Why no extensive break in required on OEM gears?

Just an honest question cause I’m curious:

When you install aftermarket Yukon or Nitro gears in your Tacoma there is a strict break-in process you need to accurately follow, including short periods of driving, periods of cooling down, and multiple oil changes, etc to break-in the aftermarket gears.

While Toyota does recommend varying speeds and avoiding towing when the truck is brand new, the prescribed break-in for the OEM Toyota gears is nothing compared to the break-in required for aftermarket gears. There isn’t even an oil change required.

What gives with that? Does Toyota break in the gears at the factory, or do they just make a higher tolerance gear set? Why the difference?

 

I’m pretty positive they are somewhat broken in from factory. Although, It’s not uncommon to see some metal shavings in a new diff after some miles

 

Yes, but the various videos I’ve seen from Nitro and others seem to indicate you can burn your aftermarket gears up and void warranty if you do not do exactly what is required. I’m surprised at the contrast in recommendations. Certainly the OEM gears are known to last and probably 90% of stock truck owners don’t follow any break in recommendation.

Maybe Toyota or their vendor can simply achieve a higher tolerance part than what Nitro or Yukon offers, requiring less break-in for proper mating?

 

OEM gears are lapped under the same pressure driving creates.

 

Aha. So they use a different manufacturing process that the aftermarket doesn’t possess?

 

I’ve wondered this same thing myself. I wouldn’t be surprised if the manufacturing process from Toyota included a step like this:
https://www.remchem.com/services/isf-process/
Apparently, the “break-in” period is not necessary when this process is completed on a gear-set. I imagine Toyota (and the auto industry for that matter) would adopt this process or one similar as an industry standard.

If money wasn’t an issue, I would have had this done to my gears (Nitro) prior to having them installed (along with cryogenic treatment). I went so far as to source a vendor that would be able to polish (and cryo treat) my gears. It was going to cost some money though...
In the end, to be cost effective, I just had the gears installed and followed the break-in procedure. The shop that was able to do the treatments catered mostly to the racing community. My truck isn’t a trophy truck so I figured I’d save the coin and follow the instructions for break in.

 

Thank you for the informative link! I've been paranoid, thinking and wondering about this while putting the 400 current miles on my truck. I clearly remember what i had to do breaking in the gears (twice) on the TJ i built. It was crazy to see how hot the iron diff cover would get during the first few trips! I mean like 200*F! drive 15 minutes in town, stop and cool. Again. Then two more times a litle longer and faster. Then keep the load light and speed under highway speeds for 500 miles. Change gear oil. Now you're good. That sucked but was worth every minute when the gears were silent with a beautiful wear pattern after 30,000 mi!

Now, the requirement to break-in may be equally to do with the material and also the ratios we who re-gear for performance use. Think about the force- both direct pressure on the teeth and the slip along the face or root and then the increase in speed of all of that the pinion sees in a 4.56, 4.88 or 5.29 compared to a 3.00, 3.29, 3.42, 3.50. That pinion shrinks dramatically for a change like that. I know in the narrow width Dana/corporate realm, when you start getting really deep in ratios with high power, you sometimes get booted into full widths because that 400hp LS swap you did will happily rip off one of the whole 9 pinion teeth you have in your D44 when you're locked on 8psi in 4:1 4Lo and you get pissed off and start bouncing on rocks. A bigger axle gives those equally few teeth (because math is math) way more size and strength.

I work in manufacturing and I think I'll have to ask around if the rear diff supplier comes around. The front ends are corporate, might be harder to get info. I know in real performance versions, the engine and rear diff have legitimate break-in formulated oils in them and have to be changed at 1500mi or so.

 

I’m sure they heat cycle and run in the gears at the factory. There’s likely a end of line tester at the gear manufacturer that runs them in. The test setup is probably expensive, but the cost is spread out over all the trucks they make. Aftermarket companies probably don’t invest in the test equipment cause the volumes are much lower. Just speculating based on my experience in other high volume product manufacturing.

 

I wanna know why the diff whine doesn't set in for a few hundred/thousand miles.

This is a mystery that has even Scooby Doo perplexed

 

I suspect it's a marketing thing. If Brand A needs a more extensive/precise break-in than Brand B, then that's buying decision. Today's I-want-it-right-now generation may think "Why should I have to do that? I wonder what else the factory hasn't done right?" Financially better for a mfr to do more at the factory to generate more/easier sales, plus it ensures break-in was done correctly to mitigate warranty issues.

 

I can't speak about the big issue with Tacomas but in the outside world, it takes that long for a less-than-perfectly-set ring and pinion to start making noise. That's why people think setting gears is a mystical, crazy thing. It's not. You just have to be precise or it will haunt you. It's easy to set gears to work and never break. Getting them to be silent isn't hard either. It's just really frustrating and time consuming. I know that rage and frustration very, very well. Dana and their case spreading and under-the-bearing shims... what a stupid design. Well, the shims will never slip out anyway. I have pics of patterns and a couple of main shims that were chewed through from a bad setup and not enough preload that allowed the shims to walk out. (The shims were not under the bearing which is actually not that bad of a thing if your case preload is to spec when done with a case spreader. They'll probably stay right there forever. Seriously though, just dremel out some bearings and make a setup set and do it right, for real. Don't get cheap or lazy here.)

I learned to set gears because I trusted someone else more than myself. They screwed it up. I unscrewed it... with new gears and more money.

Edit: I'm an illiterate idiot. You right, you right. What I missed was "gear manufacturer." People around here (not you) seem to think that Toyota makes every single part of the vehicle.

All aftermarket gears that I've dealt with are very verbal about breaking them in properly and it's on you if you don't.

I looked up the break-in on my truck and it covers it well if you know what's meant. It's not spelled out like the aftermarket companies do but if you know what's needed, it makes sense. No hard braking for 200mi (hinting at don't give it hell with the skinny pedal either), lower speeds, non-constant speeds and low loads for 500mi, no towing for 1000mi. with a more normal ratio and very controlled patterns and backlash, it makes sense.

Sidenote- holy crap my MT has 4.30 gears though! Didn't expect that!

 

Yes, but as reported here and elsewhere, the aftermarket gears cause the diff oil to go to extreme temps when breaking in. That is not true of a new Tacoma driven off the lot.

 

Wasn't it speculated Toyota puts a lapping compound in the diffs? And that's why u get the sludge on the magnetic drain plug? I wonder if that plays a part on the diff howl, people changing their oil early etc.

 

If they put lapping compound in the oil I would suspect OEM to have higher temperatures than aftermarket during break in.

The most plausible answer so far, at least to me, is what @Ronzio said, that the OEM gears are lapped at the factory under high pressure, which would possibly pre-treat the surface just as driving around does for aftermarket gears.

 

Not only are the volumes lower, they're spread out across the dozens of axles the aftermarket companies support. Having equipment for each of them would be ludicrous.

 

Yea. Exactly what I was thinking.

 

Yukon and Nitro are going up against a guy named Chet or Brain that just installed a set of gears in their square body chebby sitting on 37's with a potent 327 that came out of a wrecked Corvette. The warranty dept wants you to take it easy because said diffs were built on the kitchen table while the wife was at work. Don't worry he covered the table with Pizza Hut and Busch beer boxes, but the wife still came home and asked why the house smells like rancid gear oil. Welcome to America, there's a reason why Gorilla Glue is advertised on TV, because Chet's buddy John weighs about 300# and broke one of the bar stools. It took about a case and half of Busch to get er done and both men carried it out into the driveway, the install won't commence until football season is over

 

It still does not explain why aftermarket gears make your diff housing smoking hot during break in and OEM doesn’t.

 

I stick magnets on the diff cover when I do axles. You should see the crap that comes out of them. I just changed the rear end oil in my (sold for Taco) Silverado and at 105k, you should have seen the fine sludge on the magnet... oof. There wasn't a thing wrong with it but it still makes me squirm. Not as bad as the goo on an automatic trans pan's magnet though.

Another note is that there's likely some sort of schmutz for gear marking purposes left over in the gear oil too, if you are seeing some kind of coloration. I use yellow usually, white is common too and that'll really quickly blend into the slurry of gear oil.

 

They know Chet is going to set them up wrong, so better tight vs loose