Rebuilding the original 100,000 mile Moss gearbox with overdrive
Eliminate oil leak(s) from rear extension

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There is, in my opinion, a serious design flaw in the rear alloy extension used when an overdrive is fitted to the gearbox. The tapped hole in the extension into which a 5/16" BSF setscrew is inserted to hold the layshaft and reverse gear spindles locking plate is not a blind hole, and oil from the gearbox can leak past the setcrew to the outside world. There is no provision for a gasket or other sealing method on either side of the locking plate, and the Jaguar parts manual shows a spring washer under the head of the setscrew, not a copper washer. In any event, a copper washer under the head of the setscrew cannot prevent oil from leaking out of the joint between the rear of the alloy housing and the front face of the locking plate.

I do not know if the same design flaw exists in the rear extension of a non-overdrive Moss gearbox.

The rear extension on my overdrive gearbox was not properly assembled by the last person to work on the gearbox. It was clear that the mechanic did not take due care to seal the rear end of the gearbox. A lack of understanding was confirmed by the fact that the seven setscrews that hold the rear extension to the  gearbox case had copper washers under their heads - totally unnecessary since all seven setscrews screw into blind holes and should have had spring washers.

The two shafts are held in their longitudinal position by a hardened steel locking plate seating in a machined groove in each shaft. The locking plate is itself held in place by a setscrew that screws into a tapped hole in the rear extension that is not blind. As stated, this is a serious design fault.

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When I first dismantled the rear end of the gearbox, I noted that:

  • The gasket sealant used for the rear paper gasket was a coating on the front side of a red silicone make-a-gasket material, with zero adherence to the rear face of the case. The coating on the rear was a brittle shellac-style coating and in poor condition. Conclusion: The paper gasket had not been replaced when the gearbox was last dismantled.

  • The original felt sealing ring was present on the reverse gear spindle, but appeared to have shrunk and, in the absence of any modern sealant, was almost certainly leaking.

  • The felt seal on the layshaft spindle appeared to be in reasonable condition, but there was no modern sealant present.

I pondered how to best assure the leak from the three possible sources within the rear extension did not occur again.


Front face of rear extension showing non-blind hole for locking plate setscrew

rear locking plate

The 5/16" BSF setscrew screws
into a non-blind hole

The locking plate setscrew is, like the two shafts, always below the oil line.

The solution I devised was to convert the tapped hole in the rear extension to a blind hole using a short length of 5/16” BSF bolt. I cut the bolt so that I had a 6mm long piece of 5/16” BSF threaded bolt, and I filed the cut end smooth and flat. Next I made a slot in the filed end that would allow me to later use a screwdriver for assembly. I now had a 5/16” BSF plug, albeit with a parallel thread, not tapered.

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I thoroughly cleaned the setscrew’s tapped hole and finished off with lighter fluid as the best cleaner. Likewise, lighter fluid was used to clean the 6mm plug. I then coated the plug with Permatex black silicone adhesive sealant, smeared the same sealant into the start of the tapped hole on the forward side, and screwed the plug into the tapped hole towards the rear side, stopping when the plug was flush with the front face of the circular recess in the rear extension. I then centre-popped the plug and tapped hole junction in four places so the plug could not unscrew out towards the reverse gear wheel.

During this process, it was important to ensure that there was no excess sealant on the rear end of the plug. Too much sealant would mean it would be hard to properly tension the setscrew when it came time to lock the layshaft and reverse spindle shafts in place later.

A test confirmed that the locking plate setscrew did not bottom out against the new 5/16” BSF plug in the tapped hole

During the later assembly of the gearbox as a whole, when it came time to assemble the locking plate for the layshaft and the reverse gear spindle, I took the precaution to not rely solely on the 5/16” BSF plug I had inserted in the forward end of the tapped hole.

To seal the layshaft and the reverse gear spindle where those two shafts pass through the rear extension, I used modern synthetic rubber O-rings that take the place of the felt material used in the 1950s.

• Layshaft – The O-ring slides over the shaft, and is later deformed on assembly in a cavity on the forward face of the rear extension (see photo above).

• Reverse gear spindle – The O-ring sits in a rectangular groove machined in the spindle and then the O-ring sits in hole in the rear extension when the spindle is in place.

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The O-ring sizes are listed here.

The O-ring for the reverse gear spindle was slid onto and into the groove, and then coated with Permatex black silicone adhesive sealant. Then the reverse shaft was slid into the rear extension.

Then the layshaft spindle was also slid into the rear extension, and the locking plate (see below) was assembled on the rear extension.

To assemble the locking plate, I used a new 5/16” copper washer under the setscrew head, and I used plenty of the Permatex black silicone adhesive sealant on the washer, the setscrew thread, and coated both sides of the locking plate itself with the sealant. This approach meant that I now had a belt-and-braces sealing arrangement for that poorly-designed non-blind tapped hole for the locking plate setscrew.

Next, the layshaft O-ring was slid down to the rear of the layshaft, and coated front and back with Permatex black silicone adhesive sealant.
Then, before the sealant on the two O-rings was able to set, the rear extension and the rear gasket with sealant was mounted on the rear of the gearbox casing, the shafts were slid into the case using the soft strap to line up the cluster gear, the dummy hollow tube layshaft was pushed forward by the real layshaft, and the rear extension setscrews with spring washers tightened progressively home.

After finishing rebuilding the Moss gearbox and mating it to the overdrive unit, I filled the gearbox with the specified quantity of 5W-30 synthetic multigrade to ensure the layshaft and reverse gear spindle rear ends were fully oil immersed with this “thin” oil.

Time passes ….

After two months, the troublesome leak area of the past was completely oil-free. Of course the real test will be a long test drive, but that might not happen for years. Until then, I’m happy that the chronic gearbox oil leak location has been sealed off.

5-16 plug
plug inserted