TH350c Questions.

[BlazingBlue305]

Let me start off by saying transmisisons are not my strong points with cars. I have a TH350c in my '81 Blazer and am looking to replace it with a new rebuilt one. Its starting to slip, and as far as I'm aware the previous owners of the truck before me didnt service the transmission ever. Im having a hard time finding any lock up style TH350 transmissions around online. Let alone a 4WD option that will work with my NP208 T-case. I've had others suggest I swap it out for a 700R4, since it can bolt up to my 305 engine. However, I know some other parts will need to be adjusted. I guess my questions that I have are:

Should I just learn to rebuild my current TH350c?

Is there a website anyone knows where I can get a lock up style TH350? If not, can I purchance a non lock up th350 with a non lock up torque converter and use that?

Or should I just swap it with a 700R4? If so, what would I need to change I need to do to accommodate this?

Sorry for all the questions. Any input is appreciated.

 

[Blue85]

Have you checked to see if the vacuum modulator is leaking or broken? Is the fluid level correct and clean?

It sounds like you're saying you can't have the vehicle down for very long. Otherwise, why not have yours rebuilt?

If you go 700R-4, you'll need the driveshaft lengths changed, but both transmissions should have the same 27-spline, so by swapping/moving the cross-member the NP208 can work.
You'll also need a new throttle linkage/bracketry for the TV cable (critical!) and new wiring for lock-up using a brake pedal switch and vacuum switch. The information has been posted, so you should be able to find it here (it's actually easier sometimes to use Google by using "site:ck5.com", versus the built-in forum search.

 

[tarussell]

The reason it’s difficult to locate a lock-up version of the TH350”C” is that they were only made ( in trucks at least ) from ‘81~’84 and when the TH700R4 was introduced in ‘82 the TH350-C was only available on the basic models ( unless specified ) until it was phased out all together at the end of 1984.
And to add to the scarcity of what you need is that a 4WD version with the mounting provisions for the lower torque converter cover that uses the motor mount stiffening bars was only made in great quantities in ‘81 models - the 2WD’s did not use that type of cover or have the retaining holes to mount it.

Yes you can have a non-lockup TH350 in its place - this is actually a little more durable over the “C” version but the 4WD TH350 cases were only made from ‘78~’80 ( ‘69~’77 cases did not accept the upgraded converter cover ).
You can use an earlier case but it’s not generally recommended.

To answer your question of if you should attempt to build the transmission yourself - if ya gotta ask than most likely not.
Learning to wrench on your own stuff is great but for most folks tearing into an automatic transmission is not generally recommended.
The TH350 is by far one of the most simple designs and easiest to build but there is still the need for recognition of potential issues that a novice won’t catch ( a good builder does more than just replace parts ) and ya might not have all the required tools needed.

It sounds like your simplest solution is to have your transmission rebuilt by a qualified and reputable shop.
A TH700R4 swap is a great option if you’re alright with the basic mods to make it work - one thing to be ware of is that most early eighties trucks with non-overdrive transmissions came with very low numerical gears ( example 2.56, 2.73, 3.08 ) and sometimes those ratios make using an overdrive useless and even counterproductive.

Welcome aboard to CK5 - this is a great 4WD site with a wealth of information and knowledge that can be trusted as opposed to what is available out on the internet which is hit and miss.

 

[82355]

If you go 700R-4, you'll need the driveshaft lengths changed,

Incorrect. They are relatively the same length. They made up the difference in the adapter.

Martin

 

[tarussell]

Incorrect. They are relatively the same length. They made up the difference in the adapter.

Martin

Didn’t the 208 adapters that go to the TH350’s & 700R4’s share the same dimensions ? ( note: I said this wrong so here’s the correction - the early TH350/208 adapters were longer than the 700R4/208 adapters ).
In this case there will be a set back of the crossmember due to the added length of the 700R4 if used with the TH350/208 adapter but not if used with a 700R4/208 adapter.

All the adapters after ‘85 made for common positioning no matter the trans but prior to ‘85 that’s not in effect for every transmission offered.

 

[sweetk30]

@Greg Ducato is our trans guru and ships all over .

 

[Blue85]

Incorrect. They are relatively the same length. They made up the difference in the adapter.

Martin

Didn’t the 208 adapters that go to the TH350’s & 700R4’s share the same dimensions ?
In this case there will be a set back of the crossmember due to the added length of the 700R4.

All the adapters after ‘85 made for common positioning no matter the trans but prior to ‘85 that’s not in effect.

I propose you two arm wrestle? I remember that the adapters are different. I've never done this swap, but I can't remember anyone saying they'd done it with the same driveshafts. I bow out to whatever knowledge will be dropped here.

 

[tarussell]

I propose you two arm wrestle? I remember that the adapters are different. I've never done this swap, but I can't remember anyone saying they'd done it with the same driveshafts. I bow out to whatever knowledge will be dropped here.

I’ve done a few of these - you are spot on correct . The shafts will need to be modified.

 

[82355]

Shafts are the same. It's why you have to buy a Fitzall spacer to use a TH350 in place of a TH700R4 with the factory NP208 adapter.

Martin

 

[tarussell]

The shafts are both the same diameter and 27-spline and both trannys use the same 4-bolt pattern but there are options of tail shaft output lengths for both 700R4’s and TH350’s to work with either dimension NP208 adapters - in this case that A/A spacer would not be used because it’s going from a TH350 to a 700R4 not the other way around.

The short adapter should be GM # 14038053 and the thicker adapter should be GM # 14020853 if I remember correctly.





Not trying to dispute at all, just pointing out that it is possible to use the early TH350/208 adapter and connect to a 700R4 but that would require the crossmember to be moved back - which is what I was referring to originally.
But, if the shorter 700R4/208 adapter is used ( with the correct output shaft dimension ) then the crossmember will remain in the same location with no aftermarket spacers needed - I think this is what you were originally referring to ?

Anyway, it’s all good info that will hopefully help someone out in the search mode one day.

 

[imiceman44]

I propose you two arm wrestle? I remember that the adapters are different. I've never done this swap, but I can't remember anyone saying they'd done it with the same driveshafts. I bow out to whatever knowledge will be dropped here.

I did the swap and it was about an inch or so longer.
I had to do the driveshafts and obviously move the cross member

 

[82355]

He can take his TH350C and TH350/NP208 adapter out, and replace it with a TH700R4 and TH700R4/NP208 adapter. The TH350 is shorter, but the TH350/NP208 adapter is longer.

Martin

 

[Greg Ducato]

I am of the opinion that you should have your 350C and converter rebuilt by a reputable shop in your area, there must be dozens of these in that part of the state. This trans is pretty special with the provision for the aluminum bolt on dust cover , the locking torque converter and a two wire plug with simple control system.