frank24

Hi Yall,

Just finished putting the drivetrain back in my 81 4-spd. Had it all out for a gear change in the diff, new gaskets/seals/bushing in the tranny, re-seal and paint the engine, rebuild the q-jet, 6 new u-joints, new 360lb rear spring and new 550lb front coils.

All is well except a vibration in the rear that starts about 45mph and is nasty at 90. Definatley comes from the rear and shakes the shifter wicked. This vibration was there before I removed/installed the drivetrain.

I've read some stuff re u-joint angles being equal and the engine offset 1" to the right to accomadate the driveshaft angle. There is not a lot of room for adjustment or change from original in these cars. The differential is located by the one peice rear cover/carrier and front mount, front stock motor mounts and only adjustible piece is the tranny mount. Maybe 1/2" in any direction. Tried that, no difference.

I more suspect rear wheel bearings. There is some play and the are original w 110K miles. Before I went ahead w rebuilt wheel bearing assemblies from vbp, thought I would stir up a couple thoughts from some who have been there.

Thanks

427-390

You may want to see if the alignment is correct and have the tires re-balanced. I would think if you had bearing problems, you would get some sort of growling or grinding noise. Your description of the problem does not mention this. You also mention there is some "play". Is the play at the 12 - 6 o'clock or the 3 - 9 o'clock position? How much "play" is there?

frank24

Thanks 427/390,
I do get some low growling noise, only audible at real slow, 10-15 mph and seems rythmic w the rotation of wheels. Have new NASCAR Eagle tires w fresh balance job.

Play at 12/6 is when I pick up at the 6 and the 12 rocks back to the car about 1/4" at the top of tire.

Play at 9/3 is less, maybe 1/8" at tire edges. I did the front end alignment myself as we do the race car and is right on and steers great. Have not checked rear alignment, thought i would when i remove trailing arms to do bushings and wheel bearings if i go that way.

Any more thoughts?

69vettester

[QUOTE=frank24;1559743251]Hi Yall,

Just finished putting the drivetrain back in my 81 4-spd. Had it all out for a gear change in the diff, new gaskets/seals/bushing in the tranny, re-seal and paint the engine, rebuild the q-jet, 6 new u-joints, new 360lb rear spring and new 550lb front coils.
QUOTE]

If the Tranny Bushing was Badly Worn , Its possible for the slip yoke to be Out of Round and cause vibration at specific RPM ranges.
Keep testing the simple stuff first, Maybe it will end being that. Especially if your wheel bearings are still good.

427-390

Based on the fact that the bearings have over 100K miles, you have some 12/6 end play, and at low speeds you can actually hear some growling from the rear of the car, my money is on the rear wheel bearings. Others with more knowledge and experience can weigh in. I hope this helps.

big_G

My money is on the driveshaft being bent or out of balance.

PNK

Chased a vibration in a friends C2 for a long long time, defiantly felt like it was coming from behidnd the car, showed up to be the clutch even thou it was brand new.

blctalon

I hope you find the problem because I'm currently chasing vibration issues and I know it's a pain in the butt, especially when almost everything is new on the car.

Most logical thing sounds like the wheel bearings providing everything else is in good condition from its "new" form wherever you bought the stuff from. Let me simply say that when I first got my 78 2 years ago I chased a really bad growling/grinding noise. It only happened at low speeds taking off. Turned out to be the clutches in the rear end..you would not believe how bad these things grind when they go. It felt like the wheel was actually rubbing on the brake caliper and sounded like a pile of rubbish being drug across the ground. This didn't cause the vibration, but maybe your problem lies in more than 1 area. My main vibration at that time was tires with blisters on them.

When you filled the diff up after changing the gears did you happen to noticed what the clutches looked like or did you replace them? Did you also put posi lube additive in with the gear oil?

I would also check the driveshaft or take it somewhere and see if it and the slip yoke at the tranny are out of balance. I guess you could roll it on a smooth level concrete floor to see if it's bent?

Dr.Corvette

You should check the slip yoke and the trans tailshaft bushing. I've been working on nothing but Corvettes for 17 years and have seen alot of worn tailshaft bushings and yokes. This mostly a problem with the 71-74 Muncies. Haven't seen a lot of Super T10's go bad but then again I don't get that many of these in the shop. I would also closely inspect the driveshaft for any dents or better still have it balanced.
I had a 70 LT-1 that would only vibrate in 4th gear sometimes. Turned out to be a worn pilot bushing.

Dr.Corvette

435/427BB

my 65 has a viberation and have found it to be tailshaft bushing. i was losing some trans oil out the rear seal but then again mine was BAD!

0Corvette Engineering

Often after people have the rear end out they don't get it re-aligned perfectly causing highspeed vibration.

At least thats my therory.
I had a vibration after my rearend rebuild. but I fixed it by moving the transmission over about 1/2 inch.

frank24

Thanks Doc, Has new tailshaft bushing, slip yolk is not worn, check w mic, had driveshaft balanced. My next thought is to get the car up on 4 stands. Remove the rear axel shafts so i'm just turning the motor/tranny/diff. seperate the rear shafts and wheel bearings all together. Go through the gears and see if i feel the vibration. If not, I say its in the wheel bearings. If so, it must be in the driveshaft or driveshaft alignment w diff. Its not in the engine/clutch/tranny because on the highway, when I'm at speed, I can let the engine idle down and tranny in neutral, vibration is still there.

frank24

Thanks, which way did you move it? I did try that, moved it over to the passenger side maybe 3/8", whatever was in the slots and it did help. Can you tell me anything else re getting the u-joint angles the same? I have an aftermarket poly tranny mount. Someone said they tend to be higher than stock.

croaker

Hey Doc on the bushing did you pull the tailshaft or did you use a special tool I seen one on the snap on website,also the slip yoke if it isn't grooved on the outside how would you check the splines?

skydaddy

I had a similar vibration on my '79 after I had the rear end apart. I put it back together and had everything just right - so I thought. It vibrated like mad when it hit about 40-45 and kept going till 65-70mph - I would not push it any further. Kept chasing everything - and everything had either been gone through or was new from the tranny back (including wheel bearings). Vib was not there before I tore it apart. Had d-shaft & 1/2 shafts balanced with new joints. Ended up being the snubber bushing for the rear end (new as well - guess the old one had seated itself & new one had different dimensions). I could not get the angle right for the d-shaft. Finally after some messing around with the set up - we got it right. Thankfully - my paw-in-law is a mechanic and had a lift - we went up & down 10 to 12 times that day. Try something & drive it - switch it & drive it again till we got it right.

good luck chasing it - hope this helps you out -

0Corvette Engineering

I think I moved to towards the drivers side... But it's been almost 10 years. I had to slot the crossmember with a file. But it made a huge difference.

that was before I came up with the theory about the diff being not square.

I also was running puly mounts and removed them for rubber. That also helped a little, but it wasn't everything.

At least I can see out of my mirrors now

frank24

O.K.
So i put it up on 4 stands, disconnected the half shafts and tie wrapped them out of the way. Reved it up in fourth gear to 75mph. I got the same vibration without turning the axel shafts or wheel bearings. So then I moved the tailshaft on the tranny up/down/left/right till i found the sweet spot, which is pretty big. I only got worse vibration when it was extreme left/right.

So now i think i narrowed it to the driveshaft, even though balanced, ran out .040 at center and one end of shaft. I suspect a bad yolk, must not have the u-joint centered. I going to have a new shaft made.