66 Differential Diagnostic Help Wanted

[John R.]

Gentlemen,

I've been hearing a loud metallic click from the rear of my 66 327 / 300. While driving from Long Island to Carlisle this past week I heard it while slowly backing up in a gas station after a few hours on the highway. I heard it during my next stop as well. When I arrived at my hotel I turned into the lot and the sound actually scared me. It was very loud and I'm not sure, but I think I felt the rear end of the car step out to the left as I made the right hand turn at low speed. I've included a video link showing the spot where the half shaft connects to the differential and to my untrained eye there seems to be a lot of play. I have the tire in my hand a 12 and 6 and I am rocking it. Could this be the cause of my "click"?

https://www.youtube.com/watch?v=oesqUzCejxM

Holding the tire at 9 & 3 and rocking it doesn't seem to indicate a wheel bearing issue, but again, I'm not expert.

Thanks,

John

[Timothy B.]

Re: 66 Differential Diagnostic Help Wanted

John,

Did you check the u joints.

[Joe L.]

Re: 66 Differential Diagnostic Help Wanted

Gentlemen,

I've been hearing a loud metallic click from the rear of my 66 327 / 300. While driving from Long Island to Carlisle this past week I heard it while slowly backing up in a gas station after a few hours on the highway. I heard it during my next stop as well. When I arrived at my hotel I turned into the lot and the sound actually scared me. It was very loud and I'm not sure, but I think I felt the rear end of the car step out to the left as I made the right hand turn at low speed. I've included a video link showing the spot where the half shaft connects to the differential and to my untrained eye there seems to be a lot of play. I have the tire in my hand a 12 and 6 and I am rocking it. Could this be the cause of my "click"?

https://www.youtube.com/watch?v=oesqUzCejxM

Holding the tire at 9 & 3 and rocking it doesn't seem to indicate a wheel bearing issue, but again, I'm not expert.

Thanks,

John

John-------

You have way too much end play on the yoke axles. Most likely you have excessive wear on the ends of the axles which will require their replacement. It could also be wear on the posi clutch pack. Either way, you need to remove the differential and have it rebuilt by a competent rebuilder. Gary Ramadei who posts to this board is an excellent source for this work.

When you remove the differential carrier you should also remove the half shafts and inspect the u-joints. Most likely, they'll be shot, too. However, whether they're bad, or not, I'd replace them with new, high quality u-joints (e.g. top-of-the-line Spicer).

[Patrick B.]

Re: 66 Differential Diagnostic Help Wanted

Scary clunking sounds in a low speed sharp turning maneuver after driving at speed is also characteristic of the posi plates sticking and releasing. If this is the case it can be remedied by changing the fluid and adding two bottles of GM positraction additive. You should try this simple fix before doing the hard things.

[John R.]

Re: 66 Differential Diagnostic Help Wanted

Thank you men.


Patrick, I will give your suggestion a shot. For quite a while there has been a clunk (different than the scary sound) in the rear when I back out of my driveway. I wonder if your suggestion will help that as well?


Timothy, my father and I replaced the rear spring about 12 years ago. While doing that job we replaced bushings, strut arms and serviced the u-joints. He was the expert and an excellent mechanic so I trust they are okay as the car hasn't been driven much in that time. That said, I did turn the wheels from under the car and the joints seem to move quietly and without play. Visually they look fine too. If there is some test I should perform please let me know, I'm here to learn.


Joe, yes I thought that might be too much, but I wanted to verify this with people who know better. I was thinking I'd have to remove the differential and rebuild it which I really don't mind except for the fact that fall is my favorite driving season and I'd rather be driving than rebuilding. Is driving the car like this for a few hundred more miles likely to make things worse?

[Gary R.]

Re: 66 Differential Diagnostic Help Wanted

Hi guys
John, sorry you missed my seminar on Friday I had gone over differentials in detail with a cut away on display. But no problem we can go over it here.

There is a lot of axle wear there and that is not typical of an original 63-72 diff because they were hardened, the 72? to 79's were not. Now there were plenty of other issues with these diff's over the run from 63-79, the 80-82 I won't even mention.

So my first question, is this the original diff? If you are not 100% sure look at the casting date located at the lower LH side of the diff and the stamping on the bottom edge of the diff. The 63-68 posi cases were not the best, the 63-4's were Dana's and weak. The 65-68 were Eaton and the casting design lead to cracks forming and breaking the posi case and everything else inside. It is not uncommon to think the diff is original and later find out it is from another car. A young man brought me his 79 diff to rebuild once and told me it wasn't acting right, that was because it was a 67 open diff that was put in his car after someone blew up the original.

So you really want to know what you have there because if it is a 73-79 then that would explain the axle wear. I can see the axles are std meaning they use U-bolts to retain the u-joints. The end of the bolt sticks out from the back of the yoke. What will happen with worn axle faces is that will continue to wear and the axle will move inward facing off the 1/8 lip by the seal and the back of the u-bolt will hit the housing pretty much like a fly cutter on a Bridgeport Miller cuts into metal. Get the car back in the air and look close to see if one or the other axle is hitting the diff. The LH side usually wears more. If you find that STOP driving the car or you will risk wrecking the housing or having it actually lock up.

Now if there is still plenty of room you can try sucking out the old gear oil and installing fresh oil and 2 bottles of GM additive. For years I never needed to use more then one bottle but I think the chemistry has changed in the past few years so 2 bottles work now. I use Lucas 85-140 gear oil but you can also use GM 90 wt gear oil. There is always a debate on the brand and type of oil to use. I never use synthetic in the diff but I know plenty do. There is even a boastful guy on CF that will tell you Lucas turns to water. I have not seen that happen so far. The popping and hammering in reverse and at corners is typical of the additive not getting into the clutches or the simple fact the oil needs to be changed. When I build diffs I add in a magnetic drain plug to make oil changes a lot easier. I recommend changing the oil between 8-10k miles, that is what I do with mine. Do the oil and additive, then drive in figure 8's in an empty parking lot, nice and slow at full lock to lock. I am more concerned about the axle and diff year you have.

Axle free play is caused by 1 to 3 things.
1- worn axle faces
2-poor posi set up- even in stock form
3-worn cross shaft holes in the posi case. This is not too common but ask Pilot Dan about his 72 diff.


Here is a link to an older thread on DC but shows what I am referring to.

http://www.digitalcorvettes.com/forums/showthread.php?p=826788#post826788

[John R.]

Re: 66 Differential Diagnostic Help Wanted

Patrick, I did as you suggested, and I believe it may have helped. More testing needed, but thank you.


Gary,

Thanks for the detailed reply.

I tried to read the numbers but it was difficult, however the NON -X digits I'm sure of: XXXX47X-N. I know I should have 3876476-N so the digits I'm sure of match but I can't do much better until I have it out. I do believe this to be the original differential. One of the first things my father did when he got the car back in '88 or '89 was bring it to a local Vette differential guy. I remember going the the man's house, his garage was in the back yard and there were hundreds of diffs on shelves all around the perimeter. We left it for a few days and he said there was nothing to do, just put it back in the car. I don't recall there being an issue with the numbers and I doubt my father would have brought it to a specialist if the casting number or date code was wrong. He was a numbers guy.

By the way, do the 63/64 differentials look the same as a 66 from the outside, or are there very obvious differences?

I changed the gear oil and added 1 & 1/2 bottles of the GM additive as suggested by Patrick. I did the figure eights then drove normal for about an hour and a half. 20 minutes highway, then local, then back on the highway and local again. No scary sounds, but more testing is needed. Not sure, but it seemed quieter when going into reverse from a stop, or starting forward from a stop. Based on what Patrick wrote, I think this may have been the sound I was hearing.

Axle end play: Gary, I have read a lot of your threads and articles over the last 24 hours, thank you for sharing so much information. Here is a picture of the clearance between the u-bolts and the housing at what I believe will be the first interference point. Thanks for the heads up on that.



I want to make sure I understand the issue and terminology. Is what we are calling the axle in this thread the same as what the 1966 Chassis Overhaul Manual calls the "side gear yoke?"

What limits the in / out movement of that side gear yoke? is it the snap ring on one side and the face of the shaft meeting the pinion shaft on the other side?

Looking at my car from the rear, I think I'm seeing too much negative camber. Is this related to the end play issue we are exploring here?

Thanks again for the help guys.

John

[Gary R.]

Re: 66 Differential Diagnostic Help Wanted

Hi John,
Glad to help, if I can.

It "sounds" like the posi chatter issue from your first description. That is common with an Eaton posi and has been since 1965. The additive part number changed a few years ago and I believe, strictly my opinion, the chemistry did too. I never needed to use more then 1 bottle of additive in any of my diff builds until this new part number. So good that you used the 1.5 bottles. That might resolve the chatter but you do have a lot of play it seems in the axle I would like to see just how much. If you can push in the axle, install a dial indicator on a mag base, zero it and pull it out to measure how much play there is that will help. You might not need to remove the diff. I can see the lip at the seal so it's still clear of impact.

To answer your questions
I think you are looking at the casting number on the diff and not the date code. There would just be a letter and 2-3 digits= A128 would be Jan 12 1968 or 1978. Again this is on the lower LH side of the housing under the LH axle.

Yes side yoke = axle.

As far as your Dad's specialist from years ago, everyone has different opinions on how to build these diffs, some strictly stock others like me will modify them to make them stronger. The 65-68 posi are notorious for cracking so maybe that was checked and found to be ok and left. I really can't say.

Yes 63-64 carriers ( housing) can be ID from external appearance. Look at the forward bracket bolts. If it's 63-e 65 those will be tapped with hex bolts in each side. If there are through bolts then it would be a mid 65-79 housing- unless the hole was drilled through for using the long bolt found in the 2nd hole back. I have done that a couple of times with 64's.

The axle travel is limited inward by the posi case cross shaft. The cross shaft should be about a 60 Rc for hardness. The axle face should be 55-57 Rc. There are snap rings as you mention retaining the axle from outward movement. Many on CF refer to them as C clips such as used in other GM axle but I'm glad you see you know the difference. The amount of play between the axle face in the outward position and the cross shaft is the endplay of the axle and I never found a spec on this for the 63-79 axles. The play results from the 3 things I outlined earlier.

Negative caster can result from endplay but also from basic alignment, spring condition, bushing wear.

I am concerned about the very loud noise you heard. That sounds like an impact noise but maybe it was the posi chatter and caught you by surprise. I usually recommend the oil and additive change out since it resolves most of those noises. The fact you think it's better leads me to think in that direction.

However I did a 67 diff a couple of months ago that has 700hp at the wheels. The driver missed a high RPM shift and heard a similar noise. He did the oil change to no avail and pulled the diff for a look. He found just about all the ring gear bolts sheared, I got that in pieces but was able to salvage and modify it for some more abuse. I am not saying this is your problem but the only real way to know is open it up if you hear anymore noise.

I am glad you were able to use the information I list on these sites, that is why I spend the time writing them.

[Joe L.]

Re: 66 Differential Diagnostic Help Wanted

Hi John,
Glad to help, if I can.

It "sounds" like the posi chatter issue from your first description. That is common with an Eaton posi and has been since 1965. The additive part number changed a few years ago and I believe, strictly my opinion, the chemistry did too. I never needed to use more then 1 bottle of additive in any of my diff builds until this new part number. So good that you used the 1.5 bottles. That might resolve the chatter but you do have a lot of play it seems in the axle I would like to see just how much. If you can push in the axle, install a dial indicator on a mag base, zero it and pull it out to measure how much play there is that will help. You might not need to remove the diff. I can see the lip at the seal so it's still clear of impact.

To answer your questions
I think you are looking at the casting number on the diff and not the date code. There would just be a letter and 2-3 digits= A128 would be Jan 12 1968 or 1978. Again this is on the lower LH side of the housing under the LH axle.

Yes side yoke = axle.

As far as your Dad's specialist from years ago, everyone has different opinions on how to build these diffs, some strictly stock others like me will modify them to make them stronger. The 65-68 posi are notorious for cracking so maybe that was checked and found to be ok and left. I really can't say.

Yes 63-64 carriers ( housing) can be ID from external appearance. Look at the forward bracket bolts. If it's 63-e 65 those will be tapped with hex bolts in each side. If there are through bolts then it would be a mid 65-79 housing- unless the hole was drilled through for using the long bolt found in the 2nd hole back. I have done that a couple of times with 64's.

The axle travel is limited inward by the posi case cross shaft. The cross shaft should be about a 60 Rc for hardness. The axle face should be 55-57 Rc. There are snap rings as you mention retaining the axle from outward movement. Many on CF refer to them as C clips such as used in other GM axle but I'm glad you see you know the difference. The amount of play between the axle face in the outward position and the cross shaft is the endplay of the axle and I never found a spec on this for the 63-79 axles. The play results from the 3 things I outlined earlier.

Negative caster can result from endplay but also from basic alignment, spring condition, bushing wear.

I am concerned about the very loud noise you heard. That sounds like an impact noise but maybe it was the posi chatter and caught you by surprise. I usually recommend the oil and additive change out since it resolves most of those noises. The fact you think it's better leads me to think in that direction.

However I did a 67 diff a couple of months ago that has 700hp at the wheels. The driver missed a high RPM shift and heard a similar noise. He did the oil change to no avail and pulled the diff for a look. He found just about all the ring gear bolts sheared, I got that in pieces but was able to salvage and modify it for some more abuse. I am not saying this is your problem but the only real way to know is open it up if you hear anymore noise.

I am glad you were able to use the information I list on these sites, that is why I spend the time writing them.

Gary------


In this case, I don't think measuring the end play is necessary. Watching the youtube video linked in the first post is enough to convince me that there's way too much end play.

Also, while the earlier stub axles may be hardened better than later ones, the ones on my 1969 were seriously worn at about 130,000 miles. They had not worn down to the snap ring but once the case hardening is worn through, further wear will occur rapidly.

[Michael W.]

Re: 66 Differential Diagnostic Help Wanted

It's true that the end play is way beyond that of a new diff and that it should be addressed, but the OP wants to know if he can continue driving for the fall season.

Driving with a diff in this condition does not pose a safety hazard as the yokes are loaded inwards with approx. 300 lbs of force while the car is going straight ahead. The inward load reduces on the outside wheel when the car is going around a corner but will only go to a negative value under extreme conditions.

Here's an interesting video demonstrating this. Note that the spring clip was completely detached from the yoke.

https://youtu.be/FIFZMa1pyX0

[Gary R.]

Re: 66 Differential Diagnostic Help Wanted

Hi Joe, Mike, John

Joe -I agree completely on the endplay but based on usage thru the Fall they most likely will be ok. You are 100% correct on the metal wearing faster past the case hardening, plus all that dust ends up in the gear oil. I have also seen worn yokes on pre 73 axles such as yours but many are still good on those early cars. I certainly don't doubt you, just surprised you had that much wear at 130k. That's why it's good to check them from time to time on original cars. With the HD 30 spline axles I have to get them hardened & tempered by a heat treating facility because of the same issue, yet some will ignore that and use them as is for a street car. There is a well known diff guy on CF who stated I heat treat them with my torch. While a torch can be used I have the guy who does them every day heat treat them.

Mike- Interesting video, I was concerned with the closeness of the spring bolt to the tire sidewall that I saw. I don't know what he was speaking about with axles being spliced originally, I think he was mixed up between an original axle and rebuilt one.

John- I would monitor the car to see if the posi chatter is gone and if so decide just how much you want to drive the car. The concern is not from the axle coming out of the diff but for it to continue to wear and cut into the housing.

[Edward B.]

Re: 66 Differential Diagnostic Help Wanted

Being a straight axle guy with little first hand knowledge of the topic under discussion I must nevertheless commend Gary Ramadei for the depth and clarity of his responses and suggestions. It is not everyone who has the ability to take a technical issue and describe it in words that can be understood by all.

[Russ S.]

Re: 66 Differential Diagnostic Help Wanted

Watching your video the first problem is bad bearings in the trailing arm. There should only be .015 clearance in the bearings.

[Michael W.]

Re: 66 Differential Diagnostic Help Wanted

Watching your video the first problem is bad bearings in the trailing arm. There should only be .015 clearance in the bearings.

Bearing clearance is max. 008" not .015" but even if it was set at .001" it would have no effect on the yoke being pulled out of the diff caused by the side load on the tire. The trailing arm bushing will allow the entire arm to twist as required. If that's not enough, the arm itself will twist.

[Russ S.]

Re: 66 Differential Diagnostic Help Wanted

Watch the video again. The spindle moves in and out of the trailing arm a great deal. That is bad bearings and or clearance.

Bearing clearance is max. 008" not .015" but even if it was set at .001" it would have no effect on the yoke being pulled out of the diff caused by the side load on the tire. The trailing arm bushing will allow the entire arm to twist as required. If that's not enough, the arm itself will twist.