Would it be wise to install a used ring and pinion gear in a rear diff?

 

As long as it was set up right the first time, has a good pattern and not worn out it'll be fine.
Will

 

I can't think of a single good reason to install a used set of gears in a diff. Especially if they are oem Gm gears. Unless it's an emergency.
Mike

 

Installing a used set of GM diffy gears would be fine if:

1. You have a Magnaflux machine that can check them for micro-cracking;

2. You have the equipment and experience to set them up properly.

In case you've not done that, it is a relatively difficult skill to master setting the contact pattern on a differential gear set. You don't just bolt it up and go....

 

Mike can you expound?
I try to find GM gears of the correct ratio because they are absolutely quiet.
They don't whine at all.
After market gears almost always make some noise no matter how perfectly they are set up.
Just curious what your reasoning for this is in a street car.
I agree that if it is a drag race or something like that than a pro gear of 9310 material would be a better option, and in high horsepower applications it would be a must or they will break.
Ask me how I know?

 

Sure.
Gm gears are soft. They break in quick and also wear quick. This can be good or bad. Gm gears are based on 37 tooth ring gear except for 3.55 which has a 32 tooth. The lower the tooth count the faster the wear and more heat is created. For a show car or sunday cruiser it's not much of an issue. I have yet to see a re-usable set of 3.55's with over 50K on them.
Like 7T1 vette said, gears are hard to set up correctly. With a used set of gears to get a good pattern you have to match the pinion depth and backlash of the previous set up. The old "worn in" pattern makes it very hard to do.
All carriers have run-out, even brand new ones. Gears get worn in to match the run out of that particular carrier. Much like machining a rotor to true it up on a spindle with run out and then putting it on another car. The only way to check the runout is to set the gears up in a diff and check for varying backlash. To much variance and you have to pull it apart and rotate the ring gear.
Any new decent set of gears is made from 8620 steel. And they are based on a 41 tooth ring gear. And they are less forgiving on set up. Close won't get it. U.S.Gear gears are probably the hardest to set up but when done right you get a really good pattern and they are quiet.
So for the person wanted to spend the money only once and have a set of gears that will last forever(within reason) buy new good gears.
Everyone complained about the old Richmond gears being noisy. I never had a problem with them. But, most people don't know how to set up Richmonds. If you set them for the best pattern at 7 -8 backlash they will make noise. Richmonds are honed at .010 backlash and that is where you set your best pattern. Then reduce the backlash to 7-8.
Mike

 

I would use an original GM gear that is in good condition over any after market
gear set.

 

Thank you for the very informative response. I knew that you know your stuff, and wanted you to elaborate.
I personally hang on to all the popular ratios of GM 12 bolt stuff that are in good condition, and run them in the cars I restore, again because they are almost always quiet, and yes they don't see any serious miles put on them.
So what in your opinion is the best quality gear set to use if you are going after market, and maybe list a few in order of your preference.
Thanks in advance

 

Right now it's a short list.
USGear for nearly all except Dana diffs. Use Dana gears.
The new Richmonds I am unsure about. But I have installed several sets of the 3.90 ratio with no problems and no complaints.
Motive: I only use them in restorations. Haven't tested them under race conditions but they do set up easy.
Yukon: used to really like them but had a set fail miserably. But not positive they actually were Yukons, possibly a Korean imitation. Company I bought them from had a shady reputation.
Mike

 

And how do you identify a used set in "good condition"?
Mike

 

I totally respect your standard of work, and know of your quality, but I have to say, that since I don't drag race, but enjoy an occasional burnout on a street still.....after over 1/2 century......I have 'rebuilt' more than a few diffys and had no troubles over used gears.....

no comment over some other 'issues' though.....sure as hell wasn't from OEM gears though.....

 

How "soft" were those GM gears? They wouldn't make it out the door unless they were at least 60C Rockwell at the surface. And, you don't want the interior to be hard; you want it to be tough.

So what did these "soft" GM gears measure on your tester?

 

Hi Gene,
I've put used gears in my own cars and I probably will again at some point. Like you, if I found a problem with the old gears while setting them up I wouldn't have used them. I have barrels of used gears that are not usable and about a dozen sets that are re-usable.
The Punisher's question was "would it be wise to..." and I listed reasons why it wouldn't be. There's just not a "good" reason to reuse gears. For someone that does re-use gears, especially a novice, they need to beware of the issues I mentioned.
As for doing the occasional burnout....every chance I get.
Mike

 

If there is no significant wear on the gear set (except for surface polishing at the points of contact between the gears) and the contact pattern looks good, there is no reason NOT to re-use good GM diffy gears. Of course they have to be shimmed properly (as with ANY gear set change-over).

 

Soft enough to roll the metal off the end of the teeth when the pattern reaches the heel.
Does any remember the factory warranty on these cars? 3 months or 3,000 miles, whichever comes first. As I said before, soft gears are quiet and break-in fast. For the same reason why front rotors on Mercedes, BMW's and most Fords wear out before the brake pads. Soft rotors.. no brake squeal.
Of course I didn't check them on a tester. Could I? Yes. Will I? No. Do I have access to a tester? Yes. Why waste the time? I see wear patterns every single day. What do I base my opinions on? 41 years of building diffs on everything from chevettes to F700's. Unlike several well known companies we are all familiar with, I actually warranty my work.
Ask Alan Rothman about improperly hardened gears. I think his video is still on youtube.
Mike

 

Bingo! Except that is a mighty big "if". If you get a good pattern, minimal backlash variance, and you don't expect the gears to last 100K, then use the gears. If it is that important to save $200.
I didn't ever say "don't reuse gears". I did say I can't think of a single "good" reason.
Mike

 

So I'm planning "saving" to have my what I think it untouched rearend freshened up.

In this case the 3.36 will get tossed?

 

oh, I forgot you are the gear expert.

 

He is.

 

Not sure if this is sarcasm, but he actually is a expert in this specialized field.