Sorry about the long post, just trying to give enough info.  The car is a '74 auto, TH400, GM crate motor and stock transmission. 

I've installed the motor, transmission, driveshaft, and rear end.  All of the components that came out of the car except for the transmission went back in, although overhauled.  I used a TH400 from another C3 (74 or 75).  I didn't drive the car after I bought it, except to back it into the shop to begin the tear down.

With the car in the air or being pushed (I haven't fired the engine yet), the driveline "thumps" and slightly binds with the tranny in neutral and the rear wheels turned in the forward direction.  Backward, no noise.  With the driveshaft dropped from the transmission yoke, all components turn freely with no noise (transmission, diff, etc.).  So there is something in the driveshaft makeup that is causing it to bind or otherwise not turn freely.

There is almost NO slop in the yoke at the transmission.  To get the driveshaft in, I had to remove the cross member and lower the transmission.  When the transmission was jacked back into place and the cross member bolted up, the geometry shifts and effectively shortens the driveline.

Question: how hard has it been for you in the past to pull your driveshaft?  I'm beginning to wonder if the car has the correct length driveshaft.  I seem to remember (with other cars) having enough slop in the transmission yoke to unbolt the u-joint, slide the yoke forward, and the driveshaft dropping right out.  That is NOT the case here... I can't get the driveshaft out without dropping the cross member and lowering the transmission.

Another question: Am I missing something obvious?  Can the motor slide forward somewhat, or the rear suspension slide back?  I've had the entire rear suspension out, including the rear diff and cross member.  If I put the rear cross member (holding the rear end) on backwards, are the bolt patterns such that it could have shifted the diff forward?  I don't believe that I did so, just trying to think of what could cause this.

Scratching my head on this one.  Just about ready to drag the car to the street and put a "free" sign on it...

Thanks,
Darryl




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"Let them that don't want none have memories of not gettin' any."
- Brother Dave Gardner

Well, I think you found the issue but what caused your drive shaft to now be too short? Yes you should be able to slide the slip yoke in and out, particularly at suspension droop. At droop the slip yoke should be pulling out a bit to make up the increased distance between the axle and trans. You should see a wear mark on the slip yoke from the oil seal. This is where the yoke runs most of the time with the car driving at ride height. Also, you should see shiny smooth metal on the yoke rearward of this point from wear when the suspension runs through its range. If the yoke is jammed in the trans beyond these marks, ya, its too long. Maybe the wrong u-joints are installed? Never worked on the rear suspensin so since you pulled it that would be a place to look too. Then again, maybe there are more than one length of Turbo 400's. Different tail shafts? All stuff to examine. Keep posting....I'd like to know what you find.
Dave

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My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

Thanks for responding.

Dad and I went over it again today.  Can't see anything obvious, everything looks great.  Yoke shiny part is lined up perfectly on the oil seal.  I didn't think about the suspension droop issue, but that will make it worse when I take it off the jack stands. 

I thought about the tailpiece length possibility.  The old transmission and this one appear identical.  The "long" tailpiece is over 9 inches long, this one is just over 5 and same as what came out. 

Greased all of the ujoints and it seems much better.  Tomorrow I'm going to get the hoist and try to pull the motor forward in the mounts.  Grasping at straws...


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"Let them that don't want none have memories of not gettin' any."
- Brother Dave Gardner

Where did the driveshaft come from? I have never had any problems with taking the straps off the front yoke, sliding the yoke into the trans, and then removing the shaft. In fact, on 4 speed cars, the crossmember is welded in, so there's no way to lower it to have more room. The yoke should slide into the trans far enough to move the shaft to the side, and pull it out. With the shaft in place, how much space is there , or how far will the yoke slide into the trans? It should bottom out on the seal. If it is not moving forward enough to bottom, then you may have the wrong yoke. If it does not move forward at all, then there is definitely a problem with the shaft being too long. GM trucks also use this type of front yoke, but they have a bolt/washer that hold them solidly to the trans shaft. They look similar to the Corvette yokes, and will fit into the Corvette trans, but they will NOT interchange. The truck yokes are open at the rear, to allow them to "bolt on". The Corvette yokes are closed...no hole. It's possible you have a truck "bolt-on" yoke, since they are a bit longer, overall.

The rear crossmember(for the diff), afaik will work in either direction. It's possble the engine mounts are not correct, which could shift the drivetrain back a little, but I don't know if it would be enough to cause this issue. From the description, it sounds like the driveshaft itself is bottoming out between the trans and diff. The only reason I can see for this is:
Driveshaft too long
Trans yoke not correct
or...horrors...the frame is damaged...

Can you get pics of the trans yoke with/without the driveshaft connected?


Dave, the driveshaft in these cars does not move in/out with the suspension. The diff is mounted to the frame, so it does not move up/down with the suspension, so the driveshaft itself is stationary. The front yoke has straps that hold the u-joint, and is designed to slide into the trans with the straps removed, to give enough room to remove the shaft, leaving the yoke in the trans.

 







|UPDATED|1/12/2014 9:01:57 AM (AZT)|/UPDATED|


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Joel Adams
C3VR Lifetime Member #56    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Right right right! I keep forgetting I now own an independent rear suspension car. I'm still thinking parallel leaf.
Thought, if the trans lines up/bolts up to its cross member than there are no motor mount issues. Is it possible the rear axle is forward somehow?


(fixed quote box-aapple)

|UPDATED|1/12/2014 12:53:53 PM (AZT)|/UPDATED|


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My Christmas present to ME! Bought Sat 11-30-2013. 72, 350 4 spd, Silver with blue interior...Can't you just hear Pablo Cruz playing in the background?

The diff assembly bolts directly to the rear crossmember...and it has a front "snubber" mount that bolts to the frame. This means that the diff is pretty much solidly mounted to the frame of the car...not much way for it to move out of position. At least not enough to cause this particular issue.



Take a look HERE to see the bottom side of a nice '72 4 speed car.


|UPDATED|1/12/2014 1:12:21 PM (AZT)|/UPDATED|


______________

Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Right, I forgot about the IRS thing too.  Car in the air won't affect driveshaft geometry.  I'll try to get some photos of the yoke tomorrow, pretty sure it's a Corvette piece as it looks just like the one you showed and it matches the one I have from the parts car.

Measured the driveshaft, it's 29 1/2" from center of the yoke.  That lines up with what I think it should be.  Driveshaft length probably not the issue. 

Tonight, I removed both half shafts and the driveshaft turns VERY smoothly with no binding or bumping.  I removed the half shafts to get to the zerk fittings on the u-joints (two are inaccessible with the shafts installed); all four u-joints are new and move freely. 

Frankly, I'm stumped.  I'll grease the remaining two u-joints and put the darned thing back together.  At this point, I'm a bit worried that the problem is in the rear end, although with the half shafts out it rotates freely with no noise.  I didn't rebuild it or mess with the ring and pinion (all was good with good wear patterns), but I did change both yokes and all of the seals. 

Thanks for the help, all.

On a better note, Dad came by with the '69 427 Roadster today to remind me what the hard work is all about.  It was 72 and sunny in South Louisiana today, perfect Corvette weather (with apologies to those of you from the Northern tribes!).




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"Let them that don't want none have memories of not gettin' any."
- Brother Dave Gardner

Are the U-Joints any good ??? 

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1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI 

Darryl, I've based all of my ideas on you saying it would do this on the ground, also. With the car in the air, and the rear suspension hanging, it WILL bind up as you turn the wheels, whether by hand, or with the trans running. The angle of the halfshafts hanging will cause a bind in the u-joints, and a "thump-thump" as the wheels turn. This is NORMAL, and one reason you should never run the car in gear with the rear in the air. Some are worse than others, depending on the rear alignment, but it's hard on the u-joints on all '63-'82 cars.
With the halfshafts IN, if you turn the rear wheels, can you see the trailing arms/wheels move left/right with the thump?


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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Awesome!  Maybe I have no issue at all...  It was very hard to push while on the ground, but that may be that everything is still very tight from being rebuilt.  I'll put the halfshafts back in tomorrow and see if the trailing arms are moving laterally.  That would certainly indicate u-joint binding! 

I've never done an IRS car.  Lots to learn.

Thanks, Joel.  By the way: I looked at the undercarriage of your car on your earlier link.  Incredible! Looks brand new.  Well done.


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"Let them that don't want none have memories of not gettin' any."
- Brother Dave Gardner

That's not my car in those undercarriage pics...it belongs to another member here that posted those pics a while back, but I can't remember who it was.  The first pic is Ben's(cthulhu) '69, iirc. The others are of a '72.
Cars with positrack are a bit harder to push anyway...


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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"