Boom #?, twisted T56  mainshaft, and a mysterious vibe

Intro  Boom #1  Boom #2  Boom #3  Boom #4  Boom #?  Boom #6  Boom #7  Boom #8  Boom #9  Boom #10

Somewhere along the line, the output splines (the splines the driveshaft slides onto) of my T56 mainshaft got twisted.  Maybe when the 1LE driveshaft let go (Boom #2)?  One result of the twist is that the new Mark Williams driveshaft, which was supposed to solve all my problems, ended up too short.  To measure for a custom driveshaft, it's common to push the slip yoke on as far as it will go, pull it back out 1/2 to 3/4 inch, then measure the center-to-center distance between the u-joints with the rearend at ride height.  The T56 mainshaft has a bumper on the output splines that the slip yoke should bottom out on.  But if the splines are twisted, the slip yoke "bottoms out" on the twist, not the bumper.  Result: my length measurement was wrong.

The pic below shows how much difference there is in pinion yoke length.  Keep that in mind when you start changing the driveline!  On the left is a stock-length 1350-series yoke.  The Mark Williams billet yoke is on the right.

The too-short driveshaft was part of a vibration that's been driving me crazy for months.  Steps taken to solve the vibe included:

• balance all four wheels/tires

• try different set of four wheels/tires

• try positive, negative, and zero degree pinion angles (I tried setting the driveline up so that the pinion and tranny shafts were exactly parallel.)

• rebalance flywheel and clutch

• try stock steel driveshaft

• swap in stock tranny mount (I usually run a poly mount)

• swap in stock torque arm

• replace rear mainshaft bearing and slip yoke bushing in T56

• buy Mark Williams chrome moly driveshaft and rear yoke

• have MW driveshaft rebalanced at a local shop

• rebuild rear end, replacing pinion bearings, and checking axles and housing for straightness

• 4-wheel thrust alignment (my LCA's are adjustable length)

• balance brake rotors

• swap in Jody Shapiro's T56 for a day (Jody's T56 reveals that mine has a bent mainshaft)

• re-measure for driveshaft after my T56 is rebuilt, have Mark Williams stretch the 'shaft one inch

• using a dial indicator, I measured 0.016" of runout at the rear of the driveshaft

• also, the pinion yoke appears to be placing the u-joint off center by 0.007", which could explain the runout, so...

• sent pinion yoke back to Mark Williams for analysis

01/16/01 update: got a call today from Mark Williams himself (hard to beat that kind of customer service! :-) saying that the yoke checked out OK.  He said the runout I measured can be caused by the fact that the pinion shaft splines don't precisely locate the pinion yoke.  He suggested I reinstall the yoke and repeat the measurement.  If the runout is unacceptable, re-clock the yoke on the spines and try again.  Repeat the re-clock/re-measure steps until I find the minimum runout.  I'll post the results here.

It was Jody's T56 that revealed that the MW driveshaft, which slid an inch further onto Jody's T56 output splines than it did onto mine, was too short (notice the careful avoidance of double-entendre terminology!).  So for the second year in a row, the T56 came completely apart, this time for a new mainshaft (last year it was for a new input shaft - see T56rebuild).

The left pic is the full, naked mainshaft, the right pic shows the twist.

4/24/01 update:  The car was still shaking after I reinstalled my rebuilt T56, so I tried some more stuff: 

• swapped in my stock LCAs and panhard to see if softer bushings would help.  They don't.

• swapped in Jody's Strange 12-bolt, driveshaft, and T56.  This combo was truly smooth, so I could definitely rule out tires, rotors, alignment, pinion angle, and engine/clutch/flywheel imbalance.

• put my T56 back in - still smooth

• put my 12-bolt and driveshaft back in - vibe returns

Here is where I decided to get serious about pinion yoke runout, since it's now completely clear that the 12-bolt is causing the problem.  The 2 pix below show my first cut at measuring pinion yoke runout.  The dial indicator plunger has to be carefully retracted and the driveshaft rotated 180 degrees between the 2 measurements.  With the spicer yoke shown (the Mark Williams billet yoke that I returned for analysis still hasn't been reinstalled), the yoke runout is 0.010".

Here's where a bit of frustration motivated a questionable tactic.  I decided to use a bottle jack to push the u-joint to zero runout.  Amazingly, it worked for a short time.  I pulled the dial indicator off and put the bottle jack ram right on the u-joint end cap (which is what the indicator tip is touching in the pix above).  After a gentle shove, the runout measured 0.001".  And the vibe was gone!  But not for long.  The runout was back to 0.010" after too few smooth miles.

Vette shop owner Dave Herlinger suggested I measure the yoke itself rather than the u-joint end caps.  So I did using the setup in the pix below.  The left pic is supposed to be a better angle on the measurement setup.  The other 2 show that the 0.010" runout is definitely all in the yoke.  The driveshaft and u-joint are now pronounced blameless.

Dave also suggested I measure runout of the pinion splines by replacing the yoke with a tube that fits tightly over the splines.  Being the lazy sort, I figured just measuring the cylindrical, machined part of the yoke should do the trick.  Using the setup shown below, I measured the runout to be between 0.001" and 0.002".  I think this confirms Mark William's theory of the the pinion yoke being crooked on the pinion splines.  Once again, I'm promising to try his re-clock re-measure process.

02/09/03 update.  Last year I had Craig (Top of the) Hill set up the 12-bolt after having Tom's Differential's treat my Eaton posi to their "Best Of Everything" treatment.  The vibe disappeared.  I happy, needless to say.  Now the plan is to sell the 12-bolt before I break it and put in a 9".