The heaviest I have or have tried is T-14, and I have little use for it other than king fishing on the Kannektok. The heaviest Skagit heads have enough grains/foot to turn over T-14, but I don't think any manufactured fly line weighs 20 gr/ft. So turnover, if it occurs, won't be smooth. Like Stonefish, I wonder what a person would use T-20 for. As the grams per foot increase, T-* gets closer and closer to being glorified elongated pencil lead.

Sg