rebuilt differential break in

[stevolwevol]

I'm going to have my differential rebuilt for my 77. Is there a protocol for breaking in the gears?

 

[dougelam]

yes, no heavy acceleration for 500 miles if you replaced your ring and pinion
If not have at it
It helps to add an EP additive with your gear oil as well

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[Hib Halverson]

Drive easy for 500 miles.

No additional "EP additives" are necessary because GL5 gear lubricants already contain a lot of EP additive.

What you may need is a friction modifier additive to quiet limited slip squeal. Use as necessary.

Change the axle lube every three years or 36,000 miles.

 

[stevolwevol]

yes, no heavy acceleration for 500 miles if you replaced your ring and pinion
If not have at it
It helps to add an EP additive with your gear oil as well

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Yikes! 500 miles sounds like an eternity in my vette. Yes, I probably will have the ring and pinion replaced. I have been listening to a whining noise for as long as I have owned the car. I can hear it over the exhaust when the tops are on. The best way to describe it, is it sounds similar to a manual tranny in reverse when I am driving. The posi still works, the differential works fine as well, but the noise gets on my nerves. I'll do what it takes to break it in properly, so If I have to drive like I'm going to church for the first 500 miles, then so be it.

 

[SVO]

Noise in the rear axle, especially when coasting, could be a bad bearing.

 

[dougelam]

At the Cadillac dealership I would put the first 20 miles on a new gear set with very low load, highway at 55.
Some customers would burn rubber leaving the place, others would tell me that they were going to drive it daily for a couple of weeks. I always wondered if anyone waited 500 miles

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[dougelam]

Those were CTSV rear end repairs

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[stevolwevol]

Noise in the rear axle, especially when coasting, could be a bad bearing.

It makes noise both when I am accelerating, and coasting. It could be a bad bearing, but the gears could have been wearing new flats because of it. I will know when it gets pulled apart. If I don't have to buy new gears, then I'll be very happy, but I'm not optimistic.

 

[dougelam]

Sounds like the carrier bearings to me
Who's doing the repair?

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[stevolwevol]

Sounds like the carrier bearings to me
Who's doing the repair?

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My mechanic whom I have a fair amount of trust for is doing the rebuild. Are you saying that if it is the carrier bearing, then the ring and pinion gears could still be good?

 

[GTR1999]

Yes as mentioned there is a procedure for breaking in new gears. This is what I recommend with every vette diff I build from 250 hp cars to 1000hp 12 bolt conversions.

1- Use 2 bottled of the GM posi additive. This is not the same as it once was and 2 bottles are now what I use and recommend.
2- Use 90 wt or 85-140 Gear oil, non synthetic. I know, there are plenty who will tell you that synthetic is ok but I never needed it since the mid 70's. I use Lucas 85-140 as my choice. You can use GM if you like but I have never had an issue with the Lucas product.
3- Go on your shakedown cruise. Easy driving for 20-30 minutes tops. Stop the car and let the diff cool at least an hour. Do this cycling for the next 10 drives.
4- Change the oil and additive at 500 miles and again at 1000 if you want to critical.

Now building a diff varies among vendors and local rebuilders. You can do what you like but I polish and tune every posi I work on. I recommend you look over Tom's video on You Tube. Again this is something your locals are not going to recommend or even know how to do but it works better then a stock posi build and I have 100's of them around the globe.
Sounds like those gears were not set up correctly based on what you said, but could be a bad bearing. You will know by looking at them if they have been replaced or not. The original bearings will be NDH and the ring gear will have the date and GM stamped in the side.

Here is what I recommend you have your guy do at a minimum

1- Polish and tune the posi.- no springs or plates used.
2- Use sold steel clutches NOT the fiber coated ones or snowflakes some still sell
3 - Tap a drain hole and fit a magnetic plug in the housing
4- Fit the caps to the new races for 001" rock
5- Use Timken bearings and a CR pinion seal
6 - use socket head cap screws in place of the hex bolts in the caps
7- Blast, etch, POR15 and top coat the housing
8- set the axle end play for 005-008"
9- Check the axle faces for hardness and dimension
10- If you need gears use only Tom's buy them direct from Tom
11- Use ARP ring gear bolt and fit them to the posi case flange

I have plenty of write ups on this work if you need it. If you follow my build it will be better then any vendor or rebuilder offers you and much stronger.

 

[stevolwevol]

GTR, It sounds like you know your way around these pigs inside and out, backwards and forward. I would have to print your post, and bring it to the man doing the rebuild.

Why not synthetic?

If I use GL5, then why do I need the additive? Waste of money, or to slippery?

What about aftermarket gears? Are they inferior?

I'm sure you have your good reasons for how you go about things, but it's a tad over my head.

 

[GTR1999]

GTR, It sounds like you know your way around these pigs inside and out, backwards and forward. I would have to print your post, and bring it to the man doing the rebuild.

Why not synthetic?

If I use GL5, then why do I need the additive? Waste of money, or to slippery?

What about aftermarket gears? Are they inferior?

I'm sure you have your good reasons for how you go about things, but it's a tad over my head.

Again I am sure we would get 10 different answers if we asked 10 different people. I base mine on what has worked for me and what Eaton recommended, that is non synthetic. I know others use their favorite gear oil and some don't add in the limited slip additive and life is good. I can say I once tried a well known brand with posi additive in it on my 69 and it chattered right away. That was a stock spring posi setup.

With a tuned posi, no springs or plates, I used to use 1 bottle of posi additive up until the part number and packaging changed. Then I got a call from a guy I built a diff for saying he had chatter in the diff during road racing. Knowing how I set up that posi I was very surprised to hear that. We talked for awhile and I asked if he had reached the 500 mile break in period and followed my procedure. He said he was well beyond that period and was into about 5k miles already. Then I suggested he change the oil to Lucas 85-140 or 90 wt and add 2 bottles of the present GM limited slip, that resolved the chatter. That was several years ago, whenever the new GM additive came out. I have no idea what changed but I am convinced the chemistry did change. My own 72, that also has a tuned posi, started to chatter after a long day of driving, 2-3 hours or more. I probably have 10k miles on it and did the same thing, oil change and two bottles of the additive, work it in by loading the posi clutches in a parking lot slowly, no issue. I didn't see any sign the gear oil broke down but it was black so possibly it did over time. With the tapped drain plug in the diff draining is simple. adding the new oil can be tricky so I just got a long piece of tubing for the qt bottle and squeezed it in until the oil was level with the fill plug.

Gears - GM no longer makes them to fit C2-C3's. So where do you get them? Vendors will rebox them or some will tell you which brand they sell. Few of them know how to set them up. Right now the only gear made in the USA are Tom's made by US Gear in Chicago. The others, even though they may still carry a USA name, are imported. Tom's are the best in my opinion and the only ones I'll use- same as many vette rebuilders do. I must know something because I was selected to build one of the original 5 Grand Sport diff's and steering box, that is about a $5,000,000.00 car these days. I used new Tom's gears in it with my tuned posi.

If this is over your head don't worry about it. Anything new would be. The way Tom and myself build these diff's is different then anyone else. Here is one of several links to how I build them. Take a look and again look at Tom's video on You Tube. It works, regardless of what the common rebuilders will tell you- BUT it has to be done correctly. This is not just shimming spider gears, there is a lot more to it and that is why many will not or can not do it. I have coached guy in Australia, South Africa, Japan, Europe, and all the states on how to do this and they all were able to. You need good tools, room to work and access to a machine shop or know of one you can trust.

The common built diff's are not anything like these, that is a fact.

Differential Rebuilding, '63-'79 Iron Units - Corvette Forum : DigitalCorvettes.com Corvette Forums

 

[stevolwevol]

Following up on this thread, my mechanic discovered it was a bearing, and not the ring and pinion. Everything else looks like it's in very good condition. He told me the gears were replaced, but were not shimmed properly causing the bearings to go bad. They are 3.23's, and I'm leaven um like they are, cause they're good enough for my purposes. I'm pleased that I don't have to grandma drive for 500 miles, and I am spending MUCH less than for a complete rebuild. This is why I trust this guy.

I edited this post cause I got my vette back today. The culprit was a bad pinion bearing. The gears were GM, not made in China crap, and there was no visible wear. I still believe that the gears are 3.55's because it revs 2600 rpms @60 mph. Anyway, no noise! It was getting on my nerves.

 

[GTR1999]

Glad to hear you have the car back and you're on the road.

GM gears will have GM, the date, and tooth count stamped in them. They are a good gear set. There are 6 bearings in the diff, 2 axle, 2 case, and 2 pinion.

The axle bearings are not shimmed. The case bearings are, in stock form they are either steel shims for 63-66 or ground iron shims for the 67-79's. These shims set the lash and preload. Too loose and the lash would be sloppy and the diff noisey. The pinion is shimmed under the large bearing to change the pinion engagement depth. I have seem them all over the place.

Gear ratios for 63-79's 273( aftermarket obsolete now), 308, 336,355, 370, 373, 390, 411 433 or 456. The 373,390, and 433 are not factory ratios. The 373's are better then 370, 390 was once a GM counter gear, 433 are not too commonly used.

 

[stevolwevol]

Glad to hear you have the car back and you're on the road.

GM gears will have GM, the date, and tooth count stamped in them. They are a good gear set. There are 6 bearings in the diff, 2 axle, 2 case, and 2 pinion.

The axle bearings are not shimmed. The case bearings are, in stock form they are either steel shims for 63-66 or ground iron shims for the 67-79's. These shims set the lash and preload. Too loose and the lash would be sloppy and the diff noisey. The pinion is shimmed under the large bearing to change the pinion engagement depth. I have seem them all over the place.

Gear ratios for 63-79's 273( aftermarket obsolete now), 308, 336,355, 370, 373, 390, 411 433 or 456. The 373,390, and 433 are not factory ratios. The 373's are better then 370, 390 was once a GM counter gear, 433 are not too commonly used.

I drove a stock L48 with 4.11's and it reved at 4000 at 70 mph. Yikes, I felt like a push rod was gonna come through the hood. In my opinion stock engines don't rev high enough to take advantage of that tall of gear. What I have is good for all around crusing in my area. I quit street racing back in the 70's anyway. Too many blue suits in ghost cars lurking about.

 

[Otter]

Too many blue suits in ghost cars lurking about.

That's for sure

 

[GTR1999]

Yeah, 411 & 456 are st gears for sure. They came with the optional engines and CR trans. A solidly built SB or BB will hold up fine with them. We ran them a lot in the Ole days! My buddy blew apart his 68's 411 on the street and he went to 456's. It was a stout 400hp 350 but once on the highway he was cruising at 55! The only thing faster was the gas gauge!