I have taken a touchdown apart and have several different size sacs but none seem the proper size. What is the best size sac for a Sheaffer touchdown?

I believe it depends on whether it is a slender Touchdown, or one of the "fat" ones....
For the slender one, I use a size 15 ½ rubber thin-walled sac and for the fatter one a 17 ½ rubber thin-walled sac that I buy from Woodbin . Martin Smith of Woodbin makes and sells the thin-walled sacs specially made for Snorkels, Touchdowns and PFMs.

Try cathedral pens. If I remember correctly, they have classifications for sacs on their catalogue.

Thanks for the info . Very helpfull Mark

pendemonium also has a very useful size chart... although when the time to buy comes, it's out to the Wood Bin for me.

Yeah, Martin's service is great, isn't it?

Oh, that Pendemonium pen sac chart is derived from The Pen Sac Company (pen sac size links on this page: http://www.pensacs.com/New%20Pages/catalog.html ). They are the company that make the sacs that Pendemonium sells.....