.005" backlash too tight? - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 65 (permalink) Old Jul 5th, 20, 2:53 PM Thread Starter
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.005" backlash too tight?

I have a '67 12 bolt that I bought used in my '66. The rear end sat for a l-o-n-g time before I bought it, like maybe 30 years. But luckily - Still with its gear oil and axles in it and closed up tight. Has a 3:55 posi that coincides with differential ID callout. I think it has never been serviced, but of course I don't know for positive.

I have what I would describe as a bearing noise coming from it. A "whirring" noise that only comes on at like 55mph and continues to stay with increased speed - maybe even worse noise as I go faster, but its hard to tell with the increased road noises too. This whirr is enough to make the rear view mirror image vibrate along with the noise.
This noise has stayed with the car throughout many changes to the car:

rebuilt entire front end & disc brake conversion
replaced M20 with a M21
replaced clutch & flywheel
new u-joints & slip yoke and had driveshaft balanced
I installed aftermarket axles (and new outer axle bearings), but removed the axles and reinstalled factory axles to see if that mattered

None of the changes eliminated the noise. Last week when I reinstalled the factory axles I took a backlash measurement and it was at .0045 / .005. Is this too tight of a backlash and maybe the reason for the whirr? (running 80-90W with GM additive) And if so - I don't know what could cause a larger factory backlash setting (assuming the factory didn't set it at this tight number) to come down to such a tight measurement? I do not currently own any yellow marker paint to check pattern, but if consensus says I need to go buy some I will. In hindsight - I should have bought some before I swapped out back to the factory axles!


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post #2 of 65 (permalink) Old Jul 5th, 20, 5:10 PM
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Freddie
 
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Re: .005" backlash too tight?

That is too tight for the old style gears. They should have .007 to .009, or as much as .011 if they have wear on them.
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post #3 of 65 (permalink) Old Jul 5th, 20, 5:26 PM Thread Starter
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Re: .005" backlash too tight?

Thank you Freddie!

Do you think that could be the cause for my "whirr" noise?

To me my whirr noise seems to get more pronounced as the cars fluids heat up. (10-15 miles)


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post #4 of 65 (permalink) Old Jul 5th, 20, 9:25 PM
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Re: .005" backlash too tight?

It could be. I've opened up the backlash on noisy gears before and got less noise.


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post #5 of 65 (permalink) Old Jul 5th, 20, 10:47 PM Thread Starter
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Re: .005" backlash too tight?

Quote:
Originally Posted by big gear head View Post
It could be. I've opened up the backlash on noisy gears before and got less noise.
Ok, thanx Freddie. If this could shed any light on my issue: I did take pictures of the ring gear when I had the cover off. (even though I had no yellow marking ink)



Since the backlash is at .005 - would it be safe to assume that this differential probably has been "tinkered with" at some point?

Because I wouldn't think it left the factory at .005, and I do not know what could cause the backlash to get smaller. Like you say; should get a bigger number because of wear, not smaller.
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post #6 of 65 (permalink) Old Jul 5th, 20, 11:37 PM
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Re: .005" backlash too tight?

That may fix the noise...but if you've got a moderate to high frequency vibration that makes the rear view interior mirror "buzz"/blur the image that's a different problem, the gears won't cause that.

I'd check driveshaft run-out as installed in the car....I've checked a lot of rear pinion yokes and factory yokes are rarely very good in that respect.....worst I've ever seen is more than .050" runout at the rear weld of the shaft caused by the pinion yokes.

I checked 6 stock used yokes just to find one good enough(under .010" run-out) to use with 4.11 gears in my Nova 25 years ago. When I put a Denny's Driveshaft in and the vibe still didn't go away i had to look deeper, and that was it, pinion yoke run-out.

Check the front end of the shaft too....I've seen where it doesn't matter how good it was balanced if there is too much runout it will vibrate...most local shaft places can't do a high speed balance, just a low speed.
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post #7 of 65 (permalink) Old Jul 6th, 20, 6:55 AM
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Re: .005" backlash too tight?

If the dial indicator is not set up correctly it will give you a lower number than what the backlash actually is.

Does it still have thee original solid spacers on the sides of the differential?
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post #8 of 65 (permalink) Old Jul 6th, 20, 7:23 AM
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Re: .005" backlash too tight?

David, I'm just going thru this. 5 is really tight. I reset mine up at 8 and i think its tight for used gears. I'd like 10. After work today, I'll use some Permatex Prussian Blue ( I couldn't find the nice yellow stuff over the weekend - maybe today) and check the pattern to see what it says before I pull it again, measure all the shims with a mike this time and then calculate how I can restack them to get + .002"-.004".

Was wondering just the other day what happens with then diff get warm? Does .005 become .003?

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post #9 of 65 (permalink) Old Jul 6th, 20, 8:57 AM Thread Starter
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Re: .005" backlash too tight?

Quote:
Originally Posted by Ericnova72 View Post
That may fix the noise...but if you've got a moderate to high frequency vibration that makes the rear view interior mirror "buzz"/blur the image that's a different problem, the gears won't cause that.

I'd check driveshaft run-out as installed in the car....I've checked a lot of rear pinion yokes and factory yokes are rarely very good in that respect.....worst I've ever seen is more than .050" runout at the rear weld of the shaft caused by the pinion yokes.

I checked 6 stock used yokes just to find one good enough(under .010" run-out) to use with 4.11 gears in my Nova 25 years ago. When I put a Denny's Driveshaft in and the vibe still didn't go away i had to look deeper, and that was it, pinion yoke run-out.

Check the front end of the shaft too....I've seen where it doesn't matter how good it was balanced if there is too much runout it will vibrate...most local shaft places can't do a high speed balance, just a low speed.
Great suggestions, Eric. I have a lift so the driveshaft runout will be easy to check. The front yoke is a new Spicer, but still worth looking at. I wanted Denny's to balance the driveshaft but they said they were booked up 5 weeks on new shafts, so no time (& resources) for outside repair work. My local FleetPride did the balance and I don't know if it was high speed or low speed balanced.

Quote:
Originally Posted by big gear head View Post
If the dial indicator is not set up correctly it will give you a lower number than what the backlash actually is.

Does it still have thee original solid spacers on the sides of the differential?
Good thought about the indicator, Freddie. I did work in a machine shop / tool room / model shop so I am familiar with proper indicator setup.
I am unaware of the 3 solid spacer scenario. Can you please inform me?

Quote:
Originally Posted by 427L88 View Post
David, I'm just going thru this. 5 is really tight. I reset mine up at 8 and i think its tight for used gears. I'd like 10.

Was wondering just the other day what happens with then diff get warm? Does .005 become .003?
Yeah Gene - I am wondering about the effect of heat expansion on such little backlash as well!


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post #10 of 65 (permalink) Old Jul 6th, 20, 10:07 AM
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Re: .005" backlash too tight?

If this is a factory set up then there will not be any shims on the sides. The factory used select fit solid spacers that were between .220 and .260, no shims. If you have shims then this is not a factory set up.

I just realized that this may be a '65 or '66 rear end, which used spacers that were between .060 and .090. It's been a long time since I set up one of those and I can't remember if there was just one on each side or if there were more than one. I believe that it was just one per side.
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post #11 of 65 (permalink) Old Jul 6th, 20, 3:31 PM
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Re: .005" backlash too tight?

did ANYTHING you have done change this -at all ?
Think about frequency … is it wheel speed or engine speed ? Try running the car in second gear up to the rpm the noise occurs in third. The fact it shakes the mirror leads me believe its not the rear end
And if you find clues its the untouched factory set up , I would leave it alone.....or plan on new gears/bearings.
Granted its too tight , but it has run many miles this way
A thought -if driveshaft runs pretty straight , the trans bushing/ slip yoke clearance could be excessive
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post #12 of 65 (permalink) Old Jul 7th, 20, 7:36 AM
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Re: .005" backlash too tight?

I know this is a wildcard but if you can't find marking compound near you, in a pinch I've used thermal grease used on pc chips with great success. Usually have a computer parts/repair place nearby and you can find the grease easily. Brakleen cleans it off with no problem before you seal it up.

As mentioned above, pull your caps to see if solid spacers or little shims installed. The whirring could be the gears binding up when heated.

How hot does the diff get after a drive? How did the fluid look when you drained it? Was there metal on the magnet?

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post #13 of 65 (permalink) Old Jul 7th, 20, 8:40 AM
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Re: .005" backlash too tight?

David, I picked up some Prussian blue by Permatex @ NAPA. Not my preferred compound, but got a really nice pattern with it yesterday, so I buttoned mine up at 8 thou and will run it. And replace the frozen wheel cylinder I found yesterday!

I'd see what the pattern looks like.

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post #14 of 65 (permalink) Old Jul 7th, 20, 12:01 PM Thread Starter
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Re: .005" backlash too tight?

Quote:
Originally Posted by big gear head View Post
If this is a factory set up then there will not be any shims on the sides. The factory used select fit solid spacers that were between .220 and .260, no shims. If you have shims then this is not a factory set up.

I just realized that this may be a '65 or '66 rear end, which used spacers that were between .060 and .090. It's been a long time since I set up one of those and I can't remember if there was just one on each side or if there were more than one. I believe that it was just one per side.
Thank you. It's looking like I need to pull the cover and caps to see what's up.

Quote:
Originally Posted by driveshaft-texas View Post
did ANYTHING you have done change this -at all ?
Think about frequency … is it wheel speed or engine speed ? Try running the car in second gear up to the rpm the noise occurs in third. The fact it shakes the mirror leads me believe its not the rear end
And if you find clues its the untouched factory set up , I would leave it alone.....or plan on new gears/bearings.
Granted its too tight , but it has run many miles this way
A thought -if driveshaft runs pretty straight , the trans bushing/ slip yoke clearance could be excessive
Nothing I have done has really made a difference - except after I re-installed the factory axles; it appears the noise is minimalized until I drive the car enough to bring everything up to temps. Then its back to its noise at speed. It is definitely wheel speed - no question. RPM means nothing to the noise.
Right now I have no idea if this is factory setup or a PO was in there, so I don't yet know if it has actually run many miles this way as I bought the rear end used. And the mirror shake isn't excessive - yet still noticeable when looking at the image or you can feel it if you put your finger on the mirror.
And it has a new slip yoke (& u-joints), and I've had a M20 & rebuilt M21 in it with no noise differences.

Quote:
Originally Posted by Hotwire View Post
How hot does the diff get after a drive? How did the fluid look when you drained it? Was there metal on the magnet?
It does get very warm, but I wouldn't at all say hot. I put in new gear oil and additive when I installed the aftermarket axles (about 800 miles ago), and now the fluid has blackened and the magnet caught very, very fine particles. Felt much finer than toothpaste.

Quote:
Originally Posted by 427L88 View Post
David, I picked up some Prussian blue by Permatex @ NAPA. Not my preferred compound, but got a really nice pattern with it yesterday, so I buttoned mine up at 8 thou and will run it. And replace the frozen wheel cylinder I found yesterday!

I'd see what the pattern looks like.
I ordered some differential yellow paste form Amazon and it'll be here tomorrow!
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post #15 of 65 (permalink) Old Jul 7th, 20, 1:35 PM
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Re: .005" backlash too tight?

Quote:
Originally Posted by big gear head View Post
If this is a factory set up then there will not be any shims on the sides. The factory used select fit solid spacers that were between .220 and .260, no shims. If you have shims then this is not a factory set up.

I just realized that this may be a '65 or '66 rear end, which used spacers that were between .060 and .090. It's been a long time since I set up one of those and I can't remember if there was just one on each side or if there were more than one. I believe that it was just one per side.
Yes. Typical total shim thickness on a 1965-66 Chevelle 12-bolt is 0.170" (a single 0.060" to 0.100" selective shim per side), full size Chevy/Camaro/late1967-72 Chevelle total shim is usually in the 0.484" range (a single cast 0.220" to 0.270" per side from the factory). GM selective shims were available from 0.060" to 0.100" in 0.002" increments. Back in the day (before super shim kits) when gears were changed in late 1967-up Chevelles etc, GM offered an 0.170" base shim which was used in conjunction with the standard selective shims on each side of the carrier.

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