# Rear end rebuild



## kjk990 (Sep 1, 2010)

I am in the process of restoring my 67 GTO. I have had the engine rebuilt, the car painted, and the interior done. I need to focus now on the rear end and suspension/ brakes. I am thinking of starting with the rear end. Should I rebuild the original dif or by a new one? I am working with a limited budget and am trying not to piss off the wife anymore than I already have.

My engine has a somewhat aggressive cam, 
Gross Valve Lift
.488 Intake, .491 Exhaust
Duration @ .006 Tappet Lift 274 intake 286 exhaust

I have a 2400 RPM Stall torque converter, Headers, MSD ignition, Turbo 400 and Automatic trans.

I am looking for a powerful street car. I am mostly driving around normal. Not street racing or anything. What gears would you suggest and what posi would you go with. 

Check out the link below for progress. 

Thanks

Picasa Web Albums - Kevin Kaney - 67 Cabra


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## geeteeohguy (Feb 2, 2008)

What's wrong with the differential in the car? Many of these units go for 100's of thousands of miles without issue with preventive maintenance: change the lube every now and then and check the rear axle bearings. A good performance gear ratio for the street is what your GTO probably came with : 3:36. That was the standard ratio for TH400 GTO's in '67.


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## 66tempestGT (Nov 28, 2009)

:agree


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## Eric Animal (Oct 28, 2007)

:agree or go to a 3.55:1 for a little more "off the line" with that cam.....


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## kjk990 (Sep 1, 2010)

No Safty trak. I spin the right rear tire for days. Really thats the only reason. But while It's open I figured thats a good time to make any needed changes.


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## pontiac (Mar 6, 2011)

add a posi carrier once you check the gear ratio.


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## geeteeohguy (Feb 2, 2008)

You may have a Safe-t-track, just a worn out cone clutch. They can be rebuilt. What I'd do: pull the cover and inspect it. If it is an open rear end, and you like the gear ratio, you can get any number of good posi units brand new that will "bolt in" to your unit. Have a reputable shop do it if you're not feeling confident about it, though. It HAS to be "right". As for new inner bearings, if quiet, I'd leave them alone. They can last forever, nearly, and are bound to be better than brand new Chinese replacements.


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## old66tiger (Nov 2, 2011)

I have an auburn in mine and with 500+ HP, it still lives.


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## BearGFR (Aug 25, 2008)

:agree If your rear is otherwise good and has the ratio you want, then the quick path would be to just install an aftermarket diff unit (like the Auburn).

Even if you want to change ratios and need to buy gears, you'll likely spend less money going that route.

Bear


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## peahrens (Jun 7, 2010)

I had my mechanic rebuild the rear a couple years ago...it spun a wheel like yours...new bearings... cleaned up & painted...plus an Auburn unit. Works great!


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## kjk990 (Sep 1, 2010)

"You may have a Safe-t-track, just a worn out cone clutch."

Is there a way to tell whats inside without taking off the cover? The pumpkin scares me.


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## 66tempestGT (Nov 28, 2009)

taking the cover off is easier than replacing valve cover gaskets. just pull the bolts and look inside. no way to know for sure without looking.


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## Knuckledragger (Nov 25, 2011)

kjk990 said:


> "You may have a Safe-t-track, just a worn out cone clutch."
> 
> Is there a way to tell whats inside without taking off the cover? The pumpkin scares me.


From your original post, I would say that you probably have an open rear end. Jack up the rear of the car with the trans in neutral, brakes off. Grab one wheel and spin it. If the other wheel spins in the opposite direction, you have an open diff. If it spins in the same direction you have posi and your unit needs work. 

I have been under the impression that cone clutches in the safe-t-trak units are not serviceable, now someone says you can rebuild them. Anyone know the real deal?


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## geeteeohguy (Feb 2, 2008)

Knuckle, the cone clutch can be rebuilt. 20 years ago, Larry Woltzen of California Differential rebuilt the one in my '65, and it's still working perfectly. The procedure can be found on the internet, and involves machining the bottom portion of the "cone" so it no longer bottoms out and the cone portion of the clutch and once again make full contact with the internal portion of the unit. It's basically restoring a tapered fit. There is a bit more to it, but, as I said, it is do-able and the info is on the 'net.


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## kjk990 (Sep 1, 2010)

Tomorrow I am starting my four wheel brake conversion. I am also replacing all the bushings, springs and shocks. I have to remove the differential cover to remove the axles and then the drum brake assembly. I will look for a Safety track. Can someone explain how to figure out the gear ratio. I know I have to count teeth and do some math. But thats all I know. Also, is there anything I should inspect. I have never had it open before. 

Thanks


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## FlambeauHO (Nov 24, 2011)

Drivers side axle tube will have a faint stamp of two letters. If it is Safe t track the first will be a y if it is open it will be a w. The stamp is like two inches from the differential cover on the axle tube. It will give you ratio as well. My WH is a 3.55. Lots of articles out there for decoding it. Here is a pic of my faint stamp. Use a wire brush to see it better.










Pretty hard to see in the pic.


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## 68greengoat (Sep 15, 2005)

^^^^ That was like looking for "Waldo". The 2 letter code will tell you the ratio as long as the gears weren't swapped out at some point. The best way to determine the ratio is to count teeth and do the math, or, spin the rear wheel one complete revolution for a posi(twice for a peg leg) and count the driveshaft revolutions. 3.5 revolutions would equal a ratio of 3.55 etc.....


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## 68GTO4004Spd (Jun 19, 2008)

You can also determine the ratio if you take the pumpkin cover off, the ratio is stamped in the ring gear. In this case 39 divided 11 = 3.55


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## 68GTO4004Spd (Jun 19, 2008)

dooh forgot the pic


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## kjk990 (Sep 1, 2010)

So I jacked it up off the ground. marked the wheel and driveshaft. When I turned the wheel the other turned the opposite direction(open rear end). I turned the wheel just less than 7 times to bring the driveshaft in one full rotation. So am I correct in saying it has a 3.36 gear ratio? 

Thanks for all the comment and picks.

After I wrote this I went and double checked. During the first test I didn't watch the driveshaft as I turned the wheel. I just made a turn then looked for the tape. I did it again and watched the driveshaft. I noticed that sometimes as I turned the wheel the driveshaft would start to turn then roll back an inch or so, then started rolling in the correct direction. Is this normal? Or am I doing something wrong?


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## leeklm (Mar 11, 2012)

Someone put a 12bolt with 3.90 gears & open diff in my car... go figure. When pepping up my 400 next winter, I will definetly be adding the Auburn carrier & some lower numbered gears, 3.55 max.


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## Goatnut (Jan 17, 2012)

I've never done it on a peg-leg rear before but seems I remember that you have to have one wheel in contact with the ground to do the "spin the wheel and count the revolutions" method.


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## leeklm (Mar 11, 2012)

Goatnut said:


> I've never done it on a peg-leg rear before but seems I remember that you have to have one wheel in contact with the ground to do the "spin the wheel and count the revolutions" method.


You are correct. If the wheel actually turned 7 times against 1 turn of the drive shaft, you would have a major over-drive ratio... Great gas mileage, but probably a little low on power 

If the ratio was 3.36:1, the driveshaft should turn 3.36 times to every rotation of the tire.


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## kjk990 (Sep 1, 2010)

I took the rear end in to have it rebuilt and an eaton posi put in. He showed me how to do the ratio test. I had both tires off the ground. So I was all wrong. I have an open rear end with 2.56 gear ratio. I decided to go with 3.36. That should give me a much better launch but still able to cruise. 

I'm glad I took it in too. When removed the axles he said the bearings were rolling around in the axle tube. I did something rolling around in there when I dropped it off to him.

Thanks for all the posts


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