I was told by a very experienced jeeper that installing a used set of gears in a differential will generally always be noisy because it is almost impossible to get the gears adjusted exactly like they were worn in previously. I see a lot of low mileage 3.73's for sale so I was thinking of picking up a set to replace my 3.21's but don't want to risk having a noisy set of gears. I priced out new factory gears and by the time you get the gears, bearings and front carrier you are at close to $1500 in parts alone. how many miles does it take for gears to develope wear patterns that would cause them to be noisy? also why do they say that on new gears they need 500 miles of running slow and easy to break them in and the fluid needs to be changed when on a new vehicle this is not an issue? thanks jeff

 

I would suggest picking up a set of 4.10's instead of 3.73's. That way you'll have a little more pep and freedom to go bigger if you'd like to on the future. You won't notice a difference really in the 3.73 and 3.21. But difference in 4.10 and 3.21 is beautiful.

If you have 3.21 gears now, you'll still need a new carrier or the lower gears. If your bearings and seals are fairly new, and your mechanic is careful, you can reuse some of them. Save some money there.

If the new gears have less than 1000 miles, I'm sure they won't be too noisy granted you follow proper procedures and do a gear oil change after first 500 miles, then maybe another one just to be safe.