Originally Posted by paxfish
My front cross beam is made from an aluminum mast
section. The builder
through bolted the cleat in the center, with an aluminum
backing plate inside. How he positioned the plate and held the nuts while tightening I have yet to figure out, since it was about 9 FEET down the tube.
Anybody know what sort of trick might have been used?
Drill the beam for the cleat's bolts, then invert it so that the holes face the ground. Insert the backing plate with an indexed tool. One with witness marks, so as to correctly position the plate. Put the cleats in place on the beam, along with their bolts. Add the nuts to the inside of the beam via a custom, indexed tool, which also acts as a fixed wrench for them. And tighten the bolts externally with a standard wrench.
Note; when I say an indexed tool, to make it you can do something as simple as to take a plece of flat aluminum stock. Machine hexagonal holes into it for the nuts, with the holes spaced to match the cleat's footprint. Followed by adding another piece of aluminum over the backside of the holes, to keep them from being pushed out when you're lining them up blind. And then add precise witness marks to this (DIY) "fixed wrench's" handle, so that it lines the nuts up perfectly with the holes in the beam when it's inserted.
Also, I'd build this fixed wrench to be thick enough so that it barely fits into the beam in the vertical direction. Or rather, that thick primarily underneath of the nuts. So that when it's slid into the beam the nuts are already lightly pressed up against the pre-positioned backing plate. That way it'll take very few turns of the bolts before they begin to grab the threads in the nuts, thus pulling everything together.
Though along with making the head
of the fixed wrench the correct thickness to hold the nuts at the proper vertical distance. You might have to use a another trick or three in order to keep it pressed up against the inside of the beam. Be that temporary solid shims slipped into place underneath of it, an inflatable
bladder used to add pressure, or a lever, etc. And to some degree gravity will be assisting you, as the beam will still be oriented such that the holes in it are facing downwards.
And if you wanted to get fancy, you could add a remote
or fiber optic camera
inside of the beam to help you with positioning things at some points during the assembly.