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TH350 rebuild/replace OR 700r4 swap - read for context please

Gtlackey

Gtlackey

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Ok guys, back for my second post, I am having a tough time deciding about transmission rebuild or swap?

this is for Jonathan's 1974 Jimmy with stock 350/350. tranny needs a full rebuild and we want to move from a NP203 to a NP205 I picked up this weekend so he can go to part time 4wd (last post was on the slugs, thanks for all the replys).
From what I see with the NP205, I already have to move the cross member, get new drive shafts (need to do anyway) and replace and move the 4wd shifter, so why would I not pick up a 700r4 that needs to be rebuilt and do it all at once? I feel we might eventually go to the 4 speed anyway...
what else am I missing in this swap idea?
 
Gtlackey said:
Ok guys, back for my second post, I am having a tough time deciding about transmission rebuild or swap?

this is for Jonathan's 1974 Jimmy with stock 350/350. tranny needs a full rebuild and we want to move from a NP203 to a NP205 I picked up this weekend so he can go to part time 4wd (last post was on the slugs, thanks for all the replys).
From what I see with the NP205, I already have to move the cross member, get new drive shafts (need to do anyway) and replace and move the 4wd shifter, so why would I not pick up a 700r4 that needs to be rebuilt and do it all at once? I feel we might eventually go to the 4 speed anyway...
what else am I missing in this swap idea?
Click to expand...
I know I was considering the 4 speed transmission for my 77. Would like some info, on that.
 
if i am not mistaken you need to get the right adapter and sleeve, hopefully someone will chim in that knows for sure
whats the output shaft splin count on the t350 with 203?
 
Another member is doing a great write up in his build thread on a 700R4 upgrade (2wd but similar upgrades internally).

79' GMC C15 - Spruce Caboose

Hey dude, I hope you kept the receipt for the pipe bender. I hate to be the guy to tell you but those benders are the biggest POS sold at tool stores. All they do is crush and ruin anything less than sched 40. For exhaust work you are better off buying a box of bends and piecing it together...
ck5.com ck5.com
 
kennyw said:
Another member is doing a great write up in his build thread on a 700R4 upgrade (2wd but similar upgrades internally).

79' GMC C15 - Spruce Caboose

Hey dude, I hope you kept the receipt for the pipe bender. I hate to be the guy to tell you but those benders are the biggest POS sold at tool stores. All they do is crush and ruin anything less than sched 40. For exhaust work you are better off buying a box of bends and piecing it together...
ck5.com ck5.com
Click to expand...
Thank you, will use all resources I can get.
 
Output shaft is 27 spline that sticks out. I have the adapter for the TH350, but since I am having to change all these things anyway due to the Tcase, I wasn't sure how much more was involved? just deciding if it worth going with a 4 speed for him and bite the bullet now or just rebuild the TH350 and add the NP205...
2022-10-04_11-50-18 TH350 type.png
 
Output shaft is longer on the 700R4 but you can get a new adapter for the swap or use a spacer. I would go with the new adapter if money isn't an issue as it is stronger with less possible failure/leak points. Advanced adapters sells the adapter. I'm not sure who makes the spacer.

You also need additional wiring for the lock up converter. There is a kit for the wiring available. It may be from TCI or a similar transmission company.

Other than that, upgrade the transmission so you are not replacing it again in 6 months.
 
My 77 was a th350 np203, with slugs. It is now a 1991 "K" case 700r4, np205, with locking hubs.
Advanced Adapters makes a 4wd 700r4 out put shaft.
www.advanceadapters.com

OUTPUT SHAFT - STOCK 700R4 4X4 13.25

GM 700R4 4 Wheel Drive Transmission Output Shaft
www.advanceadapters.com www.advanceadapters.com
You need to find a K case, from 91 up not sure about end year.
I used the tall th350 np205 figure 8 adapter and the flat cross member.
Be sure to use the engine to bell housing braces and the bell housing to t case.

Use Bow Tie Overdrives for the torque converter lock up wiring. Be sure to get a quality torque converter, approved by builder for warranty. Update the trans cooler, it is my opinion automatic transmissions should stay as cool as possible. unless you live below 0.

My first 700r4 was built by Bow Tie Overdrives in Hesperia CA, My second (same case) was done by The Toy Store in Pomona, CA.

th350-np205-adapter-adapter-20002-470877262.jpg

IMG_20181230_115202.jpg
 
I should add, the 4.10 ring gear and 33 were good for cruising, but could accelerate from 50 to 65 w/o a down shift. Since I tow alot, I swapped up to 4.88 ring gear, K5 and 35s will be good at 4.56.

Get a temp gauge for it too.
 
Wes, great information thank you!
I have the NP205 with the TH350 adapter, but not sure it is the "tall one", will look in a bit. Not sure what you mean by flat crossmember? I know I have to move and potentially cut mine or buy a new one, but haven't looked into new ones yet.
 
Iirc I only moved my cross member back 1.75"

I guess it's not flat but compared to the other 2 it is.

cross member.jpg
 
Interesting note here on the shortened output shaft and driveshaft for earlier models like mine. note sure if that means I should use this and make it easy to find the new driveshaft, or go the opposite direction and use the spacer just so there is not limited space between the driveshaft and pan?2022-10-04_15-15-47 short output shaft warning note.png
 
Yes this is an issue I need to deal with still. I have had a Bluetooth front drive shaft for years.
I made it worse by adding a Derale deep cool pan on mine.
 
The spacer that some folks use has issue of its own. Adds 2" above the 1.75 of the case. Need long bolts to mate to adapter.
Now there are only the 4 bolts holding t case adapter to the trans case. These bolts are prone to coming loose in factory set ups, add the spacer only compounds this issue.
 
yes, ORD version here
www.offroaddesign.com

700R4 spacer, 1-7/8 Inch with bolts

700R4 spacer, 1-7/8" with bolts Makes shaft stick-out the same as a 4x4 TH350, allows use of NP203 and NP205
www.offroaddesign.com www.offroaddesign.com

which way you feel is better? I will have a 2.5" leaf spring lift, but don't this that is going to make a difference on that clearance even with angle change...
 
Me I do not like the the spacer.
Not sure how I am gonna solve front clearance issue.
I had In mind to change to a longer flange, like np208.

Maybe just run the small 1310 u joint drive shaft.

My np205 is a 72 unit and uses u joint yoke at front so I am already set up for that
 
you have a pic of that yoke setup? would like to compare it to what I am looking at
 
why don't you just use the NP208 that came behind the 700R4? has a better low range.
 
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