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XK120 Moss Gearbox Update 3

To: "British Cars" <british-cars@autox.team.net>,
Subject: XK120 Moss Gearbox Update 3
From: "Rob Reilly" <reilly@admail.fnal.gov>
Date: 25 Mar 1994 13:52:01 -0600
REGARDING                XK120 Moss Gearbox Update 3
I'm still studying my box of Mossy gears and I think I am faced with a
combination of normal wear and a basic design flaw. They are working together
to allow the second synchro sleeve to move forward when the first gear is too
far back.

First to be explored was the possibility of a worn ball/plunger.
The balls are all .3125" diameter. The plunger is .3122" dia. and .4895" long,
with one spherical end and one flat straight-cut end. Adding the ball and
plunger together = .802"
The mainshaft is 1.645" dia. at the dimples. The ID of the first gear at the
peaks of the internal teeth is 3.250". That leaves a space between of .8025",
so the problem is not a worn plunger.

Three alternative repair schemes have been suggested:
1. A stop pin such as was incorporated in later boxes
2. A washer/spacer on the mainshaft to limit first gear travel
3. A washer/spacer on the shifter fork actuator rod to limit fork travel

A dilemma: I don't want to reduce the amount of tooth engagement between the
first mainshaft gear and the first idler gear on the countershaft (layshaft).
Alternatives 2 & 3 would reduce tooth engagement. As it is now, from neutral
the first gear must move .300" to the rear before it begins to engage the
idler, but if it moves back more than .900" in relation to the second synchro
sleeve, the plunger ball pops partway out, allowing the sleeve to move forward,
allowing the ball to pop out some more, allowing the sleeve to move some more,
and we're stuck in first. Subtracting for the roundedness of the teeth, we have
only around 1/2" of tooth face contact as is. Alternative No. 1 begins to look
better.

I examined a partially disassembled box from a Jag Mark 1 at a shop near me
called JK Restorations, an E-type specialist. The stop pin is nothing more than
an ordinary rivet, short enough to allow the relieved part of the one special
internal tooth in the first gear to pass, but long enough to stop the
unrelieved part of this tooth.
This rivet has a 1/8" diameter solid shank. It is stuck through a drilled hole
in the outer "rim" of the second synchro sleeve, centered 3/16" from the rear
of the rim, in line with the plunger hole, and hammered down a bit to keep it
from falling out.

My plan is to try to drill a hole in my second synchro sleeve and put in a
rivet, but first I'm going to double check the location is right for it to stop
my first gear. Can't assume the Mark 1 is exactly-exactly the same. The idea is
to stop the first gear just short of where the ball plunger would roll off the
forward end of the internal tooth. But that forward end is now a bit worn off
from engaging second gear.

More study over the weekend.
Rob




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