What is a NP231 Crawl Box?

[SSTJ]

I'm entering my info into GrimmJeeper.com and am confused about Step 3: Underdrive. One of the options is "NP231 Crawl Box". Is this some sort of add-on option for my NP231-J transfer case? Googling now, but you all are always so much more helpful.

 

[Jerry Bransford]

No worries, your TJ 231 does not have a crawl box. Skip over Step 3.

 

[jjvw]

The RubiCrawler for the 42rle is an example of a crawl box or an additional underdrive.

 

[mrblaine]

I'm entering my info into GrimmJeeper.com and am confused about Step 3: Underdrive. One of the options is "NP231 Crawl Box". Is this some sort of add-on option for my NP231-J transfer case? Googling now, but you all are always so much more helpful.

The crawl boxes are made from the planetary and a new or some portion of the 231 case to put it between the back of the trans and front of the current t-case. Adds about 8" of length and a 2.72 additional ratio which gives you 1-1, 2.72-1 2.72-1 and 7.39-1 for ratios depending on how you shift each case.

 

[SSTJ]

Nice, thanks all.

 

[Gunnersmate]

The crawl boxes are made from the planetary and a new or some portion of the 231 case to put it between the back of the trans and front of the current t-case. Adds about 8" of length and a 2.72 additional ratio which gives you 1-1, 2.72-1 2.72-1 and 7.39-1 for ratios depending on how you shift each case.

Is this similar to the the 203/205 doubler

 

[mrblaine]

Is this similar to the the 203/205 doubler

Probably, they are referred to as doublers.

 

[Wildman]

I'm entering my info into GrimmJeeper.com and am confused about Step 3: Underdrive. One of the options is "NP231 Crawl Box". Is this some sort of add-on option for my NP231-J transfer case? Googling now, but you all are always so much more helpful.

There use to be a few companies that offered a crawler box kit for the 231 where it used the front section of the t-case with the gear reduction and you cut off the side for the front drive.

It went between your transmission and t-case to act as a reduction box. Most of these companies either are no longer making them or no around anymore.


There are also reduction boxes like this https://www.northwestfab.com/blackboxunderdrive

 

[Gunnersmate]

Anyone interested here is a 203/205 doubler (Crawl Box)
MIght work in in anLJ a TJ might be a little short