not sure why more don't follow gazim and find something from a scrap yard, there's a lot of "brand name" ones available, my local yard had a 50' long wall full of all sizes, all pre-tested, guaranteed and less than 1/2 new price
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
