After installing a set of shorter gears on your truck, you may be tempted to to immediately find some dirt and put your foot to the floor and enjoy all that new-found mechanical advantage and final drive ratio. Yeah, don't do that. You're break a lot of stuff that you just went through the hassle and/or expense of installing. A crucial part of any new ring and pinion gear install (whether you're going shorter or taller) is the break-in, and not doing a gear break-in properly can seriously ruin your day, not matter how high-quality of a ring-and-pinion gear kit you've put in your truck. We recently put a Complete Nitro Gears 4.11 Front & Rear Gear Package along with an Eaton TrueTrac LSD in our (as of this writing) 226,000 mile 2011 F150 XL Project Truck to help give it a new lease on life and overhaul some very tired differential hardware while we were at it. While we were successful, the agony of break-in came first.

Nitro Ring & Pinion Gear Package Break-In

Why do gears need to break-in anyway?
Before we into the "hows' of gear break-in, we do need to touch on the whys. No matter what gear set you use from the lowest-tier manufacturer to the highest-grade ring-and-pinion sets on the market, all of them require at least some form of break-in procedure before you can drive normally and especially before you hammer down on the throttle. The main reason has to do with how gears and gear steel reacts under the heat and load of everyday driving versus during the manufacturing process. While gears are machined, lapped, and hardened during manufacturing, they're not put under the same loads as the constant heat cycling and vehicle load that driving on the street puts them under, much less banging around off-road or towing heavy. This heat cycling causes the gears to expand and shrink by a several thousandths of an inch and until they're fully hardened during a gradual break-in. Driving too aggressively too soon means that instead of hardening, the gear steel will actually soften and expand due to the high levels of heat to the point where any backlash set during install gets thrown off and causes excessive noise, if not eventual gear failure.

With a proper break-in, the gears undergo a more gradual heating and cooling cycle that the gear manufacturer's call "work hardening" that makes the gear steel even harder than it came from the factory and ensures that the gear steel doesn't expand excessively even under heavy loads or high speeds. The work hardening of gears can only occur if overall heat doesn't peak too high and if the gears get enough time to cool between drive cycles in enough gear oil to keep them properly lubricated. If done correctly (and paired with a good install that hits recommended backlash specs), then your new set of gears should give you fairly quiet operation and years of excellent performance and reliability.

2011 F150 with 4.11 Gears & Eaton TrueTrac LSD

Nitro's Recommended Break-In Procedure
Now that we know why we should break in a set of gears, it's on to how. Nitro recommends taking the truck only 25-30 miles on its first drive post-gear install and not exceeding 50 mph. After stopping the first time, the gears will need to cool at for at least 20 to 25 minutes before proceeding onward. After the first drive, you'll need to avoid hard acceleration and not exceed 50 mph for 500 miles. After you hit that 500 mile break-in mark, all you have to do is swap out the gear oil with a high-quality, full-synthetic gear oil (Joe, our installer down at On-Point Performance recommended 75W-140 for both the front and rear) and then you can drive like you stole it. If you're towing, Nitro also recommends a 15 mile initial drive before a 24-30 minute cool down, and then repeating the process until you hit the 45 mile mark.

Back to our Project Truck
On our 2011 F150 XL Build, we weren't doing any towing, though the break-in process is still a bit of headache, especially if you tend to have a heavy foot on the gas (like me). The initial part of the break-in was done on the drive home, where I took surface streets back to me house about 25ish miles from Joe's shop. After I got home, I parked the truck for the night, letting the proper cool-down period take place. Then came the real agony. Having to squeeze 500 miles out of my truck while not going over 50mph was painful. Even surface streets around these parts tend to have 40 to 45mph speed limits, and having old ladies in Honda CR-Vs blow past me stung the ol' ego pretty good. Those of you in areas with lower speed limits will have less of an issue.

Once the break-in period, was up, it was time to change the gear oil. I started out with the front differential, which is notoriously annoying to change the fluid on. I used the below video from fordtechmakuloco's YouTube channel as a guide. The easiest way to do this is to snake a suction tube past the front differential and into the bottom of the diff housing to suck up all the oil. I strongly recommend using an air bleeder or other powered fluid extractor to do this. I used a manual hand suction tool, and while that method does make for one heck of a deltoid workout, it's not much fun.

Changing the oil on the rear of my truck was much easier. Mostly thanks to the G2 Axle & Gear Rear Differential Cover installed on my truck. The diff cover has a very convenient drain port located on its bottom that lets you easily drain out the old gear oil. The G2 cover's dip stick port doubles as a fill port, and the G2 cover also has a backup fill port and level port if something happens to the dipstick.

2011 F150 Rear Gear Oil Change with G2 Diff Cover

Don't be super-surprised if there's some glitter and glistening in your gear oil after removing it from your differential. That's normal during break-in as the phosphorous coating/machine oil comes off the gears and any metal particles that may have stayed on the gears during their initial machining and lapping. If you do have large pieces of metal or serious sludging, you may have a problem that you or your mechanic may want to look into.

Rear Drain Plug with Small Particles

With the old oil out, it was time to get new oil in both the differentials. Using Joe's recommended 75W-140 full synthetic, I started at the front, slowly squirting gear oil from its container through the fill hole. It took a while, but eventually oil dribbled out the fill port, which indicated a proper fill. I cleaned up the port, added some thread seal to the plug, and reinstalled it.

Filling Front Differential on a 2011 F150 5.0L

Filling Front Differential on a 2011 F150 5.0L

Just like with draining it, filling the rear differential is much easier, thanks to the G2 Cover. The large port on its top meant that I could cut the nozzle on the gear oil bottles fairly short, stand them up on their ends, and empty them with only a few squeezes.

With the gear oil changed, it was time to actually enjoy the truck and its new gears. With a proper break-in and Joe's spot-on installation, the gears did not make any more noise than stock and the jump from OEM 3.55s to 4.11s made a huge difference with my truck's excellent LT305/65R18 Falken Wildpeak A/T3Ws, even without tuning the truck for the tires or the gears. We'll get in-depth on that at another date, and do an in-depth review of the Eaton TrueTrac at a later date.