I began construction of my header tank way back in time when fish came ashore and grew legs. This tank was fit to the fuselage and secured in place with hinge pins on each side and while all this was taking place, my engine was still a figment of Lycoming's imagination. Some years later and with the engine installation complete, I put the header tank in place and attempted to insert each hinge pin.
They were very difficult to insert. My latest theory is that 300 odd lbs hanging off the firewall causes the longerons to flex inward just enough to cause a slight misalignment between the hinges bonded to the tank and their mates bonded to the fuselage.

I'll get to test this theory shortly during final installation of the tank by using the hoist to remove some of the engine weight from the firewall.

Angier Ames
N4ZQ