How to check gearing in diff?

NCHornet

New Member
Joined
Sep 13, 2006
Location
Mt Airy
I am looking for a new Dana 30 with disc brakes to install in my 77 rebuild. I know you can count the number of turns of the drive shaft for every 1 revolution of the tire. Do I need to pull the tire to this correctly? It seems the larger the tire would throw off the number. This is now sounding like a stupid question. From what I have told it should be 3:73 or 3:54 that's pretty close when you're counting revolutions. I couldn't find any markings on the diffs or metal tags. Thanks for the info.
Kevin
 
NCHornet said:
I am looking for a new Dana 30 with disc brakes to install in my 77 rebuild. I know you can count the number of turns of the drive shaft for every 1 revolution of the tire. Do I need to pull the tire to this correctly? It seems the larger the tire would throw off the number. This is now sounding like a stupid question. From what I have told it should be 3:73 or 3:54 that's pretty close when you're counting revolutions. I couldn't find any markings on the diffs or metal tags. Thanks for the info.
Kevin
Click to expand...

No need to pull the tire, it will rotate the same with or without a wheel on :shaking:.

If you are fairly confident the front and rear axle match, checking the AMC20 may be easier. This writeup says the stamp is on the LH side, I thought mine was on the RH side, but its been a few years since I looked for it.
http://www.jeeptech.com/axle/amc20.html
 
BTW...if rear end is open diff, keep one tire on the ground when ya turn the DS! More reliable reading, I think.:rolleyes:
 
Thanks for all the great info. I will rotate the tire and count the driveshaft revos. Good to know most were 3:54 as I am hunting a new axle set up for disc brakes and would rather swap axles then pull parts. How do I tell if it is a open diff? Sorry y'all I have never torn a vehicle down like this before. I appreciate the info. Were not building a rock crawler just a DD and a nice trail Jeep.
Kevin
 
Tape a piece of string on the top of the DS, let it hang to one side, rotate tire on revolution, count how many wraps (and partial wraps) of the string on the DS. No trying to keep up with the revolution count.
 
How do I tell if it is a open diff?
Click to expand...

Jack both tires off the ground, put trans in park, and turn one tire by hand, if the other side turns in an oppisite direction then its open, if they both turn the same way its a limited slip or spool or something
 
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