Here is some actual feedback. I have them in my D20. I have had them for several years. The guy who controls the product is Jack O'brien...or at least use to control them anyway. I haven't talked to him since the 35th EB aniversary. I use to live down the street from him and got his first set of kryogenitcally treat gears.
I am running 4.11 in the diff's with an NP435 which is 6.69:1 and the 4.85:1 Obrien gears (Also called Rock eater gears or JP eater gears or as originally called Jeep eater gears). I run 36" swamper Sx's with lockers and alloy axles all the way around.
Anyway, That gives me a final ratio of 135:1. I rock crawl 95% of the time. I beat the hell out of my rig. I went wheeling in the year 2003 on 28 of the 52 weekends. I have only missed 2 weekends this year.
How do I like the gears. I love them. Has anything broke...yes. I spun one rear output shaft...in fact the spud which I tore off is the one Jack brings to vendor shows for a demo. But...how did it break is important. I had the vehicle standing straight up trying to crawl a ledge and bouncing up and down. The shaft very likely snapped due to binding at the CV as much as anything. The point being...I was not being nice. After It broke, I replaced it with another stock shaft and ran that without failure for several seasons through places like little sluice on the Rubicon and many more very difficult spots. I have since replaced it with an Advance Adapters HD shaft (32 Spline version...Sorry Chuck...it was nearly free so I went that way). I have had no issues
Things to consider with an NP435...There is a big difference in Granny low to 2nd, but not too bad. I would like to add a NP203 doubler not for steeper gears, but more choices. Overall though, I have not found that to be a significant issue and 135:1 is plenty low for 36" sx's that I run now.
Some will say just buy an Atlas. If I had the money at the start to do that, I would seriously consider it...but don't for get adapter and D-line changes/costs. If you are like most of us, getting the JP gears first and adding the output shaft later may make it possible.
I will say this...going from 2.46:1 to 4:85:1 is like night and day when crawling in rocks. In many ways, you will create less stress on the drive train because you are going much slower and can control what the vehicle is doing much better. Further, you will learn you tip over point and be able to get much closer to them with a steeper gear. This will make you a better driver. But, by doubling the T-case ratio in low you are definitely adding pressure down stream and a heavy foot in the wrong spot can and will break things.
As for the Automatic Tranny issue of not being able to stop... I can't confirm this, but I could see that being an issue with some torque converters...Fact is I sometime have trouble stopping mine/getting the engine to die just using the brakes with the clutch out.
Hope this helps
