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Hi all. Got a question. I want to install higher ratio gears in my 56 chev that has 3:70 gears now, with 3:36 gears. Someone has these supposedly.
The numbers that are stamped on the case are as follows.
3725899 J361 T4 I want to make sure I`m getting 3:36 assy. that will
bolt into my housing. Thanks
 

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The 899 casting is common to all open differentials 57 and up, any gear ratio.

J361 means June 3, 1961 was the day it was cast. There should be a stamped code on the other (passenger) side of the casting that shows the gear ratio, assembly location, and date. Of course any gearset and even a posi differential could have been put in that case after it was manufactured.
 

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The J361 means Oct. 3rd 1961.

The fail safe way to know is count the teeth on the ring and pinion gears and divide the pinion number into the ring gear number. That will give you the ratio. No matter what casting you have, the ratio could have been changed.
 

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It took more than one hand to count so I missed it. :rolleyes: Actually the "J" month is September not October because they didn't use the letter "I".
 

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Another way to compute the ratio without tearing it down is to jack it up, mark 1 tire at the top and mark the drive line. rotate the driveline counting the revolutions until the tire makes one complete revolution. 3.55 turns of the driveline to one revoltion of the tire would be a 3.55-1 rear end ratio. Dave
 
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