You should really try to put about 500 miles on them in short spurts. Drive 10 miles or so at a time and let them cool in between. If you don't drive very far to work, that usually works out well. The idea is to heat cycle them. Get them hot, cool them, rinse and repeat. And then change the oil. Mainly what you don't want to do is put them under a heavy load or get them really hot for a long period of time (long highway drive) until you have heat cycled them as many time as possible (up to 500 miles or so if possible) and them changed the oil.

 

Why? Why would anything beyond the first heat cycle matter? I understand the initial wear mating at the contact points, which requires more load than most will admit, and then an oil change to get rid of the metal, but why 500 miles? Or 50 for that matter? If you are going that route, wouldnt higher gears require significantly fewer miles for a break-in than lower gears? Trying to understand the logic.

 

When I rebuilt my rear I did a few 10-15 mile drives letting it cool down for at least half an hour between runs. I then had to go to work and back (15 miles each way) a couple of times too. I ran up about 150 miles before then heading off to see friends 350 miles away where I changed the oil. Took it fairly easy on the way down stopping a few times on the way. A few warm up/cool down cycles aren't hard to do, I'm not how critical they are but they are the generally regarded method of break in. I had fine metal particles in the oil when I changed it so I wouldn't to cover too many miles with those in there. So long as you take it fairly easy for a few hundred miles till the first oil change, I don't think you'll have any problems.

 

Problems from a faulty R&P installation won't necessarily show up within the first 500 to even 1000 miles. New gears installed into my TJ went bad probably 2000 miles later (after a gear lube change at 500 miles) & after two wheeling trips. 150 miles from home, in the middle of the desert, & my new rear R&P gears started howling like a banshee, I was never so happy to make it home as I was that next morning.

 

Did the gears go bad due to a faulty installation or parts failure?

 

Problems from a faulty R&P installation won't necessarily show up within the first 500 to even 1000 miles. New gears installed into my TJ went bad probably 2000 miles later (after a gear lube change at 500 miles) & after two wheeling trips. 150 miles from home, in the middle of the desert, & my new rear R&P gears started howling like a banshee, I was never so happy to make it home as I was that next morning.

Did it howl constantly? Does the howl sound similar to mud tires when slowing/coasting or completely different?

 

I had a really good guy install my gears. Went well over 1000 miles before I changed the fluids. No signs of wear on the gear, no noticeable metal shavings. I don't really drive my Jeep over 60-100 miles in a day so my "break in" is normal drives for no longer then 45mins, get out, they cool down, get back in and drive somewhere else. So if you drive a lot of miles then I could see where a "break in" routine would be good but I never worried about how far I was driving. I didn't romp on them for a while though just to make sure.

 

The bearings will wear in through heat cycles. New bearing are a lot tighter than worn in beraings and will only get tighter as they heat up. The material from the "wear in" procedure as well as the coating that will burn and flake off of the gears as they heat up and wear in is why you want to change the oil. I'm not sure what you are asking why about?

500 miles may not be needed, and 50 may not be enough. But 500 will certainly be enough. I would suggest atleast 10 heat cycles with cool down time in between.

 

The bearings will wear in through heat cycles. New bearing are a lot tighter than worn in beraings and will only get tighter as they heat up. The material from the "wear in" procedure as well as the coating that will burn and flake off of the gears as they heat up and wear in is why you want to change the oil. I'm not sure what you are asking why about?

500 miles may not be needed, and 50 may not be enough. But 500 will certainly be enough. I would suggest atleast 10 heat cycles with cool down time in between.

This is a technique I have seen in print several times on this and other forums. I does not alway mesh with my experience nor the advice from the manufacturers and reps to whom I speak. I am trying to understand the logic and mechanics of what you are saying since it contradicts what I "know" and not let my ego get in the way of me learning something new.

My experience - circle track racing.... we break in gears on jackstands for 10 minutes, only to make sure the bearings are lubed well - this is really not necessary most of the time because I have been known for packing all of my bearing on final install. We run 12-20 laps, all out, and then I change the oil. This is all the break in we get. Only ever lost one set of gears, a 6.50 Pro set ring gear from Richmond gears. That was due to a hardening defect in the gear.

Drag racing- about a 20 minute run in on jack stands and we run them all out.

OTR trucks and equipment - no break in. We just make sure the bearings are lubed. I like to change oil in about 20-50 hours depending on availability.

Street. We run them on jack stands for a few minutes and then turn them loose. Typically, I like to change oil in a hundred miles or so.

 

"Break in" period is recommended because new bearings generate a lot of heat. New bearings are set up with more preload because as they wear in pre load loosens up. If you set up new bearings with say 12 in/lb of preload they are going to loosen up far to much and cause issues, so you usually set them up around 20 in/lbs so they loosen up to proper spec of ~12 in/lb. Usually within the first 50 miles the extra pre load in the bearings generates more heat than normal. If the installer set the new bearings up with a tad more preload than normal and you go on a long/fast drive first off, you risk burning the gear oil and ruining your new gears.

I pretty much just drove my Jeep normally and whenever I got to my destination felt the diff around the pinion to make sure nothing was getting too hot. After about 3 or 4 drives the diff was no longer getting hot. With barely a couple hundred miles on them I drove up to Georgetown (2-3 hour drive) and went wheeling, I was actually with Unlimited04. Changed the gear oil this over spring break and other than the rear oil being a bit thick with metal dust everything looked fine.

I should also mention I did the set up myself so I knew what the numbers were and was pretty confident.

 

Break in period is recommended because new bearings generate a lot of heat. New bearings are set up with more preload because as they wear in pre load loosens up. If you set up new bearings with say 12 in/lb of preload they are going to loosen up far to much and cause issues, so you usually set them up around 20 in/lbs so they loosen up to proper spec of ~12 in/lb. During the first 50-100 miles the extra pre load in the bearings generates more heat than normal. If the installer set the new bearings up with a tad more preload than normal and you go on a long drive first off, you risk burning the gear oil and ruining your new gears.

I pretty much just drove my Jeep normally and whenever I got to my destination felt the diff around the pinion to make sure nothing was getting too hot. After about 3 or 4 drives the diff was no longer getting hot. With probably a couple hundred miles on them I drove up to Georgetown (2-3 hour drive) and went wheeling, I was actually with Unlimited04. Changed the gear oil this over spring break and other than the rear oil being a bit thick with metal dust everything looked fine.

This makes sense and coincides with everyhing else I've ever been told or experienced. I use a Raytec extensively on all my gear cases. as well as temp guages when applicable.

 

I really don't think there is that much of a break in needed. When I bought my XJ off of the showroom floor, there was no discussion on break in procedure. The dealership told me to simply drive it as I normally would and follow the normal maintenance schedule.

The original rings and pinions were fine when they were removed at 75,000.

But I did flush and fill the diffs after 500 miles on the new gears. Probably out of paranoia from reading online.

 

If break in periods were so essential all new vehicles would have a break in period or warranties would be void. My gears looked fine after 1000 miles before I changed fluid and I only did so to check wear pattern which they looked brand new. I think break in "rules" are over hyped.

 

up till the 80's most every vehicle did come with a break in procedure for the gears and motors. many newer performance cars and bikes come with an initial drive recommendation as well to "condition" the motor.

no longer having this is probably partially due to improvements in materials and machining standards

 

I agree.

 

Thanks for all your input. I should have about 200-300 miles (all short trips) on my jeep after regearing before heading across country. Fortunately, there are 4 Wheel Parts stores in Utah, Washington, and Oregon I can go to in the event I have warrantly issues.

 

The owners manual describes a break in procedure which includes driving at speeds less than 50 mph for the first 50 miles and below 55 for the first 500 miles. Avoid full throttle or top speeds, steady speeds, or excessive idle during this period. Avoid fast starts and quick stops. First service period is at 600 miles.

Whilst some/all of that may affect engine break in it will help break in the gears by not generating excessive heat in the gear sets. It doesn't mention short initial runs and cool down but if the gearing we fit for larger tyres generates more heat that could be reason for that difference.

 

The heat has nothing to do with the gear ratio or even the gears themselves. When set up properly the gears themselves don't make much heat, it's all the bearings. Any vehicle with new bearings is going to generate excess heat until they loosen up.

 

More heat is worse than less heat. New axles generate more heat under similar conditions as used axles. High speed longer drives generate more heat with a new axle than a used axle.

Further evidence of stress and increased temperatures during the differential break-in period is documented in a 2005 SAE paper entitled The Effect of Heavy Loads on Light Duty Vehicle Axle Operating Temperature. A light duty GM truck towing 14,000 pounds was driven from Orange County, Calif. to the Nevada state line. The test was conducted with both a new axle and a broken-in axle. Over level ground towing, oil temperature was measured at 230 degrees F in the new axle and 203 degrees F in the broken-in axle. Oil temperature over the most grueling portion of the trip, during which a maximum 6% grade was encountered, revealed the new axle was operating at 350 degrees F and the broken-in axle was operating at 300 degrees F. Laboratory dynamometer test results simulating a truck hauling a trailer provided similar results, with level ground towing temperatures recorded at 266 degrees F with the new axle and 194 degrees F with the broken-in axle and towing temperatures (at a 3.5% grade) recorded at 370 degrees F with the new axle and 295 degrees F with the broken-in axle.

 

http://www.ringpinion.com/technical...ontentFrame.aspx?FilePath=~/Content/Parts/RingAndPinions//New_Gear_Break-In.inc

 

That first test result you quoted sounds good. I do question the extreme temperature differentials created in a lab. I'm not saying it won't happen, just that a dyno run is a far cry from the real world and is actually an artificial environment. I prefer actual working conditions and observations.