I just put new 4.10 ring & pinions in my '78 F250, and I'm trying to break in the gear sets. It's easy to put miles on the rear gears, but I obviously don't want to drive around a bunch on dry pavement in 4WD, so breaking in the front gears are a bit trickier. My question is, would it hurt anything to drive around on dry pavement in 4WD with the hubs unlocked? I assume this would eliminate the binding issue, as the wheels would be out of the equation, but I could still put a load on the front ring & pinion. Thoughts?

 

I still don't think they will have a load on them.
All they would do is spin but they are not doing any work so no load.

I only did one 4x4 gear swap and never gave it a 2nd though of front gear break in.
Been so long now I forget but want to say I run it in 4 high on the lift for a few mins.
Took it out of 4 high and ran it in 2 wheel till I needed 4x4.
Truck had a little over 12K and under a year old when I did this. I sold the truck with 230K on it and I had a lot of miles in 4x4 some of that pulling a trailer in the snow with 4 four wheelers from CT to VT for weekend fun in the snow.
I had more issues with the back rear end with bearings or no lube and pulling the car trailer, nothing with the front.
Dave ----

 

you're not going to hurt anything by engaging it with the hubs out. no different than having the hubs in and disengaging the transfer case.

 

I'd run them in 4wd unlocked …. and then locked in 2wd maybe another 1/4 mile, then if no issues, just use it as I normally do.

 

If you want to put a load on the front axle, you could remove the rear drive shaft and drive around in "front wheel drive" for a while. I had a Gear Vendors overdrive on my '97 Dodge/Cummins. It was mounted behind the transfer case. One day, while trying to accelerate hard while entering traffic, the overdrive blew up and I lost all power to the rear wheels. I engaged 4WD and made it home on the front driveshaft. Being that the front driveshaft is smaller in diameter than the rear, I was gentle on accelerating. I then removed the Gear Vendors unit. I never liked it anyway. But, yes, it is possible to use our trucks as a front wheel drive if it's necessary.

 

Good call, I should have thought of that. I had to do that with my Bronco when my rear u joints went out.

 

Sharp turns at slow speeds is when you'll experience the binding when 4wd is engaged on a hard dry surface. Its because the axle joints are at their limit of motion & the front tires are turning at different speeds in a tight turn. Apply a lot of power in this situation & breakage can occur. You won't do any harm to the assembly by driving normally on a hard surface, even with long sweeping curves.
It was explained to me by a professional that the gear surfaces need to be "work hardened" for longevity & strength. They need to have a load on them & build a little heat on the surfaces under normal conditions (no hot rodding! lol) for proper break in. A heavy shock load on a new R/P setup could very well distort the surfaces or worse, break the "teeth" off the gears.

I had my D60 front re-geared & added a detroit locker & my builder says to me "drive it 25 miles in 4wd on the pavement to set the gear surfaces" & this was on a highboy with 42" tires & no power steering....he laughed & said "do it if you want it to last"...I did, & its held up to my abuse without issues.
Good luck!

 

If they're set up right and decent quality gear sets a minimal amount of break in should be required. like basically none.

They weren't broken in when the vehicle was new . someone just jumped in and started driving.

 

On a previous bronco, the rear driveshaft was bad, kids young and money was tight, so I drove on the front end as a daily driver, vacations, for over a year in Michigan.

Just be a little easy on tight turns on pavement. Easy to squeel and do burnouts.