Old Britts

Eliminating oil leaks due to pressure build up in the Commando gearbox
by Fred Eaton


The Commando gearbox will build up pressure due to heating, and this can cause oil to leak out the seals and through the main shaft bore. This problem was fixed on the MK3, but can be fixed on earlier gearboxes.


The best way to fix the pressure build-up is to install the MK3 breather vent (06-5199). This can easily be done with a #21 drill and a 10 x 32 tap. Drill a #21 hole, in the position shown in the next picture, .8" from the stud hole and 3/8" in from the edge. Tap this hole with a 10 x 32 tap and thread in the breather vent.

Breather on the inner gearbox case
A picture of the breather installed on the inner gearbox case.

Inner cover marked for drilling
The inner case marked and punched where the hole is to be drilled.

Inner case drilled
A picture of the breather hole drilled.
Note: the angle of the hole is towards the center of the bearing.

Case being tapped
A picture of case being tapped.

Another option used by some riders is to drill a small hole in the inspection cover. This hole should be very small like a #58 drill (.042"). Replace the inspection cover with the hole towards the top of the gearbox. This is an easy and cheap fix and safe, for if the cover starts to leak out this hole, a dab of silicone sealer will plug up the hole.

Hole drilled in the inspection cover
A picture of a hole drilled in the inspection cover.

Mk3 breather vent, part number 06-5199, $8.15.
Install breather vent in customers gearbox, part number 06-5199/CU, $48.15.
Prices current as of 02/12/18.

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