Firstly, let me say that the following comments are generic to
countershaft gearboxes and those who know about this specific one should
correct me, especially if the synchro rings are not brass nor the cogs
steel.
Berry's suggestions sounds good to me, but the sticking in first that
preceded the recent fault makes me suspect worn synchro rings as well
(if it has any on first - I don't know the box at all). Before anything
else (and I may be teaching grandma how to suck eggs here), I recommend:
Drain the box, and reserve the transmission oil.
Flush it through enthusiastically with clean transmission oil, as hot as
you can stand. Ideally, have someone turn a back wheel the other being
chocked, while you do so, so all surfaces get liberally sprayed.
Reserve that too.
Let the oil settle. What's at the bottom?
Goldy dust - worn synchro rings
Silvery dust - worn cogs
little springs - a slip ring has come off.
ironmongery - oh oh.
This gives you an idea of what other problems you might have to address
before you open up the box and push the dog back where it belongs.
Also - how sloppy was the change before the problem occurred? Could the
detents have worn to the point where you could have stirred the cogs
around and jammed one tight up against the ends of the splines as you
did your push-forward-before-yanking-back-into-second trick, to overcome
reluctant synchro rings? When it went into second, was there a
noticeable crunch? Did you ever try a first-to-third, and was this
smoother? How about a third-to-first?
All will be revealed when you drop the box. But it's nice if you have
relevant spares ordered/made before you do this, and you can do a lot of
diagnosis by looking at the oil before you drop. I have been told that
experts can even look at the fragments under a microscope and tell you
which cog has worn, by the crystallography - but I take this with a
pinch of salt.
richard@shears.org
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