I just got some 4.10s installed this weekend, in addtion to new bearings and a Traction Lok rebuild. The guy who did the install, owns a speed shop and he works on Mustangs all the time, said that I didn't need to break in my new gears or clutch packs. He also put in Royal Purple gear oil instead of mineral based gear oil. My question is, do rear gears need a break in period? Just looking for 2nd and 3rd opinions. Thanks for all answers. By the way, I trust the guy, but just curious anyway.
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1992 LX 5.0, 12 inch K & N cone, SLP Loud Mouth dumps (for 98 and up Cobra), home made cold air box, underdrive pulleys, 75mm chrome Pro M MAF, bypassed airpump with short belt, Jeg's Universal Ford 2 row aluminum radiator, 16 inch electric fan, E cam, SVO 1.6 roller rockers, weld in subframe connectors, 16 degree timing, short throw shifter, Hotchkiss tubular UCAs and LCAs, Maxspeed springs, aluminum drive shaft,shorty headers, and BBK adjustable fuel pressure regulator, 3.73gears
the way to break in new gears is to drive the car until all the fluids ie; rearend fluid is at normal temp . shut down the car and let everything cool down for an hour or so and do it again . The rear gears will then be broken in .
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Never argue with an idiot. They will bring you down to their level and beat you with experience...
That was done on the day I picked it up from the shop. The gears totally rawk, its so much fun to drive now.
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1992 LX 5.0, 12 inch K & N cone, SLP Loud Mouth dumps (for 98 and up Cobra), home made cold air box, underdrive pulleys, 75mm chrome Pro M MAF, bypassed airpump with short belt, Jeg's Universal Ford 2 row aluminum radiator, 16 inch electric fan, E cam, SVO 1.6 roller rockers, weld in subframe connectors, 16 degree timing, short throw shifter, Hotchkiss tubular UCAs and LCAs, Maxspeed springs, aluminum drive shaft,shorty headers, and BBK adjustable fuel pressure regulator, 3.73gears
The gears themselves don't require a break in. Tony Gonyon who installed my gears insisted that we go for a "joy ride" right after he was done with the install. He said that shops tell you to let the gears break in because they're afraid they didn't install correctly, and don't want you to break or hear anything until later.
Now, in your case, however, the traction lock will require a proper break in. I know because I tried to do some heavy driving right after I rebuilt mine. BIG MISTAKE! The rear end made some wierd noises, and for the life of me, I could NOT do a burnout. A few days later, I got the balls to try to burn out again, and lit both the tires up!
I think I'll take it easy til this weekend or a little bit longer. So you're telling me it is only gonna get better? Awesome.
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1992 LX 5.0, 12 inch K & N cone, SLP Loud Mouth dumps (for 98 and up Cobra), home made cold air box, underdrive pulleys, 75mm chrome Pro M MAF, bypassed airpump with short belt, Jeg's Universal Ford 2 row aluminum radiator, 16 inch electric fan, E cam, SVO 1.6 roller rockers, weld in subframe connectors, 16 degree timing, short throw shifter, Hotchkiss tubular UCAs and LCAs, Maxspeed springs, aluminum drive shaft,shorty headers, and BBK adjustable fuel pressure regulator, 3.73gears
I had gears installed a month earlier. The local Mustang shop did mine and said basically the same thing. No need for a break in period or to change gear oil after so many miles on a fresh set of gears.
I've had zero problems after about 600 miles.
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1995 Laser Red GT
Kenne Bell Blowzilla (8lbs), GT 40 lower intake, AFR165 heads, Cobra 1.7 rr's, Pro-M 80mm, AFM Power Pipe, CUSTOM TUNE (SCT), 42lb injectors, FRPP pulleys, 70mm throttle body, FRPP 1 5/8" shorties, Bassani cat-x, Bassani after cat, Steeda Tri-ax, Ford 3.55's
Full street oriented Suspension (springs, shocks/struts, uppers/lowers, subframes, diff. cover, Bullitt brakes 13"/11.65", C/C plates, bump steer kit, ......
Break In Period For New Gears? 
