Old Britts – Eliminating Oil Leaks Due to Pressure Build Up in the Commando Gearbox
Download the complete PDF version using the green download button at the top right of this page for a useful Old Britts technical article explaining how pressure build up inside the Norton Commando gearbox can cause oil leaks. Written by Fred Eaton, this guide describes why heat-related internal pressure may force oil past gearbox seals and through the mainshaft bore, and it outlines practical venting solutions that can be applied to earlier Commando gearboxes.
The preferred repair covered in the article is fitting the MK3 gearbox breather vent, part number 06-5199, to the inner gearbox case. The guide gives the drilling location, the required #21 drill size, the 10 x 32 tap specification and the correct drilling angle toward the center of the bearing. Workshop photographs show the case marked, punched, drilled, tapped and fitted with the breather vent.
An alternative low-cost option is also discussed for riders who choose not to install the MK3 vent. This involves drilling a very small #58 hole in the inspection cover and positioning the hole toward the top of the gearbox. The article notes that this can relieve pressure safely and can be sealed again easily if required.
Contents Include
- Commando gearbox pressure build up explanation
- How internal pressure can contribute to oil leaks
- MK3 breather vent retrofit for earlier gearboxes
- Breather vent part number 06-5199
- Correct hole position on the inner gearbox case
- #21 drill and 10 x 32 tap requirements
- Recommended drilling angle toward the bearing center
- Photographic marking, drilling and tapping reference
- Inspection cover vent-hole alternative
- Simple pressure relief option using a #58 drilled hole
