My thoughts after installing 4.10's...

[TwoBobsKJ]

So TomMudd and I took a day to install front and rear diffs in my KJ from 4 cylinder donor KJ's running 4.10s so that I could get my KJ back to stock performance. Thought I'd give my thoughts on my Libby now that I've put about 1500 miles on the new gears in the past couple of weeks.

First, some specs on my KJ:

- 3.7 liter V6
- 242 SelecTrac transfer case
- Lifted, with OME 927/948 springs, Bilstein shocks, extra rear isos and front conduit nuts. Sitting 22.5 inches all around
- Upgraded armor, including modified, beefier BoulderBars (heavy!), 4X Guard trans skid (heavy!), Mopar engine, T-case and gas tank skids
- 2003 Jeep Rubicon Moab wheels wearing 255/70-16 Firestone Destination AT's (30.4 inches in diameter.)

Before the 4.10s I rarely used overdrive unless I was going to be over 55mph as the ****** kept shifting between 3rd and OD - very annoying. I also lost about 4 MPG's after the lift and new tires due to running higher off the ground and the taller tires messing up the overall gear ratio. My goal with the 4.10s was to gain back the power I'd lost with the larger tires and take the stress off the ****** when towing my boat, offroading and generally just driving around for my job.

So far my goals have been met.

Haven't towed the boat but over Thanksgiving I took the Jeep to Tennessee - a 1000 mile round trip - through the flats of central Ohio and the mountains of Kentucky and East Tennessee. I gained back about 2mpg's while running a steady 75mph through most of the trip - but most significantly the power and drivability are definitely improved. During a long, 7 mile climb on I-75 up to the top of the ridge just south of the Kentucky/Tennessee border the KJ maintained 70mph in 3rd gear. Doesn't sound like much but when I made the trip to Windrock for the LOST East Coast Safari on the same route, my KJ with 3.73's was only able to maintain 60mph - in SECOND GEAR! And on that trip I was carrying less than half the weight of the Thanksgiving trip - this time there were two adults and all the luggage, early Christmas gifts, two 20 pound turkeys, ice and other food to go with them. In other words, this time the KJ was loaded down a lot more but drove up the hills with much less effort.

Is it night and day difference? Honestly, no. Is the difference noticeable? Absolutely, and the mileage improvement is the quantifiable way to prove the difference. It's little things - like creeping forward at stop lights when I take my foot off the brake, where before the KJ would just sit there. It's lighting up the Firestones from a dead stop where before it would barely chirp. I'm looking forward to towing the boat in the spring and seeing how the Libby runs offroad when climbing rock piles, creeping up inclines, etc.

Thanks to Tom's expertise the swap of the front and rear drivetrains was textbook for the most part. Just a couple of little things that threw us but nothing big. I bought the front diff from a 4 cylinder KJ a couple of years ago and kept it under my garage workbench hoping to one day regear the rear axle. But I found a complete rear axle from an '03 4 cylinder KJ with only 43,000 miles at a salvage yard near Tom's - so I essentially have a V6 KJ with factory 4.10s :icon_mrgreen:

Here are some pics from our install day in no particular order. Sorry for the long-winded post - thought some of you might want to know how I felt about the regear.

The old rear axle about to come out:

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Nekked rear end!

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Tom prepping the 4.10 rear axle for install:

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Old front diff in the foreground, "new" front diff in the background getting brackets installed:

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Hole in the front end where a diff used to be. Note that with Tom's method the control arms DON'T have to be removed to get the diff out. Hint: A cutting wheel comes in real handy...

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Bob

 

[tommudd]

Good info and write up Bob

 

[Cardhu]

nice summary on the hill climb and comparison with your old setup.

If your to drop the diff without the LCA removal, which bolt did you have to butcher / replace / preorder. (passenger, driver, near what?)

 

[tommudd]

nice summary on the hill climb and comparison with your old setup.

If your to drop the diff without the LCA removal, which bolt did you have to butcher / replace / preorder. (passenger, driver, near what?)

Its a secret but you can buy the writeup for 9.95:icon_razz:
Hey I have to make a living some how :evil:

 

[Cardhu]

If i ever brave the 2550 km trip for a meet and greet, i'll buy the beer.

That or if you got a line on another 4.10 diff and want to do some cross border shipping we can work out a deal.

I cringe everytime I think about dropping 3K on a steel w 4.56.s

 

[profdlp]

Nicely written review, great pictures, nice job on the upgrade!

 

[tommudd]

Yes as much as I'd like to have 4.56s but not going to happen
Just have to keep checking around for the 4.10s, they do pop up every now and again.

In all of their wisdom when they built these there is one, sometimes two bolts that won't come out due to hitting the front mount for the LCA on either side. Remove the nut almost all the way out, cut the bolt off and then drop the diff out. When reinstalling you just put the bolt through the other way and no issues .
Funny thing is I have seem them both ways, some have one bolt others have both that you have to cut.
At least thats the way I have found to do them, lets you drop the whole case with mounting brackets down out.
Something else that you also have to swap is a bracket on the left side of the 44.10 diff. 4 cylinders and CRDs set to the left more than gassers so the brackets are different, have to make sure to flip them