I would really like a turbo 400 in my truck

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

jakewalter

jakewalter

Junior Member
Joined
Aug 23, 2023
Posts
7
Reaction score
9
Location
Northern Michigan
First Name
Jake
Truck Year
1985
Truck Model
K20
Engine Size
454
I have a 85 K20 that I built up to haul our 3500 pound pickup camper.

It currently has a pretty healthy big block that I built for torque.

The transmission is a 700R4 and a 208 transfer case.

While on vacation a couple of weeks ago the transmission started slipping and I lost reverse.

I never use the overdrive and as for mileage I threw my pencil away a long time ago.

Other than a TH400 out of a 4wd what else would I need?

I would really appreciate any input that you guys could share with me.

I need to apologize.

The guy that rebuilt my transfer case said that it is a 208 not a 203.
 
Last edited:
nvrenuf

nvrenuf

Full Access Member
Joined
Jul 30, 2010
Posts
1,981
Reaction score
2,522
Location
Mobile, Al.
First Name
John
Truck Year
1991
Truck Model
K5
Engine Size
454
You need a 4wd th400 from a 400/203 combo. The th400 tailshaft that fits a 203 is super short and only fits that application, nothing else 2wd or 4wd.

You will also need the trans to tcase adapter and kick down switch for the gas pedal.
 
jakewalter

jakewalter

Junior Member
Joined
Aug 23, 2023
Posts
7
Reaction score
9
Location
Northern Michigan
First Name
Jake
Truck Year
1985
Truck Model
K20
Engine Size
454
nvrenuf said:
You need a 4wd th400 from a 400/203 combo. The th400 tailshaft that fits a 203 is super short and only fits that application, nothing else 2wd or 4wd.

You will also need the trans to tcase adapter and kick down switch for the gas pedal.
Click to expand...

I need to apologize.

The guy that rebuilt my transfer case said that it is a 208 not a 203.
 
nvrenuf

nvrenuf

Full Access Member
Joined
Jul 30, 2010
Posts
1,981
Reaction score
2,522
Location
Mobile, Al.
First Name
John
Truck Year
1991
Truck Model
K5
Engine Size
454
You’ll still need the correct tailshaft, I think it sticks out of the case 3.5” but I’m not sure. Also you’ll need the adapter.

Another hurdle will be the tcase input shaft, the th400 is 32 spline and the th350/th700 use 27 spline. I believe the 2 input shafts for the 208 are a direct swap.
 
Old Guy Bill

Old Guy Bill

Full Access Member
Joined
Sep 10, 2023
Posts
445
Reaction score
1,372
Location
KY
First Name
Bill
Truck Year
1978
Truck Model
K20
Engine Size
400
I just changed the 203 to a 208 behind my TH400.
A 6-7” tail shaft is needed, I could only find a good used one from Novak.
 
Last edited:
nvrenuf

nvrenuf

Full Access Member
Joined
Jul 30, 2010
Posts
1,981
Reaction score
2,522
Location
Mobile, Al.
First Name
John
Truck Year
1991
Truck Model
K5
Engine Size
454
Old Guy Bill said:
I just changed the 203 to a 208 behind my TH400.
A 6x7” tail shaft is needed, I could only find a good used one from Novak.
Click to expand...

Just to be clear, you are talking about total length, correct?
My estimate was just what sticks out of the case.
 
Old Guy Bill

Old Guy Bill

Full Access Member
Joined
Sep 10, 2023
Posts
445
Reaction score
1,372
Location
KY
First Name
Bill
Truck Year
1978
Truck Model
K20
Engine Size
400
Middle one, bottom row.
The new ones sold by Jeggs & Summit are for drag racing and have no provision for turning the Governor.
 

Attachments

  • IMG_2795.png
    IMG_2795.png
    798.4 KB · Views: 24
nvrenuf

nvrenuf

Full Access Member
Joined
Jul 30, 2010
Posts
1,981
Reaction score
2,522
Location
Mobile, Al.
First Name
John
Truck Year
1991
Truck Model
K5
Engine Size
454
Old Guy Bill said:
Middle one, bottom row.
The new ones sold by Jeggs & Summit are for drag racing and have no provision for turning the Governor.
Click to expand...

Are you sure the 4.5” stick out fits a 208? I thought the stick out was less than a figure 8 205 tailshaft.
 
jakewalter

jakewalter

Junior Member
Joined
Aug 23, 2023
Posts
7
Reaction score
9
Location
Northern Michigan
First Name
Jake
Truck Year
1985
Truck Model
K20
Engine Size
454
nvrenuf said:
You need a 4wd th400 from a 400/203 combo. The th400 tailshaft that fits a 203 is super short and only fits that application, nothing else 2wd or 4wd.

You will also need the trans to tcase adapter and kick down switch for the gas pedal.
Click to expand...
I need to apologize.

The guy that rebuilt my transfer case said that it is a 208 not a 203.
 
nvrenuf

nvrenuf

Full Access Member
Joined
Jul 30, 2010
Posts
1,981
Reaction score
2,522
Location
Mobile, Al.
First Name
John
Truck Year
1991
Truck Model
K5
Engine Size
454
jakewalter said:
I need to apologize.

The guy that rebuilt my transfer case said that it is a 208 not a 203.
Click to expand...

No problem but a lot of the previously post items still apply - correct trans tailshaft length, adapter and tcase input shaft spline count.
 
Nasty-LSX

Nasty-LSX

TogetherforeverCovid19
Joined
Nov 22, 2014
Posts
1,395
Reaction score
1,739
Location
HOUSTON TX
First Name
Mi Hung Lo
Truck Year
85/86/87
Truck Model
c10/k10/k20
Engine Size
LSX
jakewalter said:
I have a 85 K20 that I built up to haul our 3500 pound pickup camper.

It currently has a pretty healthy big block that I built for torque.

The transmission is a 700R4 and a 208 transfer case.

While on vacation a couple of weeks ago the transmission started slipping and I lost reverse.

I never use the overdrive and as for mileage I threw my pencil away a long time ago.

Other than a TH400 out of a 4wd what else would I need?

I would really appreciate any input that you guys could share with me.

I need to apologize.

The guy that rebuilt my transfer case said that it is a 208 not a 203.
Click to expand...
I'm swapping to a 4l80e I NEED overdrive. I have a perfect TH400 that needs no rebuild. No slipping skipping or weird stuff if your interested. the 2500 K20 th400 are 32 spline. I also have the 208 T/C. You can swap your T/C gear from 27 to 32. Its very easy. NOW, contrary to what everyone believes, You can get your 700R4 built to do what you want. I had a 82 k10 with a 454 and a 700r4 and beat the hell out of it with no problems. You just need a builder that knows his stuff. Talking 700R4's is like talking politics or religion, everyone(almost will tell you 700's suck). Not true. as long as its not a 86 or older. They suck!!!87 and newer are upgraded internals. Here is my 85 K20 4x4 with a TH400 and 208.

You must be registered for see images attach
You must be registered for see images attach
 
Last edited:
J

Jimmyperf400

Junior Member
Joined
Mar 29, 2022
Posts
16
Reaction score
13
Location
06606
First Name
Peter
Truck Year
1990
Truck Model
Full size gmc jimmy
Engine Size
Sbc 400
I built my 700r4 with every after market performance part they make and I've towed a 10 thousand pound excavator with the 1990 gmc jimmy its in and I've never had any issues. But up to you you know what you want.
 
You must log in or register to reply here.

Similar threads

H
Can I put a np203 in my 84’ K10 that originally had the 208?
Replies
13
Views
1K
Cyrillious
Cyrillious
S
after doin front end clutch and steering column,truck feels like its in 4wd
Replies
10
Views
980
Grit dog
Grit dog
D
Sm465 sounds like a rock tumbler in 4th gear
Replies
2
Views
300
dmac2590
D
68dhayes
Building a new motor for my 79 C10 - would love some advice!
2
Replies
15
Views
2K
SQ HAF KRU
SQ HAF KRU
Ole Buck
A very expensive and dumb mistake (update I was wrong)
Replies
11
Views
2K
Tonimus
Tonimus

Latest posts

Forum statistics

Threads
44,414
Posts
956,966
Members
36,737
Latest member
Fjoel040
Top