I have seen channel iron used for forks. 4" should stand a lot They will hold quite a bit before bending. Depending on your loader ---- I would "box" the open end with 1/4" flat stock for reinforcement as an extra measure of strength. I know you want the empty weight down as much as possible, also the length of the channels will dictate load-before-bend factor. sounds like you have everything figured out for the frame. On your top pin, The center sleeve will hold if/when you use close setting on the forks.--- mine has a couple in the center and I can get forks in to the distance in the pic. These are from a Bobcat brand loader that I cut down and modified to fit one of my New Holland skidloaders quick tach which is WAY different than Bobcat. They slide on 1 1/2" bar at top and rest in 3" channel at bottom and yes they do swing when dumping. --- When digging sweet potatoes and horseradish with them I slide them in close as they will go then chain them back with little 1/4" chain ----- works for me!
"A machine you build yourself is a vote for a different way of life. There are things you have to earn with your hands."
