"McDoubler" How I built a Jeep Comanche 231 Doubler Transfer Case 231/231 Dual Cases Twin Stick

I wanted a lower crawl ratio for my MJ, so here's the "Mcdoubler." Dual 231J cases allow me to run at 3 speeds, 1:1, low range at 2.72:1 and double low at 7.39:1.
Here’s where I purchased the kit
froehlichsuspension-com.3dcart...

[@elivilla1759]

Damn, wearing flip flops and reassembling t cases fast as hell, thats a man that knows what he’s doing

[@Hvacoffroad]

Just showing everyone how to get things done in flops lol

[@1298Jazzy]

Dude you just blew my mind with this mod. I didn't even know this was a thing!

[@beargibson3100]

When I was young my dad and another guy had a small logging outfit. Instead of buying a skidder they took a 2 ton dump truck and put a boom winch on it. To get some really low gear ratios the mounted a second transmission behind the first one. Tail shaft to tail shaft. Output yoke on the input shaft of the second transmission. They called it the creature.

[@Hvacoffroad]

I love custom creations like that. From conception to application.

[@phillipgwin5032]

Great video, I am building a Comanche too, can't wait to see how you fit this in.

[@Hvacoffroad]

Length no problem, i did have to cut some of the floorboard on the driver side to make room for case.

[@jonaathansimons7938]

I have a very similar Box4Rocks 231/231 in my XJ. Rear case upgraded to 6 pinion & wide chain. Sourced dual cable shifters from JBFab. With an AW4, double low (through 4.56 axles) is 94:1. Almost takes two feet to hold it stopped even @ idle. Great in super technical rock crawling and fantastic for deep snow wheelin'! Best part for me is the 2lo. Makes life much easier in tight PNW mountain trails! I didn't add an extra mount, but did fab up a belly skid since the transfer case hangs a bit lower. Prb'ly lost two inches of breakover. Livable for what I do.

[@Hvacoffroad]

I can’t wait to try my setup out. Thanks for the encouragement. So you used the stock transmission mount only? No support for the rear two cases?

[@jonaathansimons7938]

@@Hvacoffroad That is correct sir. G'luck! 🤠

[@ITSNOTMEITISYOU]

Great stuff thanks for the detailed video! Plan for my Comanche is 241/241 since running a SBC with a 4L 80e

[@Hvacoffroad]

I haven't had any strength issues with my 231/231 combo behind the 4.0. But I agree, I'd feel more comfortable with a 241/241 behind a healthy v8.

[@samuelhansen516]

I did almost the same thing but with a dana 300 as the transfer case. Its a great set up and with the 6 pinion it really makes it strong.

[@Hvacoffroad]

I consider the d300 as the rear case. I couldn’t fine one at a reasonable price and I heard of them leaking a fair amount when you flip them. Did you flip to driver drop?

[@MacsSpecialitiesMotorsports]

Nice work man!

[@Hvacoffroad]

Appreciate it!

[@kevsbuild2807]

Good to see you have your safety sandals on.

[@Hvacoffroad]

Always. Standard issue in Florida, haha

[@H4rleyBoy]

@@Hvacoffroad Hope you were issued with safety squints too.

[@Hvacoffroad]

@@H4rleyBoy my first line of defense

[@peterhatch1583]

Truly a great video thks!!

[@Hvacoffroad]

Glad you enjoyed it!

[@alexandredemers9156]

You got me subscribed 😉

[@Georgiaguntraining]

Great stuff. Thanks.

[@Hvacoffroad]

Glad you enjoyed it!

[@alantownsend5468]

I've watched your videos on this multiple times, thank you for all the detail you've shared. How is this arrangement holding up for you?

[@Hvacoffroad]

It's great so far. I've taken it out 3 times and love it. I've tried some obstacles in "low" and couldn't make it over, lock it in "double low" and it crawls up. The extra low range is awesome! I hope to get some videos soon.

[@wagnermagalhaes00]

Nice job👍 Is it possible to convert a NV 231J t-case to 4x4 FULL TIME?

[@terrrs01]

I’m digging this! I think I’m going to order a kit. My main question is my entire drive line are 21 spline shafts. If I’m thinking correctly I can still use my front 21spline input in the “doubler” but I’ll have to change out my rear t-case input shaft to a 23spline to match the new chromo shaft that comes in the kit? Thanks ahead of time!

[@Hvacoffroad]

If I remember correctly, you can choose 21 or 23 spline. So you could use your existing trans and turn you tcase into a doubler. Then purchase another 21 spline tcase to keep it all the same. Just make sure you order the appropriate planetary gear if you arguing to upgrade, which I would recommend.

[@bonespur2728]

For the shifter fork rod, they should just drill and tap to put a large set screw through the new case half, would keep it from sliding back and forth.

[@Hvacoffroad]

I believe that is how they originally built the kit, but had issues with the set screw working it's way out. I haven't opened the case since I built it, but I also haven't had an issue with the shift fork. Maybe I'll do a video update and check out the inside of the case, and go over my thoughts after a year of use.

[@clsalcedo99]

Great video, can you do a follow up video of the Jeep crawling

[@Hvacoffroad]

Absolutely. I got a wheeling trip planned in early January. I just have to finish knocking out the details to get the MJ ready for the trip. I hope to get some footage of it on the trails.

[@thehouseman81]

Nice vid....did have 2 questions...what are those 2 open holes on the adapter for? Are they just open? How does the front half get oil...stay oiled? With no pump how?

[@Hvacoffroad]

Thanks. Those 2 open holes in the adapter are for drain and fill plugs. Those plugs didn't come in the kit. And that is also how the front half stays oiled. It's fluid is independent from the rear case and the transmission.

[@calebsurpass]

hey man, thanks for the cool video. Didn't think it was possible - - was wondering if you have any thoughts on this idea - I have the idea of buying the teraflex 2wd LO range kit and doing this set up for the greater crawl ratio .. would this doubler kit allow for the 2 lo range kit? It's not necessary, just a nice extra and since the case will be taken out and torn apart - figured it wouldn't hurt to throw it in there. if it can work thanks and best of luck with the build. For sure will be bad ass

[@Hvacoffroad]

With this doubler kit, you get a 2wd lo option. Since the doubler doesn’t connect to the front driveshaft, you can lock the front case in 4lo, and you get your “2lo.” Only the rear case will actuate the 4wd options.

[@calebsurpass]

@@Hvacoffroad i seee, thank you for elaborating, that’s awesome. And thanks for the patience in my learning 🙏🏽 lol I got a lot to learn

[@brannonmiller5548]

First time watching a double installed on a T-Case. It looks like a about the doubler could be 3 to 4 inches shorter. I realize that would mean cutting out a section between the two sides, machining both flat and welding it back together. Does that sound right or am I missing something?

[@Hvacoffroad]

I can see that being a possibility and beneficial for something with a short wheelbase and you have the ability or access to the machine work.

[@evinsteven13]

Did you end up getting this installed? I got a Comanche project and am trying to decide if I want a doubler, or just rebuild the core 241OR I have.

[@Hvacoffroad]

Yep, installed and wheeling. Here's a video from a few months again with it in the MJ kzbin.info/www/bejne/bojHl6qLlNKsnsk

[@danaweinberg1585]

What's the fluid capacity of the crawl box? With the NP231 being 2.2 quarts, I'm guessing the crawl box takes about 1.5? Did you just fill it up to the bottom of the top fill hole? Thanks for your help.

[@Hvacoffroad]

I never calculated the capacity, but I would think 1.5 quarts is close. Yep, I just fill until it reaches the bottom of the fill hole.

[@99skatebaker]

I’m really interested to know how much a setup like this runs ya, my 231 needs rebuilt so deciding on this or a rubicon 4:1 I can get local

[@Hvacoffroad]

If I factor in everything to upgrade the case, the stub shaft/doubler kit, sye, and the extra case, and shifters I'd say I'm close to $1300-$1400. This doesn't include driveshafts and mounts. I swapped to tons at the same time I did the doubler, so I needed shafts anyways.

[@rcurrie88]

Do you have any videos or pictures of your shifter setup in the jeep?

[@Hvacoffroad]

Yep, here you go. kzbin.info/www/bejne/eJjIaJRqhN-GhdU

[@bryanlooper9023]

What about fluid for the front case, or is it not needed?

[@Hvacoffroad]

Yes. There is a hole for a drain and fill plug. The plugs were not included in the kit unfortunately. 1/2”npt threads.

[@42Goopy42]

Do you know if a 242 would work as the rear case? I like having the full time option and 2lo sounds pretty usefull as well.

[@Hvacoffroad]

I’m not sure with the 242. The kit I bought said it was good for a 231 or Dana 300. You had to pick which rear case, so I imagine there’s some differences in the output shaft. I feel the 242 would bolt to the adapter, but not sure about the shaft specifics. That would be a question for the manufacturer.

[@42Goopy42]

@@Hvacoffroad thanks figured it was worth a shot before calling them

[@johnhufnagel]

TeraFlex sells/sold 2LO shift kits for the 231 and 242 cases. sadly not for the 241 though.

[@42Goopy42]

@@johnhufnagel thank ya, I assume it would work seeing as you can swap 242 and 231 within spline counts but thats also a good option

[@SLJ-0331]

Getting ready to install a doubler in my Jeep how do you know which cut of gears to order

[@Hvacoffroad]

There seems to be a gear change around the year 1996. The pre96 gears are more squared off on the teeth, the post 96 have more rounded edges.

[@lol123321123321]

How do you keep it oiled? Just a splash lubricating system with custom fill and drain holes?

[@Hvacoffroad]

Yep, the kit has a drain and fill plug machined into them to service the fluid.

[@lol123321123321]

​@@Hvacoffroad ahh, thanks for the reply. I've been contemplating this mod for years now lol. Cheaper than an atlas 🤣

[@BackyardBeaters]

Where did you get the doubler kit?

[@Hvacoffroad]

This is the kit froehlichsuspension-com.3dcartstores.com/NP231-Doubler-Adapter_p_14.html

[@ApocGuy]

im total noob in mechanics, but this is just two T-cases paired together, right? damn smart way to further reduce RPM and thus speed.

[@Hvacoffroad]

You got it. Ultra low crawling.

[@wvrockcrawlers172]

What did you use to weld the block off plate

[@Hvacoffroad]

I didn't haha. I had a buddy aluminum weld it. I've seen others use jbweld on on the plate.

[@ernestjohnson1807]

I had to flip the range box for drive shaft clearance. The part u cut off and cap would hit the cv joint.Why did you do a 231 to 231 and not a 231 to 300?

[@ernestjohnson1807]

Also I would not add a mount like people say to. Every person I have seen add a mount to the 231/231. Has broken the front output off the transfer case.

[@Hvacoffroad]

It doesn’t look like my cv will hit the range box, the way I have it clocked. Time will tell. I’ll keep that noted for flipping the box. That’ll add a new set of hurdles to overcome. I thought about the d300, but they seem to be scarce in my area, and pricey when you find them. Then if you take into account the flip kit, shifters, etc, I’d be close to the price of an Atlas.

[@Hvacoffroad]

@@ernestjohnson1807 I agree. I don’t like the way that I have seen people attach the mount on the rear case. I’m thinking attaching to the range box and let them rear case “hang,” similar to the factory setup. Then eliminating the factory transmission mount. So three mounts total. What’s your thoughts on that?

[@ernestjohnson1807]

@@Hvacoffroad I am running a atlas in a zj now. I don't even have the xj buggy the dubbler in any more. I ran factory mounts on a modified cross member. I never had issues with it other the the fact factory rubber don't last long. On my current set up the atlas is just bolted to the transmission. I have not had any reason to add a mount on that either. I think a lot of people over build simple thing like this because of bad information on the internet.

[@Hvacoffroad]

@@ernestjohnson1807 Yeah everyone's an expert on the internet lol. Hard to know who to believe. I have a modified crossmember that incorporates long arm mounts and uses the stock trans mount. I may just stick with that if you didn't have any issues. Less work that way. Just a little concerned with the extra length and leverage from the doubler setup hanging off the back. Your 231/300 was shorter, but the wight was about the same(80lbs.) Thanks for the info!

[@omar1990seattle]

Can you do this with any transfer case? Like a 98 jeep Cherokee 4.0 i6 4x4?

[@Hvacoffroad]

My MJ had a complete drive train from a 99 xj. The case i used to make the doubler range box was the original 99 case. So my setup was one year off of yours.

[@omar1990seattle]

@@Hvacoffroad ok, where did you get the cover plate and shaft from? I think this is a really cool set up good job

[@Hvacoffroad]

@@omar1990seattle froehlichsuspension-com.3dcartstores.com/NP231-Doubler-Adapter_p_14.html

[@omar1990seattle]

@@Hvacoffroad thanks for sharing , this is soooo cool

[@waltermartinez3939]

where can i buy the plate and adapter

[@Hvacoffroad]

froehlichsuspension-com.3dcartstores.com/NP231-Doubler-Adapter_p_14.html

[@brentfellers9632]

I have a 231 and a 237, would the 237 work for the range box?

[@Hvacoffroad]

I’m not familiar with the 237. The kit I purchased was for a 231j.

[@brentfellers9632]

@@Hvacoffroad oops 247

[@Hvacoffroad]

@@brentfellers9632 I don’t believe the 237 will work.

[@brentfellers9632]

@@Hvacoffroad I totally had my numbers scrambled. I have a 95 242xj and a couple of 249s out of 94, and 97 zj, so I'm thinking of keeping the spare 249 to make a doubler from it... Great video btw,and engagement 👍

[@dualsportrider3221]

Why not a 231 w a 300 on the back? 6" shorter than a stock 231

[@Hvacoffroad]

D300 would be a great option too, and i considered it. But after calculating the cost for the tcase, shifters, and the dreaded leaky flip kit, I decided against the D300. The 231/231 was cheaper and I wasn't worried about the length with the MJ wheelbase. If I already had a D300 on hand, I probably would have just slung it behind the 231. Used D300 around here are overpriced.

[@hlcwolfhunt9010]

Anyone do this to a wrangler? Very interested

[@Hvacoffroad]

I think your rear driveshaft length would be an issue with a YJ. But if your stretch the rear..... that would be real interesting.

[@channghiem5012]

update?

[@Hvacoffroad]

That's a good question. I'm going to make a video on that.

[@hiltonpineda7378]

can this be done in a JK?

[@Hvacoffroad]

I haven't seen a kit for the JK setup.

[@desertoasislawncare8342]

Who makes the kit ?

[@Hvacoffroad]

froehlichsuspension-com.3dcartstores.com/NP231-Doubler-Adapter_p_14.html

[@jesuscorralesrosales4070]

muy buen video donde se puede conseguir la barra para adaptarla

[@Hvacoffroad]

This is where I got the kit for the doubler. froehlichsuspension-com.3dcartstores.com/NP231-Doubler-Adapter_p_14.html

[@DennisWHJr]

@@Hvacoffroad how long it take for delivery from time of order?

[@Hvacoffroad]

@@DennisWHJr I’d say close to 3 weeks. They had to make the shaft when I ordered.

[@Hvacoffroad]

About 2-3 weeks when I ordered it

[@meeperrxp]

Why polish this chain driven turd? Lol atlas aren’t that expensive and a smaller package

[@Hvacoffroad]

Definitely agree the Atlas is stronger and smaller. But, they are a few months out on builds and I had most of the parts for the doubler on hand. Length wasn’t an issue, just had to build the extra crossmember. Never had an failure with any of my 231s I’ve wheeled, so I wouldn’t call it a turd.