Do re-installed used OEM gears need breaking in?
 

I just put a 12 bolt in my 65 GTO that was assembled using a 65 Chevelle housing, a used set of OEM 3.31 gears I bought on ebay, and a new Eaton TrueTrac. The assembler who owns a local respected differential shop said the gears were “cherry” and did not need breaking in, in his opinion. Initial driving impression is good and noise seems minimal. I was wondering if people here, based on their experience, think they would need any kind of break in period? Thank you in advance for your reply.

If the gears were already broken in they don't need to be broken in again. They just need set up as close as possible to the way they were when broken in. We have transplanted tons of factory gears with no problems.

Thank you. That is what I was hoping to hear.

So they are not quite set up exactly as before. Might not be a bad idea to at least keep the maximum speed and the duration of the drives a little conservative. This will allow the areas of contact that have changed a little time to polish and bed in.

Ok, thank you.

Side-lash variation for re-installation is likely no matter at all.
Pinion-Depth is all the matter, yet being a sliding-contact allows a minor variation in pinion depth, but not much. I dunno how little or much.

Reverse can reveal some noise, which if set-up good for Forward, will clear up...with wear.

Thanks for your response. So far it seems to be ok.

I've been running used GM 3.36 gears in my 3.55 case since 1990. No issues.

Before going to the 12 bolt, I had had two sets of aftermarket gears installed in my original 8.2. They all became very noisy after a few hundred miles. Rather than go for a third try, I went for the 12 bolt with used OEM gears, all of which I bought from different people, after trying to find NOS gears for a few years. After my previous experiences, I have been skeptical of all gears and those who install them, that’s why I was asking others for their experience. So far, these are good. I also think that my original 8.2 housing may have had internal problems that prevented newly installed gears from keeping their settings.

When the gear "sets" are ground, they are then " lapped" in so the wear surfaces are correct and can handle the power. After that they are then cleaned and Parkerized so they look " unused" , but they have been run in at factory, and our job is to duplicate those angles and preload as we run them. I just did a 12 bolt with used 3.31 gears that were very used. It was tricky getting it close, but not too tight. Finding 3.31 gears isn't easy, or wasn't a year ago. You'll be fine as long as the pinion seal was spaced correctly

So did the factory go through any sort of break in once the rear was in the car prior to delivery, or just the typical easy driving for 500 miles or so once you got it?

At the gear and axle plant, They have this machine and I cant remember the name off the top of my head. Its after assembly, and before brakes and its on an arm has a rubber yoke engage and spins the rear and its inspected for leaks and noise. Keep in mind, the gears are already worn " lapped" in when made. The factory gears never got parkerized but I think they were Tuftrited, but I am Not 100% on that. They did get a hardening though.

I had Larry Woltzen of California Differential do the work and he was at the time, and likely still is (he relocated) a real pro who has been doing this for decades. I personally would not have undertaken the gear change myself. And I'm an ASE certified Master auto tech.

breakin
 

When I redid my rear. I reused my low mileage 3:08 gears and replaced the open carrier
with an Auburn Posi. All new bearings were installed, including pinion and axle bearings.
I did go easy on it for a few hundred miles and followed Auburns instructions for break-in
of posi. Differential has been quiet for 10 years now.

Gerry

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