Old Britts – Fixing a Stripped Drain Plug on 1972–1975 Commando Crank Cases
Download the complete PDF version using the green download button at the top right of this page for an Old Britts technical article covering repair of a stripped 3/8 x 16 drain plug hole on 1972 through 1975 Norton Commando crank cases. Written by Fred Eaton, this workshop guide explains how the damaged threads can be restored using a Heli-Coil repair, while keeping the new hole straight, centered and properly surfaced for the drain plug sealing washer.
The article shows how the crank cases are securely mounted so the original drain hole can be drilled accurately with a 25/64-inch drill. It then covers tapping the hole with the Heli-Coil kit tap, using a spring-loaded tap centering tool on a mill or carefully starting the tap in a drill press to keep the new thread perpendicular to the crank case surface.
Further sections explain surfacing the sealing area around the drain plug hole with a 3/4-inch four-flute end mill where needed, installing the Heli-Coil insert, breaking off the coil tab and checking that the drain plug threads smoothly into the repaired opening by hand. Photographs throughout the article show the milling setup, tapping process, faced sealing surface and the completed refurbished drain plug repair.
Contents Include
- Repairing stripped 3/8 x 16 drain plug threads
- Applies to 1972–1975 Norton Commando crank cases
- Securing crank cases for accurate drilling
- Using a 25/64-inch drill for thread repair preparation
- Heli-Coil tapping procedure and centering guidance
- Mill and drill press repair options
- Surfacing the drain plug sealing area
- Installing the Heli-Coil insert
- Breaking off the insert tab after installation
- Checking final drain plug fit by hand
- Photographic reference of the finished repair
