Greg Bender
http://www.thisoldtractor.com/gtbender
The size of the o-ring used to seal the distributor shaft inside the distributor body is 17 mm ID x 2 mm Thick. Viton o-rings are best, but Buna-N will also work and are a lot cheaper. Make sure to purchase an o-ring that is sufficiently soft...something with a Shore A Hardness rating = 70 is great.