[T3] Transmisison fun

[Tim Shreve]

Soren,

What happened to your circlip(snap ring) that holds the "reverse gear" in
its proper place?
I'm thinking that the "reverse gear" you speak of is splined on the inside
and those splines connect the input shaft to the main shaft; and that is
what transfers/carries the load. I.E. the short stud doesn't carry the load.

??

Tim

-----Original Message-----
From: Soren Jacobsen [mailto:snj at blef.org] 
Sent: Friday, September 21, 2018 2:32 AM
To: type3 at vwtype3.org
Subject: [T3] Transmisison fun

Here's a new failure mode for me!  Thought you all might enjoy a bit of
schadenfreude and information about a strange design quirk of the manual
transmissions we're cruising around with.

While on a lengthy out of state trip, I hear a strange clicking/grinding
type noise from the rear of the car.  It only happens once, and I'm still
happily cruising along the highway.  No cause for immediate concern, just
something to note.

An hour or so later, I stop at a riverside park to use the bathroom.
It's the boonies, so I've left the car running, and I hop back in quickly,
throw it in reverse, release the clutch, but...nothing happens.
This isn't an "oh, you probably have a mis-adjusted shifter and hit 2nd
instead of reverse" type thing -- this has been my main car for 10+ years
and I am very much in tune with the feel of the shifter.  It's right where
it should be.  No, this is a "you engaged the reverse gear selector fork,
there just aren't any gears engaging" thing.  I'm on level ground,
thankfully, so I push the car a short distance and drive the remaining 7
hours home.  Every once in a while, I hear that same noise.  It troubles me,
but I'm tired and the car is rolling smoothly, so I keep going.

Back at home, I start tearing stuff apart to see what's up.  It doesn't take
long for something...interesting...to pop up, or should I say out.
When I pull the engine, the input shaft to the transmission comes with it.
Shoving it forward with some pliers, I manage to get the engine out and then
start freaking out.  Seems like a big deal, right?

Well, it turns out that the culprit here is something very simple: a stud
that sometimes, if you're unlucky like me, can work its way out.

The input shaft / main shaft is two pieces held together by a short stud.
On top of them goes a reverse gear, which, if things work loose, can slide
out of its proper location.  This was what I was observing when driving.
The reverse gear had slid out far enough that it couldn't engage, but
everything else was still together enough to keep the overall transmission
operational.

The problem, on a deeper level, is this: the stud holding these two shaft
pieces together is too short.  On top of that, the shafts have chamfered
edges leading up to the threads (if you've got an alignment tool made out of
an input shaft, take a look at it and you'll see what I mean).  This means
that you've got a very short section of threads actually engaging, and
enough room for the stud to rotate to one end or the other.  Why does this
only rarely happen?  I'm not smart enough to answer that.  What I do know is
that VW later fixed this issue not by lengthening the thread, but by
introducing a stud with a shoulder in the middle.  I'm not exactly sure when
this fancy stud was introduced, but my impression is that it happened during
the water cooled era.

Anyway, I guess the moral of the story is that cars are difficult to design,
and if your input shaft falls out it's probably not as big of a deal as you
think.

Soren