Having spent many evenings last winter with my XK's Moss box, I too am
surprised that all of the synchro grooves could be worn off. The
synchros work by engaging a cup over a cone- the contact surface is
slightly tapered. Usually, the synchros give up when the cone wears
down to the point that the cup lip contacts the gear side. At this
point, no more friction and no more synchro action. The wear on the
synchro grooves should stop as well.
To inspect the synchros, slide them together until they bind and
measure the clearance between the cup lip and the gear side. If there
is less the 1 mm or so, the synchros will be very weak. You can't
replace the synchros but you can revitalize them by machining the lip of
the cup back to allow sufficient interference between the cone and the
cup.
I can think of only two things that would remove the grooves
completely. If the above synchro fix has already been done once and
worn out again, the grooves could be almost gone. If the gearbox had
run out of oil, the needles under the mainshaft gears (or between the
input and output shafts) could wear and allow a misalignment that would
eat the grooves. I suppose that severely damaged shift forks could also
cause trouble.
A few caveats. Jag Moss gear boxes come in many flavors including
manufacturer (Moss or Jag) close ratio and different gear processing
groups (mine is a JLS- the S stands for shaved gears). Other than the
obvious close vs wide ratio, I don't know which flavors can be used
together.
Very early boxes also use a built-up laygear rather than a one-piece
unit. Again I am not sure about interchangeability
The shift fork shafts and the synchro hubs have include some ingenious
interlock assemblies that are not complex but do require care for
reassembly. On the later boxes, this included a pressed-in pin at the
very rear of the 1-2 slider.
When you start assembly, do not pull the 1-2 slider all of the way to
the rear. It can stick there and you will have to take the box apart
again. I don't think you have to ask why I know this. Once assembled,
the shift fork will limit rearward motion.
To install or remove the input shaft, you first have to drop the
laygear. It took me a gasket set to figure this one out. A 3/3 inch
copper pipe splicing tube is the perfect diameter for a dummy layshaft.
I have been told not to use grease as an assembly lube because oil can't
dissolve it and the needle bearings can gall. I used Vaseline.
And last but not least, once you get all of the parts, you can shim the
detent ball springs to trade slickness for synchro action. the more you
shim, the better the synchros work but shifting will require more
pressure (a "notchy" box).
If it would help, I also have some close-up shots of the gears. My box
is in very good shape and is from a 1959 or so XK150 with overdrive. It
has shaved gears and is of Jaguar manufacture.
Good luck,
Bill
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