The problem.
I get a bit of vibration on acceleration that I am trying to eliminate from my otherwise nice numbers matching '66 L-79 4speed Vert with a fresh engine rebuild. I've balanced wheels and tires, even the driveshaft, and replaced worn driveshaft shaft ujoints, which has lessened but not eliminated the vibration. When I pulled the driveshaft to balance it, I saw that the front slip yoke has a lot of wear around where I think it has contacted the brass(?) bearing in the tailshaft case of the transmission. The wear is visible to the eye and noticable to the touch.

The person balancing my drive shaft said he thought the wear was probably caused by someone driving the car with a worn out ujoint, or a driveshaft imbalance that was not attended to, that caused the tailshaft to whip, thus wearing out the yoke and the bearing it rides in. He said fixing the ujoints and/or balancing the shaft without also replacing the worn yoke and bearing would not eliminate the vibration. He was right, it didn't.

A new yoke has not been easy to find, and even if I did replace it, my local Vette pro told me this new youke would probably wear out quickly if the bad bearing in the tranny is not replaced at the same time, and it may not stop the vibration, even temporarily.


The solution(s)

Plan A - Pull the tranny to replace the bearing, find a new yoke and replace that too.

Plan B - Take my worn yoke to a machine shop and have them restore the missing steel on the yoke by welding onto it, then machine it to a larger O.D. to match it to the larger I.D. of the worn bearing.

Now for the brain teaser: How do I estimate the current I. D. of the worn bearing based on an empirical measurement of wear on the yoke? Is there a formula that can be applied to estimate what the wear is on the brass bearing, and therefore determine the required increase in yoke O.D. to get the tolerance between these two parts back to where it should be?

Thanks. My guess is Plan B would be a lot easier than A, if I can get the tolerance back to where it should be using this approach. My guess is it all depends on my ability to estimate the wear in brass bearing based on a measurement of the wear in the steel yoke.


[Modified by brucep, 11:30 PM 10/2/2003]

 

More to think about......
A bad pr. plate can cause the same thing.....
And have had other cars that needed spin ballancing
to finally get rid of certain shakes.
Not saying its not what you think it is, just mentioning things that
have driven me nuts in the past!

 

Yokes are not that hard to find. They are now being reproduced for a reasonable price. Pull the rear tail housing off and replace the bushing at the same time. No quess no mess.

 

:iagree: Plan "A" :thumbs:

 

Several of the Muncies that I've repaired have had two (or more!) bushings in the tailhousing. Only one was in use of course, the others had been pushed further up the tailshaft when a new bushing was installed.

Yes, you can install a tailhousing bushing with the transmission in the car. What you can't do is pull the old one out. A lot of mechanics just slide a new bushing on the tailshaft and use a special sleeve to hammer it home in the housing.

 

Jerry,

That sounds like a great idea-a new yoke and fresh bushing without pulling the tranny out.

-Any down sides to doing this?
-Anybody know where I can get a new yoke? I've heard they are not currently being remanufactured, and retail supply is low to no.


[Modified by brucep, 3:25 PM 10/3/2003]


[Modified by brucep, 3:26 PM 10/3/2003]

 

"Yokes are not that hard to find. They are now being reproduced for a reasonable price. "

Where ? How much ??

I rebuild quite a few Muncies and I can't find 'em either.
A few vendors have OEM yokes for $300 ++, but that is not "reasonable".

So- who's got 'em?

I agree... "Pull the rear tail housing off and replace the bushing at the same time. No quess no mess. "

You'll likely need a new tailshaft-to-mainhousing gasket... both if the second one tears also.

FYI-
To get the tailshaft housing off, you'll need to knock out (up) the shift lever pin to pull the internal fork far enough away from the gear so that the fork will slip off the gear. Then you rotate the housing slightly so the fork & housing both clear the gear & slide the whole mess off. Just make sure the idler gear shaft does not come out of its mooring (in the main case) when you do this... if it does, the internal shim will fall down and you won't be able to get the tranny back together... it will then have to be pulled to be re-assembled.

I use a cam bearing driver (remover/installer) to remove & install the tailshaft bushings when I have the tailshaft housing on the bench.

By the way... there is a special puller tool that is designed specifically for removing worn tailshaft bushings without knocking them inside the tranny. If you locate & purchase the tool, you don't have to mess around with disassembly/re-assembly.

 

I just talked to Long Island Corvette about yoke availability. They said their supplier didn't expect to be able to deliver any to them until January.

 

I just ordered a Spicer yoke here for $59.
http://www.drivelineshop.com/Default.htm

 

FYI I put a new yoke in, and left the tailshaft bushing as is. The vibration I saught to eliminate is gone. Keep this in mind when you are getting vibration through the steering wheel on acceleration. It could be your yoke.

 

I am assuming the vibration goes with the speed of the car , and not the engine.

Replace the yoke. If the vibration goes away, replace the bushing and seal.

BUT FIRST, unbolt the rear Ujoint, turn the driveshaft and Ujoint 180degrees and bolt it back to the pinion yoke (famous factory cause of vibration). While you have it unbolted, make sure the yoke has both Ujoint centering stops that hold the caps in place. I have seen them missing.

If the vibration is mitigated by disengaging the clutch. AND if you pull the trans, REPLACE THE PILOT bushing/bearing. Also, the X on the flywheel should match the X stamped in the pressure plate cover. Both may be missing - aftermarket plates and resurfaced wheels remove them.
:lurk:

 

Mgic,

Tell me more abott why rotating the shaft 180 degrees makes vibration go away? How come is that? Is there a way to tell you have a problem other than just rotating the shaft and trying it?




[Modified by brucep, 10:01 PM 10/13/2003]

 

Chevy dealers were getting a lot of vibration complaints. An engineer at StLouis went out to the chassis assembly line and noticed that the driveshafts were randomly installed with the paint marks matched. He asked the operator - who said he hadn't a clue as to the meaning of the paint marks - which were supposed to be aligned to forestall vibration. Instructions were given and the problem solved until shift change.

I assume your paint marks are long gone, and the guy who balanced your driveshaft probably repainted/rebalanced anyways - so just try turning the thing around in the pinion yoke. :lurk:

 

Interesting. But what was the purpose of the paint marks being matched? Were the drive shafts and pinion yokes balanced together then taken apart for vehicle assembly which required them to be put back togther as a set in a particular way to preserve the benefit of the balancing? Aren't the yokes balanced by themselves?

 

like the dots on the tires matched to the marks on the wheels - the factory knew where tolerances collected.

Here's another:

vibration during acceleration: Chevrolet Dealer Tech Service Bulletin 77-T-48 about customer complaints of vibration on acceleration which are not corrected by conventional methods may be caused by the rear drive wheel(s) camber angle. The typical Corvette torque shake is a vibration which occurs in the steering wheel and can also be felt in the floor pan and seat, peaking at 55 - 60 mph. This condition is also related to drive torque, that is, the vibration can be noticed during acceleration, but not during coast. Torque shake can be corrected by readjusting the rear wheel camber to 0° from the normal specification of -1/2°, (plus or minus 1/4°).

 

Interesting. Thanks. Is there a public resource for old tech bulletins?

 

http://www.ncrs.org is where I get mine - Dave, NCRS #24235