Friday, August 02, 2019

Wltoys 12428b (part 8: fixing the front drive)

So I didn't enter this RC thing with my eyes entirely closed.  I understood that things break, and i was willing and able to fix them when it happened.  Yet when this broke i was scared that I'd made a mistake.  As i sit here, i have started tearing down the front end and i breathed a sigh of calm and added Jazz (Thelonious Monk - Straight, No Chaser (thinking of you @hobbit!)):
  1. I do like fixing things
  2. Parts are available although they might take awhile to get here.
  3. I bought two identical cars in case one broke.. ;)
So now i get to see deep inside my car, albeit somewhat earlier than i thought.  I'm not really sure what's the fastest path to the front diff, so I'm just diving in, live and learn and all that.  Somewhere i got these really nice and small ziplock backs, so I'm just popping each "group" in a new bag as i go. forward a bit .. so anyhow, looks like the front diff and pinion are all intact (not that there's much in the way of grease..):

The front differential.  All good there (but needs lube)
  (BTW I'm still blowing out about how much fun this car was to drive.  And the more I sit here looking at it's insides, the calmer i feel.  It's bizarre i know.)

.. add time .. ok, so found it.  There is a very small bolt holding the "Then Cup" (don't ask me, that's what it says on the parts list! ;)) to the front pinion gear.  The cup and pinion interlock and the screw holds them together.  The screw shouldn't have been loaded rotationally and only under a static load longitudinally, so i can't see how it could shear, yet it has.  This pic shows the rear of the front pinion and if you look inside you can see the back of the head of the bolt:

Left: The "Then Cup" and right: rear side of 12t pinion gear.
.. fast forward .. i found the part numbers in the manual (I do like that this car has nice exploded parts diagrams).  The name of the broken item is the 0109 "Pan head screw" and it's sheared off inside the 0081 "Then cup".

Appropriate page in the manual..
So now, how to fix it? I guess i could go see my friend Randall and try to drill them both out and tap the cup with a larger bolt.  And/or order replacement parts.  On that front I was VERY pleased when i found multiple stockists in Australia, Albury Models and Super Cheaper Hobbies.

PS: It seems like I'm not the only one who's had issues with this particular bolt. forward a few days.. parts delayed, bored.. stuff it, I'm going to fix it myself.  I drilled out the screw head from the driving gear easily enough (with a drill press).  Next i tried to drill out the remainder of the bolt from the then cup, which went amazingly well.  I happened to have an M3 tap so i drilled it out to 2.5mm and tapped it from there, which also went amazingly well:

Tapping Then Cup 0081 out to M3.  Tape indicates 8mm depth.
So all that was left to do was to find a suitable M3 bolt, which i didn't really have, but found an M3 threaded screw with a nice solid Phillips head (I think it came from an old printer or something).  The driving gear hold needed a bit of relief for the screw, easy:

I used the longer M3 screw at the rear, cut to length.
And finally here it all is back inside the car.  White paper is to save the desk from the grease a bit:

Now putting the rest of the car back together, fingers crossed! :)

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