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View Full Version : Adapt 4x4 Th350 in Place of 700r4


HotRodPC
04-30-2011, 04:06 AM
As most of us know already, Tranfer Case adapters for 700r4 usually fit Th350 also, and vice versa. This is because the tailshaft housing bolt pattern is the same and so is the output splines meaning, they'll share the same driveshaft yoke.

Here is where the 2 differ and why you simply can't swap a Th350 in place of a 700r4 and vice versa. The reason is, because the 700r4 is longer in the main case before it gets to the tailshaft. So what happens, is the crossmember needs to come forward if your using a Th350 instead, and also you'd need a new driveshaft since it would be short.

Today I happen to be at Pull a Part and seen a 4x4 Burb. It was gutted, except I could see the trans and tranfer case hanging down from a distance. I looked inside the Burb and seen the gear shift indicator was D 321 telling me it was originally a 700r4 which is correct for an 85. Just happened to look under it and seen a Turbo 350 and NP241C. Walked off and then all of a sudden it him me. I bet there is an adapter for Th350 to 700r4. There sure was. Its an expensive adapter. I've been told about $225. It took me about 15 minutes total to get it and I bought it for $5. I also got the hardware that goes with it.

http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2711&d=1304147171

HotRodPC
04-30-2011, 04:09 AM
So what's the purpose of downgrading to a Th350 from a 700r4??? Well, calling it downgrade is a matter of opinion, and actually depends on what your intend to use the truck for if its actually a downgrade. Most will make this conversion for 1 of 2 or both reasons as I am going to for both reasons. #1, the Turbo 350 can be buillt stronger than a 700r4 and much less prone to failure, espeically in an abusive use situation.
#2, the cost to build and beef up a Th350 is much cheaper than building and beefing up a 700r4.

What are you losing by going to Th350 from a 700r4? You're losig a lower first gear. 700r4 is 3.06:1 while Th350 is 2.52:1 first gear. But you do gain a better transition to 2nd gear where 700r4 is very wide ratio to get to 2nd gear. Also of course you lose overdrive, so if its a vehicle intended for alot of highway driving, I'd avoid this conversion.

HotRodPC
04-30-2011, 04:16 AM
What does this adapter do? It equals the length of the T350 to the 700r4.

Here is a pic of the ends of the cases. Pics don't show it, but the bell housings are matched up side by side. Notice the black Th350 case is shorter than the silver 700r4 case.
http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2712&d=1304147754

HotRodPC
04-30-2011, 04:18 AM
Now the adapter is in place. Its not up tight since the bolts are not tightened, but you see the difference and what's going to happen here.

http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2713&d=1304147923

HotRodPC
04-30-2011, 04:24 AM
Are you wondering about that output shaft yet on the Th350??? Notice its to long compared to the 700r4? This is because that is a 2wd Short output shaft for a passenger car. Remember the output shafts on 4x4 transmissions are shorter stubs, just long enough to marry the splines of the Transfer case. With the adapter, an OEM 4x4 Th350 output shaft would be to short. So what's going to happen next, when I get to it, is this 2wd passenger car output shaft will be cut off approximately 1 1/2 inches, so that it will match the overal length of the 700r4 4x4 output shaft. Even with the cut-off, there will be the same or more spline length into the tranfer case and all will be well.

With this adapter, the Th350 will use the same Tcase, Tcase mount, Tcase linkage, crossmember, OEM crossmember position, and also the same driveshafts and yokes both front and rear. Now I'll be able to install either this Th350 or a 700r4 without any other modifications. Man, I love having options without grief. All for Fi Dolla !!! :High 5: It does pay to pay attention and check things out. Sure glad I tripped over that part because I sure wouldn't have spent that kind of money to buy one.

Swims350
04-30-2011, 12:03 PM
they make a kit for this with the right length shaft, northern auto parts has it and somebody else tci i think. It's what I bought. Kit is like $150-200 but it has a dipstick, kitdown cable, the adapter, gaskets, bolts, nuts, flexplate dust cover, the shaft, and maybe other things I forget. I did this in my 88 k1500.

HotRodPC
04-30-2011, 12:13 PM
Glad to hear such kit exists for those who need it. When I hear or see the word "KIT" that usually tells me its to much money. They are usually charging for assembling what you'll need to make a conversion. I got the hard to find part for $5, the rest of it i'll gather the parts up myself and make my own correct length output shaft and save a bundle. I'll run this Th350 in this truck, then the 700r4 I have set aside for this truck, I'll put in the other K10 with the 4.10's when I'll want that OD and the lower first gear. But now, this will take some pressure off me. I've been debating whether or not to pull the 700r4 K case apart for a build, even though it doesn't need it, to beef it up for strength. Now, I can do that since I won't be using it in the 85 K1500 and the rush is off.

89Suburban
04-30-2011, 12:33 PM
Good shit!!!! :High 5:

Manbearpig
04-30-2011, 06:50 PM
Looks like whoever swapped a TH350 in my K5 used the stock 700r4 driveshaft lol

Not sure if it has that adapter

HotRodPC
04-30-2011, 08:57 PM
I assume you might suggest that because 6 feet of yoke is showing??? :roflbow: Really, its only an inch and half or so differant, so if the yoke is long enough, I can see someone might make the stretch. That's all fine if your rear end doesn't drop down to low and pull the driveshaft yoke out, or get the to the point you're barely grabbing splines.
What normally would happen to make the swap, move the crossmember forward, so now your Tcase shift linkage is to long. Front driveshaft is now to long, rear driveshaft is now to short. The mods can certianly be made to make it work and be safe and functional. But, I prefer the adapter because then there is no other mods that would need to be done. I just prefer to do it the right way, so that I have more options. When I finally get this truck running, I am likley to be like I was when I was younger. I changed engine and transmission combinations like I changed underwear. So I love the idea of interchangeability. I might just put the 4.56 CUCV rear end in this truck if I find a 4.56 D44 or 10 bolt 8 lug front axle. If I do that, then I'll go back to a well built 700r4.

Swims350
04-30-2011, 11:21 PM
ok here's some links for ya on the swap kits....

http://www.summitracing.com/parts/TCI-329900/?rtype=10

http://www.bulkpart.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=2&Product_Code=7-35A&Category_Code=350-350Cswitch

http://www.compperformancegroupstores.com/store/merchant.mvc?Screen=PROD&Store_Code=TC&Product_Code=329900

HotRodPC
04-30-2011, 11:29 PM
Great info Swims. So its not as expensive as I was told. I was told $225. Maybe so with shipping and the $204 kit. But I was also told that didn't include the output shaft. I know I can cut an output shaft to work, but now maybe I should go back and get that output shaft out of that trans. Here is the bad deal. At our Pull a Part, they will not sell transmission or motor parts. Example, an output shaft, or a rod and piston. Apparently its against the rules to disassemble engines and transmissions. Someone has already jacked the valve body of the transmission, so I sure don't think I shuld have to pay for a full transmission.

Here is the aluminum adapter only. You'd still have to modify an output shaft.

http://www.summitracing.com/parts/add-7q-35a

HotRodPC
05-02-2011, 06:04 PM
I went ahead and went back to PAP today. I gutted that Th350 to get the shaft out of it too. Knowing they wouldn't sell to me if it was a trans part, the girl asked what it was, I said a driveshaft yoke and she sold it to me for $8. :roflbow:

But as I had said before, I intended on cutting down a short output shaft. Obviously it'll work, so while I have the shaft to the kit out now too, I'll go ahead and match them up, and cut the 2wd short shaft to match so I have a spare incase for a really wierd event, break the output shaft. Of all the years, I've never seen a Th350 break an output shaft, but you never know. Better safe then sorry.


http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2742&d=1304370275

HotRodPC
05-02-2011, 08:21 PM
With my precision set of power tools..... :roflbow:

http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2744&d=1304378419
http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2745&d=1304378436




I was able to cut off the 2wd short output shaft, and make me another modified 4wd output shaft for the Th350 adapter. It doesn't look it in the angle of the pic, but rest assured, its cut square and is the exact same length.

http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2746&d=1304378447
http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2747&d=1304378464

crazy4offroad
05-02-2011, 09:21 PM
As long as you went really slow and didn't overheat it, it should be fine. A wet saw is the best way to go. If the one you cut is the one on the left in the last pic, WOW it has some good, deep splines.

HotRodPC
05-02-2011, 10:07 PM
Yeah, you want to keep them cool as possible so you don't distupt the heat treat temper. Yes, I cut the one on the left. It does actually appear to be in better shape and look stronger than the one that came in the kit. I have already pushed in the Tcase adapter, nice and snug fit and went in and came out very smooth. I'm planning on using the one I cut and keep the other one for a template. You know thats not an OEM 4wd output shaft. Its the special one that works with the apapter. But doing this one, did bring something else to mind. I can easiy cut down a 2wd 700r4 shaft to be a 4wd shaft.

Swims350
05-02-2011, 10:42 PM
a word of advice to you, I bet you already know it or thought of it, but change the t-case input seal.

I did not change mine in the 88 and now it leaks trans fluid into the t-case and overfills it.

It's like a front pump seal on the trans, I guess anytime you have converter out you change it to be sure, guess the same goes for the T-case.

crazy4offroad
05-02-2011, 10:54 PM
I agree, cheap insurance while you're in there. I have a lifetime warranty clutch for my truck and you can bet if the throw-out bearing is bad I would be swapping everything else while I have it apart.

HotRodPC
05-02-2011, 10:59 PM
I intend on ordering a gasket and seal kit and resealing the whole Tcase. I am even considering bearings and races too. Just not 100% sure yet. It feels good, but I know I want to reseal it just because, and I'm thinking while I'm in it and got it apart, may as well do it all.

Swims350
05-02-2011, 11:14 PM
wouldn't hurt a bit, new bearings and races is cheap insurance and you'll have a new trans and new t-case then.

I only wished I would have changed my seal, I hate the thing overfilling the t-case.

HotRodPC
05-30-2011, 04:57 AM
Here is a comparison pic once again. From left to right. The one on the left is the one I cut for the adapter kit to make the th350 the same length as a 700r4 so it will fit both my K1500 and K10 without any crossmember or driveshaft mods since they were both OEM 700r4. I'll keep the Th350 for a back up trans for when I break the 700r4's. The one in the middle is a short 6in shaft which is the size I cut down for the adpater shaft. The short 6in shaft is usually found in passenger cars. The 3rd one on the far right, is the intermediate 9in shaft. Most will refer to it as long shaft, but is actually intermediate since there is also a 12in long shaft which is a bit rarely seen these days and not pictured here. The 9in Intermediate shaft is the size of output shaft used for Squarebody trucks starting in 73 up until the 700r4 replaced the Th350. So I started with a Squarebody 2wd transmission. Cut down the shaft of a passenger car output shaft and put in the OEM squarebody intermediate shaft case to make this Th350 for my 4x4 trucks. So the other 2 sizes of output shafts not picutred for Th350, are the actual 4x4 output shaft that does not need adapted, and the long 12in 2wd output shaft. So there are 4 OEM output shaft sizes, then the adpater 4x4 size that is made from an OEM shaft to match up to the 700r4. Confusing I know. But its actually quite simple.

http://www.gmsquarebody.com/forum/attachment.php?attachmentid=2915&d=1306742215

Zoolo42
07-10-2012, 06:40 PM
Hey guys noob here, my 700r4 just went out
In my 87 v10. My buddy has a 350/205 for me
If I want it, what I'm asking is will I have to use
This kit and also will it work for a 205 behind the th350?

HotRodPC
07-10-2012, 07:03 PM
NO, this kit will not do you a bit of good. You'll have to get your driveshafts worked over to fit this this whole new combination. Quite worth it though. The NP205 is about bulletproof being a gear to gear driven unit where the aluminum case NP208 that you have now is chain driven. The Th350 is quite strong too and will tolerate much more abuse than the 700r4. If you can afford the driveshaft work, and don't mind losing Overdrive, you'll have a much stronger combo. Also, ask your buddy if he has the front driveshaft from whatever he got the Th350/NP205 out of, that will save you half the cost.

Zoolo42
07-10-2012, 07:20 PM
Thanks for the info! I'll ask him about the front driveshaft
And what it came out of.

jelawrfarms7
04-18-2013, 05:50 PM
oh I have a 700r4 with what I believe is a np203 case and I can get my hands on th350 4x4 trans with or without tcase would this help if I use the current tcase in the truck??

HotRodPC
04-18-2013, 06:17 PM
Welcome to GMSB Chris.

If you're wanting to convert from a 700r4/NP208 to a Th350/NP208 without any driveshaft mods, exhaust mods, or crossmember mods, the Th350 you actually want to find is a 2wd short shaft. The one most common in passenger cars with a 6in tailshaft. Reason being, you're going to do away with the tailshaft, BUT, you'll have the output shaft long enough for the spacer, then you'll cut that output shaft down about an inch IIRC, to fit your current NP208 and it's apapter. You should be able to easily find a 2wd 6in tailshaft unit.

Now if you're buying the whole space kit with the shaft and all already in the kit, then it doesn't matter, just get any Th350 core you can find, and use that output shaft. Now the 4x4 Units do have the bigger bolts at the bell housing for bolting support rods, but honestly, I Think on the Th350, you can do some some drilling and make the holes bigger, or use smaller bolts and still use the support rods if you wanted. Also with this mod though, you will get to keep your support rod from the 700r4, Tcase to bell housing, so you may not need the under the bellhousing rods. Up to you.

buckethead27
06-14-2013, 01:07 PM
hey guys quick question, I am running a th350/np203 combo, Im wanting to put in an np241, Ive bought an adapter that ran in the 81-82 trucks that came with the 350/241 combo and I was wondering if there was anything else I would have to do as far as crossmember movement, driveshaft modification and so forth. Ive been searching for days now and hopefully some of you guys will be able to chime in and help me. Thanks in advance, Jordan

HotRodPC
06-14-2013, 03:14 PM
hey guys quick question, I am running a th350/np203 combo, Im wanting to put in an np241, Ive bought an adapter that ran in the 81-82 trucks that came with the 350/241 combo and I was wondering if there was anything else I would have to do as far as crossmember movement, driveshaft modification and so forth. Ive been searching for days now and hopefully some of you guys will be able to chime in and help me. Thanks in advance, Jordan


NP241 was NEVER mated up to a Th350. NP241 came about in the late 80's but does have the same mounting specs as a NP208. But again, NP208 was never mated to a Th350 either. The Th350 output shaft for a NP203 will be to short to reach the splines for a NP208 or NP241C. So that will be your first problem.

I am not familiar with the mounting location of the rear mount on the NP203, so I'll someone else answer that portion.

bucket
06-15-2013, 01:31 AM
The 208 was indeed available behind the th350, but it had a longer trans output than the th350 used with the 203 and 205.

HotRodPC
06-15-2013, 05:21 AM
The 208 was indeed available behind the th350, but it had a longer trans output than the th350 used with the 203 and 205.

Yeah I guess so huh? That would have been 81 for sure and some 82's that weren't 700r4 yet. But still, his output shaft wouldn't work having a NP203 unit.

bucket
06-15-2013, 11:33 AM
Yeah I guess so huh? That would have been 81 for sure and some 82's that weren't 700r4 yet. But still, his output shaft wouldn't work having a NP203 unit.

Right. With the short output, you only options are the 203 and 205.

00-dirt
07-24-2013, 08:50 PM
So how much did you actually cut off of the output shaft?

HotRodPC
07-24-2013, 09:23 PM
So how much did you actually cut off of the output shaft?

Ahh Hell !!! Good Question. I should have given the measurements. I have another shaft here I cut off, but it's in storage at the moment, and if I can find it... I'll do some looking around. I may not be able to tell you how much I cut off, but at least tell you what length to cut it too. I'm wanting to say it was about an inch and that included saw curf. The blade is pretty wide on the chop saw like maybe an 1/8 inch.

00-dirt
07-25-2013, 05:42 PM
I will take what length to cut it to getting ready to make this swap. Thanks. also I am mating to a 208 does that make a difference besides needing the adapter piece?

HotRodPC
07-25-2013, 06:19 PM
I will take what length to cut it to getting ready to make this swap. Thanks. also I am mating to a 208 does that make a difference besides needing the adapter piece?

This is to adapt to an NP208 so it will work fine. You'd use the same adapter that is used for the 700r4. This will also work for a NP241C.

All you're really doing is putting in the spacer which I assume you have already, then cutting down the 2wd output shaft so it's the correct length with the adapter. I still haven't made it to storage to see if that shaft is in there.

Krusty78
10-12-2013, 04:51 PM
My buddy has a 700r4 mated w/ a np208 from my research he needs the spacer for sure but whats the scoop on the output shaft can we just cut his stock one on the th350 and run the 700r4 driveshaft?

MrMarty51
10-12-2013, 06:54 PM
Hey HR, seems some good things is coming Your way, for once.:console:

jth678
12-10-2013, 11:24 AM
when making the 700r4 to th350 conversion what happens to the electrical plug on the 700r4 ? and what about speed sensors and such?

HotRodPC
12-11-2013, 01:25 AM
when making the 700r4 to th350 conversion what happens to the electrical plug on the 700r4 ? and what about speed sensors and such?

If you're talking in a 4x4, the Speedo is controlled by the Transfer case, so you'd use the same Tcase and the Speedo is not an issue.

For a 2wd, you'd need an adapter since no Th350 has provisions for the Electronic type since the Th350 output shafts don't have reluctor wheels.