In this video, This Old House general contractor Tom Silva shows how to stabilize an unsteady staircase post.
1. Cut the finish between newel post and cap with a utility knife. Pry off the cap with a thin pry bar.
2. Shine a flashlight down inside the hollow post to confirm there are no obstructions.
3. Make an extra-long hole-boring tool by joining together three 12-inch-long drill-bit extensions and one ¼-inch-diameter x 12-inch-long drill bit.
4. Chuck the hole-boring tool into a drill and bore two pilot holes into the base of the post.
5. Screw a coupling nut halfway onto a ⅜-inch-diameter x 36-inch-long threaded rod. Then, thread a hanger bolt into the coupling nut. Tighten the hanger bolt with pliers. Repeat to make a second threaded-rod assembly.
6. Use a router with slot-cutting bit to cut a ⅜-inch-deep slot around the inside of the hollow newel post. Adjust the router to cut at least 1½ inches down from the top of the post.
7. Chisel out the waste wood from around the inside of the post. Remove ⅜ inch of wood from each side, creating a ⅜-in. x 1½-inch rabbet.
8. Use a jigsaw to cut a square washer from a piece of ½-inch-thick plywood. Cut the washer to fit snugly into the rabbet cut inside the post.
9. Drill two 7/16-inch-diameter holes through the plywood washer for the threaded rods to pass through.
10. Thread two hex nuts onto the upper end of each threaded rod.
11. Set the threaded rods down inside the newel post and tighten them into the pilot holes using a ratcheting socket wrench. Remove the hex nuts from the ends of the threaded rods.
12. Slip the plywood washer over the threaded rods and tap it down tight into the rabbet.
13. Place a steel washer and hex nut onto each threaded rod and hand tighten.
14. Use a reciprocating saw with a metal-cutting blade to cut the threaded rods approximately ½ inch above the hex nuts.
15. Tighten both hex nuts with the socket wrench.
16. Apply yellow carpenters glue around the top of the newel post, then set the cap in place. Wipe away any excess glue with a clean cloth.