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Old 07-14-2004, 11:05 PM   #1 (permalink)
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np231 doubler

Its booty fab at its best but someone somewhere may find it useful, I posted it like ayear ago but busted mine on a nice pointy rock and had to start over, so here it is with pics and better detail.


homegrown np 231 doubler how to:

This is what the finished product will look like....kinda.



I used a 207 front case (freebie) this time around, after destroying my original set-up (231-231) on a nice gooner rock, I was just to lazy to pick up the free 231 my bud jay was giving me. So either a 207 or 231 can be used to do this. The second (rear) case however must be a GM 207 or 231 because it has the 27 spline input that matches both the jeep and chevy output.
Now I wont get into how to take the damn things apart (if you cant figure that out you shouldnt be attempting this mod) but you start this project by gutting the front case, leaving ony the shifting mechanism and planetary gears. This here pic is what come out or off of my front np207....except for that c-bushing that snuck into the picture when I wasnt looking



You also must cut the front drive output off the front case in order to clear the front drive shaft. I cut mine off a lil past the oil fill plug, so your left with a gutted and cut case that will look like something like this:





Well, its time to figure out how the hell were gonna get the slippery stuff to stay in this booty fabbed psycho butchered thing. We must block off the big hole we left in the side of the case, I used a piece of aluminum to do the job. I cut it somewhat shaped like the hole in the case, leaving little tabs that will be bent and used to anchor it to the case with a few self tapping screws, hopes are high that a generous amount of smooge will keep the slippery liquid inside. This ugly mess is what we got so far:



Surely a better job could have been done here but unless she's a bleeder, I myself am happy with that twisted tortured chunk of metal. Next in line is starting to put the front case back together. You will only need to put the shaft back in and the first tailshaft chunk of housing back on (like in pic below). The rear most chunk of the t-case housing can live a better life by being recycled into a beer can. So please recycle, it does more than just help the environment. Now...where were we? Oh Lets put these bitches together, leaving the future beer can off the case you may now have to cut a chunk of the tailshaft off, I had to remove 1 1/8" from my shaft. Depending on your adapter you may need more off or less (*see below for how to determine amont that will need cut off*). You should now have a chunk of scrap that looks something like this



I used an original GM tranny-tcase adapter to mate these things together. This piece (dont know no part# or what all they came on) will bolt directly to the rear case, it looks like this...kinda



Once bolted on the rear case slide the two cases together. This is how you will know how much of the shaft you must cut off the front case (this is the shit I was talkin about up above). Once all cut and slid together its as simple as getting the things welded together, just decide how you want the rear clocked. I have mine up a lil for better clearance but you will likely need a small body lift, i have 1.5". The cases will still come apart, the adapter will unbolt from the rear case and they will slide right apart but adapter becomes a part of the front case. It in no way interferes with disassembly of the front case should you ever need to take the thing apart.

IMPORTANT TO NOTE
1) The compound low WILL destroy motor mounts almost immediately, solid mounts or even a ghetto chained motor is an absolute necessity.
2) You will need two crossmembers, I also recommend adapting a brace from the front case to the rear portion of the rear case. GM used a brace that went from the tranny to t-case on a bunch of vehicles, this can be adapted easily.
3) The cases will shorten the rear driveshaft and lengthen the front by about 15"...give or take, so it is not ment for your wrangler...
4) This doubler will give you a low-low of
np231 to np231= 7.4:1

np207 to np231= 7.1:1

np207 to np207= 6.8:1

slap a 4:1 in one of the puppys and it gets crazy low. This is not a simple mod but it aint a real hard one either. The effort is well worth it, your chevy or heep vehicle will crawl with the best of em, and if you find a case cheap (or free like i did) you wont break the bank to do it.

Last edited by psychobilly; 07-14-2004 at 11:12 PM.
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Old 07-15-2004, 12:43 AM   #2 (permalink)
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Ghetto But you get an A+ for originality
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Old 07-15-2004, 07:17 AM   #3 (permalink)
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You are a crack head. Weld that plate on and you wont have any leaks at all.

But good job.
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Old 07-15-2004, 08:57 AM   #4 (permalink)
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Yet another reason to learn how to weld on aluminum and to ask yourself




Got wheelbase?

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Old 07-15-2004, 10:38 AM   #5 (permalink)
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1) I think you should make something even longer. Less drive shaft is a good thing right? edit: just realized the front shaft will be super long
2) All that and not even a SYE
3) Take all that time, energy, and ability and do something that makes sense
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Last edited by MossMan; 07-15-2004 at 10:43 AM.
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Old 07-15-2004, 11:02 AM   #6 (permalink)
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You couldn’t pay me to run that setup in my rig....

....but I would place a bet on how long it will take to grenade!
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Old 07-15-2004, 11:16 AM   #7 (permalink)
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last one made it over a year, then a lil rock took her out or it would still be going
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Old 07-15-2004, 11:44 AM   #8 (permalink)
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If you got the wheelbase, might as well use it. Not everyone runs Jeeps and Zukis.
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Old 07-15-2004, 12:14 PM   #9 (permalink)
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Cool. If the suit fits, wear it.

I thought about doing somthing similar when I was with 35s, but I'm just going to do an entire driveline swap now.
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Old 07-15-2004, 03:32 PM   #10 (permalink)
 
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Ya ..I agree ..it's total ghetto but shit if you got the wheelbase and what about $150 for everything ?...Sounds good for a budget truck to me .Add a cut and tap SYE and you have a pretty good set-up for like $200-$300.I wouldn't feed it v8 power but a 6cyl and lots of axle gear ...should last long enough to more than pay for itself.
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Old 07-15-2004, 05:30 PM   #11 (permalink)
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Cool! I never threw the old 207 away, knowing I could always use it for something interesting GM NP231, is that like in S10's?
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Old 07-15-2004, 05:38 PM   #12 (permalink)
 
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Yes!
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Old 07-15-2004, 06:22 PM   #13 (permalink)
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Quote:
Originally Posted by Amund
Cool! I never threw the old 207 away, knowing I could always use it for something interesting GM NP231, is that like in S10's?
Yes same transfercase. A majority (if not all) of the NP231Cs have a wider chain.
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Old 07-15-2004, 06:41 PM   #14 (permalink)
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Couldnt any GM th350 or 700r4 t-case be used for the rear case?
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Old 07-15-2004, 07:16 PM   #15 (permalink)
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Ignore the haters that is way cool. Getto but still a cool idea. I'm confused on what you used to bolt them together with, what adapter?
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Last edited by roctoy; 07-15-2004 at 07:23 PM.
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Old 07-15-2004, 08:06 PM   #16 (permalink)
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I've actually run some variation of this for some time now (like 4 years or so) one with a 208 and 231 and then 231-231 and now the 207-231. Strength of the internals or shaft was never an issue, remember gm cases are 27 spline. The 208-231 got busted up when my motor mounts tore apart, split the housings into pieces cuz the cases were mounted kinda solid (1/2" of rubber on each side with bolt through the mount) then the 231-231 doubler got busted up real good by some rocks. I didnt have a skid cuz I was lazy and the cases tucked up inside the frame rails, didnt think I would really need it...oops. As for the shaft lengths, I run an s10, hp 44 front moved 4 iniches to the front and I still have a shorter shaft than a YJ. The rear shaft is like 30-35 or so inches long. This set up is the cats ass in an s10 but may work in a cherokee or any buggy running a 4.3. The adapter was off some gm 4wd tranny, its just the stock gm adapter that goes from the tranny-t-case on some GM's. I dont know what all had them, I know my s10 didnt, this one was on a tranny just sitting in a junkyard, I just looked to see it was the right bolt pattern and pulled it, didnt ID the tranny it was on.


wish I had a good pic of the drive shaft clearance but I dont, this is the best I can do now....its a linky cuz its a larger file

http://img28.photobucket.com/albums/...4/P5290048.jpg

Last edited by psychobilly; 07-15-2004 at 08:22 PM.
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Old 07-15-2004, 08:07 PM   #17 (permalink)
 
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take that pic to your local wrecking yard and someone there will know what it is.
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Old 07-15-2004, 08:47 PM   #18 (permalink)
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The adapter you used looks just like the ones I have, which is any s-series automatic(4bolt) to the s-series t-case(5bolt). Be aware that in, 96 I think, the s-series went to a 6-bolt t-case wich is the same as Jeep uses. The input is still 27 spline though. If you know about where GM used things, such as the 700-208, you can make many different combos using this idea. I'm working on a 207-233 setup in my blazer but I bolted the adapter where billy welded. I can't weld aluminum good enough, and I figure grade 8 bolts and 3/8" plate is strong enough.
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Old 07-15-2004, 10:01 PM   #19 (permalink)
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excellent. could you use a gm 27 spline 205?
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Old 07-15-2004, 10:40 PM   #20 (permalink)
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Quote:
The adapter you used looks just like the ones I have, which is any s-series automatic(4bolt) to the s-series t-case(5bolt). Be aware that in, 96 I think, the s-series went to a 6-bolt t-case wich is the same as Jeep uses. The input is still 27 spline though. If you know about where GM used things, such as the 700-208, you can make many different combos using this idea. I'm working on a 207-233 setup in my blazer but I bolted the adapter where billy welded. I can't weld aluminum good enough, and I figure grade 8 bolts and 3/8" plate is strong enough
Excellent info Jamus! I didnt know the s10 went to a 6 bolt t-case?? I thought my adapter was a 700r4 but I wasnt sure enough to post it up, my nv3500 doesnt have that piece.

I dont see why somthing like this couldnt be done with the 205, but then again I never messed with one of them, someone try it.
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Old 07-16-2004, 07:47 AM   #21 (permalink)
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Yeah, I figured if this twin case setup didn't work out, I'd get one of those Jeep 231 4:1 deals that replace the front half of the case. Then swap in my Cheby 27 spline input. But I had the extra 207 laying around, so like $20 invested compared to around $1,000. Think I'll try this first, haha. On a side note, anybody know id a 208 is 27 spline output too? Ir is it 32 spline, my 233 is 32 spline output.
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Old 07-16-2004, 03:22 PM   #22 (permalink)
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cool Idea unit!

I've been looking at doing the same setup with a 13-56 and a 208.
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Old 07-16-2004, 05:09 PM   #23 (permalink)
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What options do you have as far as shifting, same as a marlin?
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Old 07-16-2004, 05:32 PM   #24 (permalink)
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Anyone know if a GM 208 has the same 27 spline input?

Justin
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Old 07-16-2004, 08:33 PM   #25 (permalink)
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If the 208 came behind a 700R4 it would have to have 27 spline input correct? I don't know about std tranny but I see no reason for it to differ.
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