NP203 to NP208 swap?

[SquareRoot]

Regardless of the swaps and mods one does to their square, the mpg will suck. It may suck slightly less, but it will still suck. Chasing fuel economy always results in going down the rabbit hole.

 

[Big Ray]

I've been contemplating a 4 speed overdrive swap in my '77 K5 as well, but using the 203 from my K30 and redrilling the face so it bolts up.

Why a 203?

 

[bucket]

Why a 203?

Simply because I have it and the 203 doesn't bother me. Plus they are easy to twin stick for ease of use. Once I clear the hurdle of heaving that big sumbitch in there, the painful part will be over, lol.

 

[Big Ray]

Never heard of a 203 getting twin sticked.
I need to look into this...

 

[ali_c20]

I did the 203/205 swap and did not recognize a big improvement in mileage. I like the rwd and that there are less moving parts that could fail. Shifting the 205 is not easy and I have twin sticks. The only way to improve mileage on our bricks is manual trans, overdrive, lockup converters and gearing ratio change.

 

[Big Ray]

Simply because I have it and the 203 doesn't bother me. Plus they are easy to twin stick for ease of use. Once I clear the hurdle of heaving that big sumbitch in there, the painful part will be over, lol.

I never knew that twin sticking a 203 was a thing. That's my only issue with it, fulltime 4wd.
Might have a change of plans here.
Hmm

 

[iamtherealJayy]

@SquareRoot i can live with 10+, but I’d rather not get 5/6mpg.

 

[bucket]

I never knew that twin sticking a 203 was a thing. That's my only issue with it, fulltime 4wd.
Might have a change of plans here.
Hmm

ORD offers a really simple and tidy twin stick setup, but I noticed the other day that it showed as currently not available on the website. I'm not sure if anyone else offers a kit, but I remember seeing years ago that folks would make their own setup utilizing parts from a second 203 shifter assembly.

 

[bucket]

But really though, if you have access to a good 208, that's not a bad setup at all. If I were to go that route, I'd use a cable shifter from JB Fab.

 

[bucket]

@SquareRoot i can live with 10+, but I’d rather not get 5/6mpg.

Back when I used to drive my '77 K5 daily, I averaged 10 mpg. That was with a part-time converted 203. However, driving in the winter with the hubs locked didn't take away from the mpg. Sounds like some tuning and maintenance may be all that's needed to achieve your goal.

 

[iamtherealJayy]

I’ve never even taken this truck down the road, I’ve had it since July and it’s only moved 2-3 miles. I’d just heard the 203 wore parts quicker, terrible fuel mileage, etc so I was set I didn’t need that because this truck isn’t going to be off-road hardly ever.

 

[CalSgt]

If I’ve got the driveshafts from the 208 and the cross member etc would it work together? Or is it enough difference to not interchange

I'm assuming the transmission donor truck has/had a 700R4, in that case the shafts from the donor would not work for the swap into a TH350 truck. The front would be too long & rear too short most likely.

IIRC the 203 uses a half-round yoke on the rear output and 208 uses a slip yoke, you may be able to switch out the driveshaft yokes add a fresh U-joint and re-use the original (modified) driveshaft. You would also probably need to figure out how to eliminate the spline slip joint on the driveshaft, tack weld or maybe drill and pin it.

The front shafts should both be flange mount though, your 203 front shaft may bolt right up but I'm not 100% sure. If not have the double cardan unit from the donor welded to your tube.

I will say it is far cheaper to have an existing shaft cut to length (shortened or modified) than it is to lengthen one. Also the rear slip yoke and front double cardan from the donor are valuable parts when its time to get a custom shaft made and will save you hundreds at the driveline shop if you end up going that route.

 

[Kurnacopia]

The 208 isn't as sturdy as the 203 is, so if strength is an issue a 208 isn't the case for you. You will also need an adaptor and drive shaft mods as well as shifter mods. The 203 is quite a beast and its large and heavy (all iron + chain). Easiest route is to pull the 203 and put a part time kit in it. Have it rebuilt and replace the old chain. Then add part time hubs. Warn and MM are good choices. easy to do by your self with you tube as a guide. Once the 203 is built you will be good to go for the next 50 years.

USA Standard Transfer Case NP203 Part-Time Conversion Kit – Jim's Drive Train

jimsdrivetrain.com

Best of luck my friend!

 

[Bextreme04]

If your only goal is fuel mileage improvement, then the only thing you need to be looking at is an AFR gauge and a carb tuning kit. Spending thousands of dollars and days of labor to change the trans/t-case is going to give marginal(if any) gains and you will still be getting poor mileage from a poorly tuned carb. You could probably spend less money to take it to a shop with people that have the right knowledge and equipment to dyno or even road tune the carb with the help of an AFR gauge.

 

[Dan Brown]

I think an NP205 will swap directly with the NP203 if you can find one. I'm currently restoring a 1974 K10 that I bought in 1974 with a Muncie SM465 and an NP205 T case.