Just picked up some Lucas brand 80/90 non synthetic for when I do the install. Do I need any other additive for the initial break in? I recall something a Ford friction modifier or some other small 4-6 oz bottle of something else being added in. Is that just for 500 break in or after break in? Thanks.

 

the only time you need an additive is if you have the clutch-type posi. none of the other diff units require anything but straight gear oil.

 

you only need to run the car on the stands in forward and reverse for 5 minutes one time, and the purpose is to get the oil circulated well before the parts are put under load. it is ok to drive the car right after this for the first heat cycle. the actual break-in process is supposed to be under load, and running the car on stands will not load it enough.

the heat cycle process is like this: easy driving- no clutch dumps, no burnouts, no excessive highway speeds or sustained mileage at once, no full throttle acceleration. you want to keep it under 45mph if possible, vary your speeds, and do not abuse it what so ever. in town, stop-and-go type driving would be a good scenario. the point here is to bring the gears up to operating temp slowly, and then let them cool slowly. cool down time between heat cycles is absolutely no less than 1 hour.

The biggest issue is overheating the gear too quickly. All gears come heat treated from the factory, but the break-in process is to actually "surface temper" the gears, while establishing a wear pattern. Surface tempering makes the very top surface of the metal extremely hard, more so than the other metal in the component.

A 9" gear builds alot of heat by it's design nature, thus making the break-in procedure that much more important with this style rearend. If you talk to the gear manufacturers, they will tell you that 95% of all the returned gears are from improper break-in. They will actually cut the gears apart and check the hardness depth to verify the situation. You can guarantee that if they find improper break-in that there will be no chance of any type of warranty, which is up to the manufacturer.

The absolute worst thing anyone can do is go out and immediately beat on the car with a new gear installed... no matter what rearend is in it. The point here is to bring the gears up to operating temp slowly, and then let them cool slowly... several times. Any clutch dumps, burnouts, excessive highway mileage, etc.before the surface tempering is done will do nothing but damage your new investment.

 

So...

#1: Put rear of car on jackstands.

#2: 15 minutes in forward gear. No cool down.

#3: 15 minutes in reverse. No cool down.

#4: No cool down. Lower car off of jackstands.

#5: 15 minutes of easy driving, less than 30 mph. Let cool for AT LEAST one hour.

#6: Repeat step 5 four times, for a total of 5 cycles. This adds up to a total of 1 hour 15 minutes of driving and 5 hours of cooldown.

#7: EASY street driving x 500 miles, less than 45 mph.

So... the shop is 1 hour by freeway from my house. Looks like I have to get it towed home once the rear end is installed?!?

 

First 4 steps are correct. For the first drive immediately off the jackstands and for the subsequent 4 other heat cycles, I didn't go faster than 15 MPH as stated in Post #17 above. The shop I did it in was only 4 miles away from my house, so I did the install late at night and creeped back home at 15 MPH at 1 am with my hazards on. LOL. Did 4 15 min cruises at 15 MPH around my neighborhood with all the cool down periods, now just cruising at less than 45 MPH.

I'd definitely flatbed tow it or trailer after the install living that far away.

 

If you bought the Yukon gears you shouldve received a booklet with your 3rd member or in the crate of your axle with the break in procedure. I know I did but I dont recall all the different low speed drive cycles mentioned. When I did my break in I just drove mine to work & back using the side roads since it was a 15 mile drive one way.