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NP231 Transfer case lower gears

ole heep

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May 31, 2013
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285
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Louisville Tn
Can anyone school me on what to do to get lower gears in the transfercase of a NP231?
I think that is what a friend of mine has in his 1994 YJ 4CYL 5speed.
He is an older guy that wants 36"tires. He has 34" LTB now. I am pretty sure he has 488 gears in diffs already and wanting lower gearing for off road.
I seen Tera Flex had a kit but looks like it is discontinued.
Any suggestions or options?
 

The Luke

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Joined
Dec 12, 2010
Messages
4,133
I don't know if anyone is even making 4:1 gears for 231's anymore.

Basically your options are:

1. Try to find a used 4:1 set

2. 231/231 doubler(or some kind of junkyard doubler combination)(probably too long for your setup)

3. d300 flip kit and 4:1 kit(if anyone still makes them, or try and buy used)

4. buy a rocktrack np241 out of a rubicon(there's one on here for sale right now) I personally think they are overpriced. But given that there aren't many purchasable options, that might be the way to go.

5. Atlas.
 

The Luke

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Dec 12, 2010
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4,133
Would the NP241 be a direct bolt up?
Yes and no. The case bolts up, but the input shaft will need changed out. On the TJ's and LJ's the 4cyl's were 21 spl and the 6cyl's were 23 spl. The automatic JK and JL rubicon cases had external splines on the input shaft. The manual had a standard 23 spl. The good news is You can pull the input shaft out of his current 231 and swap it in to any of the case I mentioned. WORST case you might have to swap the 241 input bearing also, But that's an easy swap if needed.
 

TBItoy

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Jul 2, 2010
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Dodson Branch, TN
I'd be looking at a blackbox/ecobox/klune reduction box + the 231 w/ slip yoke eliminator, or a flipped D300 (stock geared)

It's pretty dang nice having 2 low range ratios for poking around in a manual trans 4cyl rig
 

ole heep

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May 31, 2013
Messages
285
Location
Louisville Tn
I'd be looking at a blackbox/ecobox/klune reduction box + the 231 w/ slip yoke eliminator, or a flipped D300 (stock geared)

It's pretty dang nice having 2 low range ratios for poking around in a manual trans 4cyl rig
I am definitely familiar with doublers from the Yota world. I have 203/205 in my buggy now.
But I know nothing about the black box, Klune, etc.... other than hearing there names. What I take to make them work with his setup? Guessing Driveshafts and?
 

The Luke

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Dec 12, 2010
Messages
4,133
I am definitely familiar with doublers from the Yota world. I have 203/205 in my buggy now.
But I know nothing about the black box, Klune, etc.... other than hearing there names. What I take to make them work with his setup? Guessing Driveshafts and?
I forgot about those Nick. The ecobox is a good option. It’s just a range box.
 

nightcrawler4x4

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Jan 15, 2015
Messages
97
Location
foristell, mo
My buddy and I are collecting parts to do an Aw4, eco box, and 4:1 Dana 300 swap. This is going into a Toyota but same concept lower gears behind a 4 cylinder also gotta have front dig.
 

TBItoy

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Joined
Jul 2, 2010
Messages
10,606
Location
Dodson Branch, TN
I am definitely familiar with doublers from the Yota world. I have 203/205 in my buggy now.
But I know nothing about the black box, Klune, etc.... other than hearing there names. What I take to make them work with his setup? Guessing Driveshafts and?

basically all of those options are a reduction housing that holds a NP231/241 planetary (the Ecobox is a DIY kit where you use a donor case). IMO it's a much cleaner and higher value setup than the cut-case NP doublers that used to be common.

The advantage of using a D300 (or even a NP205) behind a reduction housing/crawl box is that the overall size is about the same as a stock NP231/241 (because of the the case configuration of a parallel gear driven case vs a chain drive case).

it works out better in Jeeps which tend to have long front driveshafts and short rear shafts.


I know a NP205 is super-overkill for a 4cyl Jeep (and heavy) especially one that sounds to be stock-ish. But a Ecobox+Ford 205 is the cheapest, strongest, shortest setup you'll be able to to put together for a driver-drop 4 speed tcase.
 

TBItoy

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Jul 2, 2010
Messages
10,606
Location
Dodson Branch, TN
My buddy and I are collecting parts to do an Aw4, eco box, and 4:1 Dana 300 swap. This is going into a Toyota but same concept lower gears behind a 4 cylinder also gotta have front dig.
I'd doubt you'd need the 4:1 in the 300 with the auto trans + 2.72 in the crawl box + 2.62 stock 300. Just a thought.
 

ole heep

Well-Known Member
Joined
May 31, 2013
Messages
285
Location
Louisville Tn
basically all of those options are a reduction housing that holds a NP231/241 planetary (the Ecobox is a DIY kit where you use a donor case). IMO it's a much cleaner and higher value setup than the cut-case NP doublers that used to be common.

The advantage of using a D300 (or even a NP205) behind a reduction housing/crawl box is that the overall size is about the same as a stock NP231/241 (because of the the case configuration of a parallel gear driven case vs a chain drive case).

it works out better in Jeeps which tend to have long front driveshafts and short rear shafts.


I know a NP205 is super-overkill for a 4cyl Jeep (and heavy) especially one that sounds to be stock-ish. But a Ecobox+Ford 205 is the cheapest, strongest, shortest setup you'll be able to to put together for a driver-drop 4 speed tcase.
Thanks Nick!
I will definitely pass this information on.
 

nightcrawler4x4

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Joined
Jan 15, 2015
Messages
97
Location
foristell, mo
With 538s, dual 2.28, and 39 reds it’s not enough. Could try it with a stock 300 before putting in the 4:1. Need some training wheels too.
 

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muddinmetal

Guntersville, AL
Joined
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Guntersville, AL
Adding an automatic transmission to the mix will effectively "double" your gearing due to the torque multiplication of the converter. A rig with an automatic trans and a 75 to 1 crawl ratio will feel like a manual rig with a 130 or 150 to 1 ratio.
 

Poester

New Member
Joined
Oct 25, 2020
Messages
1
I'd be looking at a blackbox/ecobox/klune reduction box + the 231 w/ slip yoke eliminator, or a flipped D300 (stock geared)

It's pretty dang nice having 2 low range ratios for poking around in a manual trans 4cyl rig
Great suggestion. 205/Ecobox here. less than $1500 including drivelines. still have 2.72:1 which I actually use most of the time in big rocks with less than optimal grip. Plus in the Sand Hollow / mud/ snow. 5.33:1 just cruises up @ 1k RPM. Doesn't shift as smooth as an Atlas but its half the $.
 

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