Chris,
In my experience, the 3-synchro box is good for about 75k - 100k miles at
best. The layshaft and it's bearings tend to fail rather suddenly and
loudly, seizing the bearings to the shaft and destroying the shaft (cheap),
bearings (cheap), and laygear (OUCH-not cheap). The tendency for these boxes
to pop out of gear is not really an indication of the layshaft condition.
The indicator is usually a loud growling followed quickly by grinding. If
you have no history of the car and you're going to have the box out, I would
rebuild it and start from scratch. What kind of shape is the 2nd gear
synchro in? Does it grind any on shifting or downshifting? That also is an
indication of what kind of shape the box is in.
At any rate, the layshaft is easy to replace. Stand the box up on it's
output flange, clamp it in place so it will not fall over. Remove the input
shaft cover, and carefully grab the shaft (half round end) and slide it
straight out. DO NOT rotate either input or output of the box. If you do,
the layshaft and bearings will become displaced and you will almost never get
the layshaft back in . Inspect shaft. If slightly worn, replace. If really
worn or pitted, it's time for a rebuild.
Any questions?
Eric Swanger
Huntersville, NC
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