston. I have managed to get things apart with out trashing the piston. The rings are toast though. Piston is a .040 over. I would prefer a new piston and ring set but if all I can get are .040 rings I'll go that route and use the piston I have. Any help will be greatly appreciated. Thanks Lloyd.
.. both negative and positive earth in the 1950s - my 1952 ES2 came with negative from the factory. For a bike I'm keeping to ride, I prefer to wire them negative earth anyway as that is consistent with modern machinery and helps to reduce brain-fade induced errors. As you say however yours would be positive earth as standard so you may want to keep it like that.
I believe that positive earth vehicle wiring went out of favour because it promotes galvanic corrosion on the salty wet roads of Northern Europe.
If you can't find specific Norton piston rings you should be able to use Triumph 650 piston rings, the bore is 71mm and are readily available. The Triumph rings are, compression 1/16" thick and the oil rings 1/8" thick. I use them in my model 40 and you get two sets for your money as the Triumph is a twin. They would not suit the modern Italian pistons as I think the oil ring in those pistons are thicker than 1/8".
Thanks Nick, I have found a set of .040 over rings from Steadfast Cycles out of Washington State. Great advice on the Triumph rings too. I hope to have all my parts here soon. The rest of the bike is sorted and ready to go. Mag is still very good with a nice fat spark.
I'll keep you posted.