NP231 Transfer case lower gears

ole heep · Oct 30, 2020

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 · Oct 30, 2020

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.

ole heep · Oct 30, 2020

Would the NP241 be a direct bolt up?

The Luke · Oct 30, 2020

ole heep said:
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.

ole heep · Oct 30, 2020

Appreciate the info, Thanks!

Big David · Oct 30, 2020

The Tera Flex 4.1 had a top rev limit of 2500 RPM or something like that.
They were known for exploding.

The Luke · Oct 30, 2020

Big David said:
The Tera Flex 4.1 had a top rev limit of 2500 RPM or something like that.
They were known for exploding.

Hell, I'd blow that up loading on my trailer.....

ole heep · Oct 30, 2020

Big David said:
The Tera Flex 4.1 had a top revlimit of 2500 RPM or something like that.
They were known for

I was wondering what the reason was that they quit making them.

TBItoy · Oct 30, 2020

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 · Oct 30, 2020

TBItoy said:
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 · Oct 30, 2020

ole heep said:
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 · Oct 31, 2020

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 · Oct 31, 2020

ole heep said:
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 · Oct 31, 2020

nightcrawler4x4 said:
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 · Oct 31, 2020

TBItoy said:
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 · Oct 31, 2020

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.

muddinmetal · Oct 31, 2020

nightcrawler4x4 said:
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.

Manual trans?

nightcrawler4x4 · Oct 31, 2020

Yep for now going with an Aw4 with a Toyota bellhousing.

muddinmetal · Nov 1, 2020

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 · Nov 3, 2020

TBItoy said:
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 $.