[FWDLK] M...m...m...my teeth are chattering

[Dan Morton <morton@RAD.UPENN.EDU>]

OK, so The World's Slowest Auto Mechanic is slow about posting too.

A week ago Sunday I brought the '55 Savoy home from Mom's house (after
dropping in a rebuilt flathead), and we made it just fine.  Had to take
Highway 95 for part of the stretch, and got some curious vibrations around
55mph -- my guess is that I need to get the driveshaft balanced (I rebuilt
one of the universals).  No big deal, I just kept it under 55mph.

But there's another noise I encountered while cruising on the street (not
accelerating, nor decelerating) that I can best describe as teeth chatter.
One would think that in the absence of rocker panels (I mean, TOTALLY gone)
I could pinpoint its location, but I couldn't.  So here's what I know, let's
play gear doctor:

1) Before I pulled the block, if I took my foot off the gas in second gear,
the column shift would drop out of gear.  While inspecting the transmission,
I saw no sign of wear -- except for the bronze 2nd/3rd gearshift fork.  I
replaced it, and now it doesn't drop out anymore.  But...

2) If I take my foot off the gas in second gear now, the shift jerks up
(counter-clockwise) a bit.  It does it in first as well, but not as
perceptibly as in second.  I can only feel it, not see it, in third.

3) I replaced the oil seal in the rear differential with the driveshaft
down.  While inspecting it, I noticed that there was a fair amount of play
in the drive pinion -- maybe 20 degrees of arc or so.  Dunno, didn't think
much of it at the time.

OK, so here's MY theory:  My assessment of the transmission having little
wear is accurate.  The "chatter" I hear is because the differential is
alternately slowing down and catching up with the rotational speed of the
driveshaft under "no load" conditions.  The action of the column shift is
backlash from the differential as the engine slows; this accelerated wear of
the gearshift fork as it tries to throw it counter-clockwise.  The reason it
does not happen to a greater degree in 1st gear, where one would expect the
torque to be even higher, is somehow because the gear ensemble is in a
better position to absorb rather than reflect the torque in first gear (1st
is at the opposite end of traversal than 2nd gear).  So my real problem is
in the differential.

Plausible?  Proof by Conjecture!  I suppose I could get a buddy to drive the
car while I sit in the back seat, but that might be too easy.  Anyway, tell
me your theory.

        adTHANKSvance
        Dan, TWSAM
        '55 Savoy