[Spridgets] Broken Halfshaft and Thunkin Punkin

[Bud Pazur]

The halfshaft broke on my street Bugeye! I thought that only happened to race
cars - I certainly don't drive Mr. White that sprightly!

It broke about an inch and a half from the end, about 1/4" before the end of
the splines. It's not a clean crack - its got a spun point on it. And a half
inch before the break the end is twisted somewhat - several degrees.
Basically, the halfshaft twisted itself in two, like the end was caught and
held stationary and the wheel twisted it off. But there was absolutely no
warning  - no stopped wheel, no sound of any kind. I had just made a turn at
about 20 mph. The turn was a left turn, and it is the driver's side halfshaft
that broke.

All of which makes me wonder - what is the root cause? I cleaned the diff guts
in gasoline. There is no obvious damage, other than some scoring inside the
cage bearing hub on the side of the broken halfshaft. But the bearings move (a
little stiffly, with no lube) and the ball bearings inside don't look damaged.
What could have caused the halfshaft to twist itself in two? Just fatigue?

A clue - the diff (a 9/38) always knocked (thunked) when I started off in
first or backed up. My Haynes says that this could be due to several things,
including pinion bearing pre-load being too low. I had torqued it to 140 ft-lb
a few years ago, but that didn't eliminate the knock. Haynes say it could also
be cause by excessive backlash. There is some backlash between the pinion gear
and the crown gear. The crownwheel is inscribed 007 - according to Haynes this
is supposed to be the backlash to pinion. This is difficult to measure, but it
seems that the backlash is only about .008. However, I have a welded 9/38 that
I used in my old red race bugeye - it has more backlash than Mr. White's diff
and it never made a knocking noise.

What is the collective wisdom of the list, aside from the fact that I need
another halfshaft? Does anyone rebuild diffs?

TIA

Bud