Rear end identification

J

joeskylark

Member
I have a 1970 Skylark convertible, and cannot identify the rear end with my shop manual. The only number I can find is 1385724-F. This number is not on the bottom of the axle tube either. Rather, it is on the front end side of the differential housing. Any help is always good. -Joe
 
You will be looking for the "code " and it should be on the front of the pass. side axle tube about in the middle .HTH
 
I am all over that axle, and the only markings I can find are stamped on the diff housing itself. Is it possible the code could have rusted over? It does have some surface rust, but nothing I have'nt already sanded off. Thanks. By the way, the differential tag is also M.I.A.
 
The only place that I have ever found the code on a GM car rear end was on the right axle tube. Most of the time it is on the front side close to the center, but I have seen them stamped on the back side. What is it that you want to know about it?
 
Joe, if the rear diff assembly is the original for the car I believe it is the 8.2 axle of the BOP design.
Tom T.
 
I'm trying to I.D. the gears. I've never known which rear I had. The car is a Skylark convertible with center console. I've had the car since 1994, and I was always curious especially now that I'm about to put in a 455. I'm about to remove the rear axle to restore the frame, and I started looking for the codes, only to not find them anywhere. I guess I'll keep looking. Thanks -Joe
 
Joe,

Jack up the rear of the car so that both wheels are off the ground. If you spin one wheel and the other one turns the same direction you hava a posi. If it spins the opposite direction you have a peg-legger. If it is a posi, see how many times the driveshaft goes around for one wheel revolution. That is your gear ratio. If it is a non-posi rear multiply the number of driveshaft revolutions per wheel revolution by two to get your rear gear ratio. HTH
 
The best way, and the only sure way to know what is inside it is to remove the cover and look at it. I have seen too many posi rear ends spin one wheel and I have seen several standard rear ends spin both wheels. This is not a sure test.

As for the gear ratio, if it is a posi rear end be sure that both wheels turn exactly one full turn to check the gear ratio. If one wheel slips a little your check will not be accurate. If it is not a posi rear end then leave one wheel on the ground and turn the other one exactly two full turns. In both cases count the number of times that the drive shaft turns. This will be your gear ratio. Just over 3 turns is a 3.08, just over 3 1/2 turns is a 3.55, 3 3/4 turns is a 3.73, just over 4 turns is a 4.10...
 
Thanks for the help. It only spins one wheel. I will have a chance to look inside to confirm that it is a non-posi as I clean up the rear before I reinstall it. Kind of disappointed. With all this torque, I guess I'll be able to pull a good donut.
 
I'd like to, but don't have much experience with swapping differentials. How difficult is it and how much (ballpark) am I looking at? I've been looking for whole rears, but even high mileage ones are 1k plus.
 
You would need a dial indicator with a magnetic base (about $40 on ebay), a micrometer or dial calipers and a torque wrench. You start by removing the cover, removing the axles and cheecking the backlash. You use the dial indicator for this. The pinion gear must be held perfectly still while checking the backlash or your reading will not be accurate. Then remove the stock differential and swap the ring gear over to the posi. You need new bearings for the posi and a shim set to reset the backlash. Install the new posi with shims on eaxh side and measure the backlash again. Make adjustments to the shims to get the backlash the same as it was before. The shims are also used to set the preload on the differential bearings. They must be tight enough that you can not pull the differential out by hand. Send me an email for the price of the posi.
 
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