View Full Version : 700R4 to TH350 Swap......what to do for transmission mount?...?

09-06-2011, 10:15 AM
Like the title says I have an 87, and I am switching over to TH350. I have the adapters and have had the trans rebuilt with a new output shaft to meet the length of 700R4. I swapped in the motor and trans last night and well its looking as if im gonna have to move the trans crossmember forward but my only problem with that is the trans mission mount isn't the same! Do I need to go find a late 70s early 80s cross member?? Im hoping someone else on here has experienced this Thanks

09-06-2011, 11:49 AM
to swap a in a th350 u need to get a 2wd short shaft th350 cut the shaft to propper lenght put in the 1 7/8 block between the trans and the cast iron adapter BTW mine is th350 to a 208 so it might be diffrent for a 203 or a 205

09-06-2011, 12:45 PM
Its the same cross member just different tranny mount.

You had a 2wd or 4wd th350 rebuilt? Not sure I understand what it is you had done.

09-06-2011, 01:09 PM
What ive swapped to is a TH350 K-Case out of a early 80s pickup, which then I also picked up the adapter piece and new dipstick for the 700r4 to TH350 conversion, im also going to be running the NP208 thats why I did the conversion to keep that transfer case! Where the original 700R4 trans mount was on the crossmember im about 3 to 4 inches away from that, and the 700r4 trans mount for an 87 does not match up to a TH350 the trans mount bolt holes are nowhere close to eachother!

09-06-2011, 01:10 PM
Yes it was a 4x4 TH350 I had rebuilt you can only find K-Case in a 4x4! I wouldnt run a 2wd TH350 the bell housing is no where near as strong!

09-06-2011, 02:38 PM
I'm still lost.

Yes only 4x4's had K cases. But what you have done is perplexing.

You took a 4x4 th350 and had output shaft swapped out to fit the 208? and you need adapter? you had 700r4/208 before right?
Now 350/208.

The output shaft on the 350 is the same as the the 700 so not sure why it had to be changed. Also same adapter. Only thing to change is the tranny mount itself not the crossmember.

You might want to try getting a th350 tranny mount for a say 82 K10 and see if that fits the bill. The K10 and K5 have two different crossmembers. The K's is W shaped because of shorter drivetrian. The K10's is a little more flat but only because drivetrain is longer. But they use two different transmission adapters. The K10's is tall where the K'5 are very short. Can you post a picture of the Adapter you have? And maybe your crossmember? Hell take pictures of it all. We will figure it out.

700r4 & th350 share a 27 spline yoke (31 spline th350's exist, but are not the norm) So tell us why the output shaft swap? Did it have the 31 spline output in it? If so you made big money mistake.

09-06-2011, 03:56 PM
You need the adapter because the 700r4 is approximately 3 inches longer then a TH350 so you get a longer output shaft and then you get a adapter to make up the difference that gos between the TH350 and NP208 which sums up the difference in length it just appears im gonna have to move the cross-member forward for trans mount but the trans mount is different between the two, and when I go to parts store too look up trans mount for an 80 k20 it shows a left and right mount for trans which has me confused!

Does anyone have a picture of a TH350 factory truck and how it mounts to the trans the turbo 350 bolts in the bottom are approximately 4inchs center to center id have to say for trans mount to bolt up where the 700R4 is about 6-7 inches center to center! My main problem is finding the proper trans mount no big deal to move the cross-member forward its only 4 holes ill have to drill in the frame rail for the move!

09-06-2011, 06:34 PM
Are you sure your not trying to place the tranny mount on tranny rather than the adapter? It belongs under adapter not the tranny.

I still dont understand why you have gone the route you have but I guess it till still work. You could have used the 4x4 th350, the adapter that was behind the 700 and been fine with no issue.

Lot of extra time and money for same effect.

09-07-2011, 12:13 AM
Here's how mine is set up... th350 in an 86 (originally 700R4) mated to an NP208. the crossmember is dropped but still in the stock location, and yes I have the 1-7/8" adapter and longer output shaft as well. http://i829.photobucket.com/albums/zz216/ahays000/491e4d55.jpg Kinda hard to see but hope it helps.

09-07-2011, 01:41 AM
Your picture is clear and good. Thanks for showing. That spacer is needed when using a 2wd case. But when using a 2wd case you normally need to shorten the shaft by about 3/4 of an inch rather than lengthen it. Not sure why the spacer would be needed if the case is already a 4wd case and still lengthen the shaft to boot.

But with picture you clearly see the tranny mount goes to adapter not the tranny if that helps with your issue.

09-07-2011, 01:44 AM
AHays. That is one dirty under carriage lol.I'm guessing pre-restorative pic?

I think I understand what he did. There are a couple variations of th350-700's some do have a shorter end cone and shaft.
I know on cars there is the long shaft for a manual trans and another shorty for auto trans.

09-07-2011, 01:47 AM
I cannot say for certain if this is a 2wd trans or 4wd... the PO did the swap, he told me they had to swap the tail shaft too, but can't tell you for sure.

09-07-2011, 02:20 AM
Ok I think I know what needs to be done now I hope!! Ill mess with it tomorrow I went TH350 cause im using this truck for off-road purposes, and it was all around alot cheaper to build a TH350, and set it up with my truck!! I pulled this thing apart so fast I didn't even pay attention to be honest and everything under my truck was so crust it was ridiculous!!

09-08-2011, 06:08 AM
My mount is bolted directly to the T-case, not the adapter.

And yes its pretty dirty under there, the PO drove around with a leaky trans pan gasket so the dust and dirt just stuck.

09-08-2011, 12:30 PM
My mount is bolted directly to the T-case, not the adapter.

And yes its pretty dirty under there, the PO drove around with a leaky trans pan gasket so the dust and dirt just stuck.

Bro. Look closer. Your mount is bolted directly to the adapter. You have a spacer and an adapter. The spacer is used for the 2wd 350 to 208 because the shaft is to long. Thats why I have to question why the PO would say he installed longer shaft. Makes no sense.

09-08-2011, 02:17 PM
Okay I see what your saying... I was referring to the "spacer" as the adapter since it has been added. But I guess they all come stock with the adapter right?