The coupling or center pole joins the rear bolster and the front bolster. They are connected with the egg-crate joint we looked at earlier. The pivot pin goes through the center of the joint and into the axletree.
The coupling pole and the front bolster are are joined together by the upper plate of the fifth wheel.
Here are the coupling pole, front bolster, and upper fifth wheel plate.
The coupling pole is a 48" 2x4. To the left is the dado for the egg-crate joint to the rear bolster. It is set 6.5" in from the end, and is 1.5" wide and 1.75" deep. To the right we see the rebate for the fifth wheel and the deeper dado for front bolster.
Cut the dado for the bolster first. It is 1.75" deep x 1.5" wide and is 4.5" in from the end. The rebate for the fifth wheel is 5/8" deep and 10.5 inches wide.
The bolster is 27" long and ripped to 3 and 3/16ths wide. It has a dado on one side centered, 11" wide and 5/16" deep. And on the other a dado for the egg-crate joint 1.75" deep x 1.5" wide. Begin by cutting a 2x4 to length, then nibble out the dado for the fifth wheel and egg-crate joint. Only after the dadoes are cut rip the bolster to 3 and 3/16ths inch wide, taking off the waste on the side of the fifth wheel dado. Look again at the picture of the assembled coupling pole, fifth wheel upper plate and bolster in place. You'll note a small gap between the bolster and the axle tree. The bolster does not contact the axle tree. The fifth wheel upper and lower plates are the only point of contact. If you leave this gap out and have the bolster and axletree in contact at the start, the eventual slop that comes from wear with use will cause them to bind in a turn.
Once the parts are cut fit the egg-crate joint and set the fifth wheel in place. The center of the fifth wheel should be over the center of the egg-crate joint. Attach the 5th wheel to the bolster and pole with 1 1/4" drywall screws. Again, do not screw through through the center.
Using a drill press if you can, or great care if you can't. Drill a hole for the pivot pin perpendicular through the center of the 5th wheel and through the joint.
For a pivot pin I usually use a 7.5" spike. This requires a hole of 11/32" diameter. You can use a long machine bolt with the threaded end cut off, or piece of rod for the pivot pin as well. Don't use threaded rod, as it will chew up the hole in use and lead to slop that will effect steering and can lead to the front wheels "wandering" which makes it a pain to pull. When you've drilled the pivot pin hole in the pole/bolster assembly, go back to the axle and drill a hole at least 4" deep through the center of the lower plate into the axletree.
A word about drill bits. Drill bits come in many flavors. Spade bits, twist bits, brad and spur points, High Speed Steel, etc.
When I used to teach woodworking to cub scouts we always spent an several evenings on the basics before we even started a project. It's not true that guys are born able to drive nails. Or that girls can't learn--this is sexist propaganda. The cubbies always looked a me funny when I handed them a hammer, a 2x4 and a box of nails; and told them to hammer nails in a straight line down the board, but they quickly learned a valuable life lesson about hubris and that even simple seeming things take practice and attention to do well.