Fishmouth repair

Since the bottom 1/2-20 UNF kingpin mount is cross threaded and misaligned I am going to drill and tap it oversize to 9/16-18 UNF. Usually the threads are just stripped and I will level the kingpin bolt hole axis and use the bubble level on my drill motor to drill the hole true to the kingpin bolt axis. Drill bits tend to follow the existing hole so this method will only make this mess worse. I need to fixture the frame to straighten out a hole that is off front to rear and side to side. I started by putting an 8″ 3 jaw Buck chuck on my drill press. Then I chucked up a 33/64″ drill bit, lowered the drill bit into the chuck, chucked it up and then bolted the chuck to the drill press table. This aligns the drill bit to the kingpin bolt hole axis.

Next I threaded a long 1/2-20 UNF grade 8 bolt into the top kingpin bolt hole that is still in good shape. I will use this bolt to line up on.

Now the frame is loaded upside down with the long bolt going into the Buck chuck. The frame is far too heavy to support by just the bolt so I have to move the drill press and frame over to a chain hoist to support the rear of the frame.

Drill the hole.

tap the hole.

Done. The thread alignment turned out perfect with at least 80% of the threaded hole cleaning up.