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Thinking about swapping a 4L80E into my ’89 K5 (currently 700R4). Looking for feedback before I start parts-hunting.

clutch2k3

clutch2k3

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I'm thinking about swapping a 4L80E into my ’89 K5 (currently 700R4). Looking for feedback before I start parts-hunting.

Truck / Use Case
  • ’89 K5, 4WD, 5.7 TBI
  • 700R4 (rebuilt ~9k miles ago — TCC clutch already glazed + shuddering)
  • Derale 17 row trans cooler (DER33604), temp gauge, AN-6 SS braided cooler lines
  • NP241C (rebuilt 9k miles ago)
  • 33×12.5 KO2s (<10k miles)
  • 10-bolt axles w/new bearings & seals
  • 4.10 gears just installed, <500 miles
  • 2" lift, Bilstein shocks
  • Driveshaft angles:
    • T-case output: –5.1°
    • Rear pinion: +6.1°
    • Rear shaft: +19.6°
  • Purpose: long highway trips in FL + towing a <3,000 lb Casita for 100s–1000s of miles
  • Mall crawler until the kids get older

My 4L80E Swap Checklist — please sanity-check this

Transmission / Drivetrain Parts
  • 4L80E built with the correct 4WD output shaft (mandatory)
  • 32-spline NP241C input gear *or* full 4L80E→241 adapter kit
  • Standalone controller (leaning US Shift)
  • 4L80E dipstick & tube
  • Cooler line adapters or custom -6AN lines (4L80E ports are further back)
  • Shifter rod/bracket adjustment
  • Move crossmember rearward (existing crossmember reused)
  • Front driveshaft longer, rear shorter
  • Possible floor/tunnel tapping near cooler line bosses
  • Verify flexplate spacing & converter bolt pattern
  • Verify starter alignment/engagement
TPS & MAP to controller
  • Simply tap existing TBI TPS sensor signal wire
  • Use a T-fitting on an existing vacuum nipple and a second GM MAP sensor - only needed if the controller requires it

VSS

Driveshaft / Angle Stuff
  • With the longer 4L80E + 2" lift, rear shaft angle gets steeper
  • May Will need a SYE + double-Cardan rear shaft depending on final angles
  • Front shaft length + slip travel needs to be checked too

Gear Ratio Note
  • 700R4 1st = 3.06:1
  • 4L80E 1st = 2.48:1
  • I’m totally fine with this — towing reliability > off-the-line punch

Questions for the CK5 experts
  • My understanding is the stock crossmember is reused — I just drill new holes to move it back. Correct?
  • Do the common 4L80E→NP241 Adapter kits include:
    • the adapter housing and
    • the 32-spline input gear?
    • Or does it vary by vendor?
  • If I order a fresh 4L80E, can a shop like Monster or Certified just build it with the correct 4WD output shaft from the start?
  • Given my lift + angles + the longer trans:
    • Is a SYE + double-Cardan rear shaft basically mandatory?
    • Anyone running the stock slip yoke with a 4L80E successfully?
    • Any issues with front shaft clearance after the swap?

If I’m missing something obvious, feel free to roast me before I spend money.
 
Last edited:
I had a 2wd 4l80e and all it needed to make it work with my 208 was the 4wd housing adapter and then the correct input shaft for the 208. Nice thing about starting with a 2wd is there is already a tone wheel on the factory shaft for the VSS.

Cross member slid back about an inch.

I had to massage the floor a small amount with a hammer.

Definitely had to do new driveshafts front and rear. I already had a SYE and double cardigan I don’t know how you’d do it otherwise. No issues on front shaft clearance.

The linkage is a bit wonky reusing the stock column shift stuff. There’s a few threads on here about it.

Honestly it’s a fairly straight forward install. About as simple as dropping in a new motor. It was a lot easier than I thought it would be.
 
I have a us shift new in box . Hanged directions and no longer need it . Pm me if interested .

And i hope you are NOT towing in overdrive with a 700r4 .

Do NOT EVER buy monster trans . :ignore:

Get ahold of @Greg Ducato he is our trans goo ruu here and he owns his own shop for decades and builds good units . Phoenix transmission in wetherford tx .
 
This may be a horrible thing to say, but I'm just reading the requirements as written. The swap list gets a lot shorter if you convert to 2WD. It also doesn't say what rear axle is there, but I'll assume something with 14 cover bolts, or that 700 isn't the weak link for towing.
 
@sweetk30 I'm going to DM you later about the shift controller and I'm also going to reach out to @Greg Ducato, thanks!!

@Blue85 going to 2wd makes total sense but I'd prefer keeping it 4wd. After driving ~9000 miles without towing and then towing the Casita only in 3rd for ~100 miles the 10-bolt rear axle is fine but the 700R4 lockup clutch is toast. I just had 4.10's installed front and back which haven't even been broken-in yet.
 
Not all torque converter clutches are equal. You sometimes also need to up the pressure. Greg may even recommend just getting a heavy duty converter built for it and keep your current trans.

You should also discuss a 4L60-E with Greg. If it's set up right, it's probably up to this task, but the swap gets much easier.
 
@Blue85 I would LOVE not having to do the 4L80E swap!

Blue85 said:
Not all torque converter clutches are equal. You sometimes also need to up the pressure. Greg may even recommend just getting a heavy duty converter built for it and keep your current trans.

You should also discuss a 4L60-E with Greg. If it's set up right, it's probably up to this task, but the swap gets much easier.
Click to expand...
 
clutch2k3 said:
I'm thinking about swapping a 4L80E into my ’89 K5 (currently 700R4). Looking for feedback before I start parts-hunting.

Truck / Use Case
  • ’89 K5, 4WD, 5.7 TBI
  • 700R4 (rebuilt ~9k miles ago — TCC clutch already glazed + shuddering)
  • Derale 17 row trans cooler (DER33604), temp gauge, AN-6 SS braided cooler lines
  • NP241C (rebuilt 9k miles ago)
  • 33×12.5 KO2s (<10k miles)
  • 10-bolt axles w/new bearings & seals
  • 4.10 gears just installed, <500 miles
  • 2" lift, Bilstein shocks
  • Driveshaft angles:
    • T-case output: –5.1°
    • Rear pinion: +6.1°
    • Rear shaft: +19.6°
  • Purpose: long highway trips in FL + towing a <3,000 lb Casita for 100s–1000s of miles
  • Mall crawler until the kids get older

My 4L80E Swap Checklist — please sanity-check this

Transmission / Drivetrain Parts
  • 4L80E built with the correct 4WD output shaft (mandatory)
  • 32-spline NP241C input gear *or* full 4L80E→241 adapter kit
  • Standalone controller (leaning US Shift)
  • 4L80E dipstick & tube
  • Cooler line adapters or custom -6AN lines (4L80E ports are further back)
  • Shifter rod/bracket adjustment
  • Move crossmember rearward (existing crossmember reused)
  • Front driveshaft longer, rear shorter
  • Possible floor/tunnel tapping near cooler line bosses
  • Verify flexplate spacing & converter bolt pattern
  • Verify starter alignment/engagement
TPS & MAP to controller
  • Simply tap existing TBI TPS sensor signal wire
  • Use a T-fitting on an existing vacuum nipple and a second GM MAP sensor - only needed if the controller requires it

VSS

Driveshaft / Angle Stuff
  • With the longer 4L80E + 2" lift, rear shaft angle gets steeper
  • May Will need a SYE + double-Cardan rear shaft depending on final angles
  • Front shaft length + slip travel needs to be checked too

Gear Ratio Note
  • 700R4 1st = 3.06:1
  • 4L80E 1st = 2.48:1
  • I’m totally fine with this — towing reliability > off-the-line punch

Questions for the CK5 experts
  • My understanding is the stock crossmember is reused — I just drill new holes to move it back. Correct?
  • Do the common 4L80E→NP241 Adapter kits include:
    • the adapter housing and
    • the 32-spline input gear?
    • Or does it vary by vendor?
  • If I order a fresh 4L80E, can a shop like Monster or Certified just build it with the correct 4WD output shaft from the start?
  • Given my lift + angles + the longer trans:
    • Is a SYE + double-Cardan rear shaft basically mandatory?
    • Anyone running the stock slip yoke with a 4L80E successfully?
    • Any issues with front shaft clearance after the swap?

If I’m missing something obvious, feel free to roast me before I spend money.
Click to expand...
I used:
GM Genuine 15729870 Automatic Transmission Fluid Fill Tube

GM Genuine 15183801 Transmission Fluid Level Indicator, dip stick

And I second Greg Ducato! who made my 4L80E


20241216_135327_resized.jpg20250110_144321_resized.jpg20250426_164306_resized.jpg
 
Spoke with Phoenix transmission today. They're saying a 4L80E would be overkill for a stock 5.7L TBI and I should figure out what exactly failed in my 700R4. It probably just needs a new, good torque convertor. Now I'm looking for a good transmission shop in Tampa Bay FL. I'd still like to buy the TC from Phoenix transmission though.
 
Greg is a honest guy and knows his stuff .

Had him do a custom stall converter for my moms c4 in mustang .

He has also done parts and modded stuff for me and hit the nail on the head first round.
 
clutch2k3 said:
Spoke with Phoenix transmission today. They're saying a 4L80E would be overkill for a stock 5.7L TBI and I should figure out what exactly failed in my 700R4. It probably just needs a new, good torque convertor. Now I'm looking for a good transmission shop in Tampa Bay FL. I'd still like to buy the TC from Phoenix transmission though.
Click to expand...
This is what I'm saying. You could ship the failed converter to them to determine the failure mode (don't know what kind of down-time is OK for you). It could be your transmission isn't supplying enough pressure to the clutch. That can be from a myriad of things from bad o-ring on the input shaft to valve body issues to improper TV adjustment. Was this converter newly manufactured, or a could it be a remanufactured older design?

Did you ever see high temps on the trans?
 
Thats exactly what they recommended - ship the failed convertor to them for diagnosis! Sounded so crazy to me I asked him if he was joking. I love the idea of performing an autopsy on it though.

The only detail I know about the convertor is that the trans shop sends in the broken convertor as a core so I'm speculating my current convertor is a reman. I attached the reciept from the April 2024 rebuild.

I talked about trans temps during its final drive with Phoenix. I had a temp guage installed but not a cooler. Highest temp I saw was 215° for under 10 minutes. Temp gauge is installed on the send line, not the return line.

My next step is deciding between finding a trans shop in FL or doing the work myself. I've replaced a manual transmission and dropped a transfer case from my garage floor but never an automatic transmission.

Blue85 said:
This is what I'm saying. You could ship the failed converter to them to determine the failure mode (don't know what kind of down-time is OK for you). It could be your transmission isn't supplying enough pressure to the clutch. That can be from a myriad of things from bad o-ring on the input shaft to valve body issues to improper TV adjustment. Was this converter newly manufactured, or a could it be a remanufactured older design?

Did you ever see high temps on the trans?
Click to expand...

signal-2025-12-03-083922_002.jpeg
 
clutch2k3 said:
Thats exactly what they recommended - ship the failed convertor to them for diagnosis! Sounded so crazy to me I asked him if he was joking. I love the idea of performing an autopsy on it though.

The only detail I know about the convertor is that the trans shop sends in the broken convertor as a core so I'm speculating my current convertor is a reman. I attached the reciept from the April 2024 rebuild.

I talked about trans temps during its final drive with Phoenix. I had a temp guage installed but not a cooler. Highest temp I saw was 215° for under 10 minutes. Temp gauge is installed on the send line, not the return line.

My next step is deciding between finding a trans shop in FL or doing the work myself. I've replaced a manual transmission and dropped a transfer case from my garage floor but never an automatic transmission.



View attachment 518174
Click to expand...
Automatic transmission is no harder than a manual to replace, I could argue the adjustment of the clutch is harder than connecting an automatic.
And it's lighter.
YOU CAN DO IT
 
Having a 4L80-E in front of a 10-bolt is a bit of a mismatch. I think if it was coming from the factory, the bigger transmission wouldn't translate to a higher tow rating, but a bigger axle might, especially if it came with stiffer rear springs. In general, the K-5s weren't intended to tow much.

If you just wanted to upgrade in general, it's a good transmission, but it would make sense to swap the rear end at the same time, and have the driveshaft made for that.
 
This thread is going in a differnet direction. Please sanity check my new to-do list (edited):

## Measure for rear DC (Double Cardan) Driveshaft
- Tom Wood's Transmissions allows you to measure for DC/CV driveshafts with the old slip yoke still installed in the t-case (!!)
- My rear pinion U-joint size is 1310

## Install a trans pressure gauge and perform a pressure test

## While the Trans is Out
- Send TC to Phoenix Transmissions for diagnosis & replacement
- Remove & inspect flexplate for cracks, ring gear wear, warping
- Replace engine's rear main seal
- Replace engine's oil pan gasket w/Fel-Pro OS 34500 R
- Inspect rear oil gallery plugs; replace only if seeping
- Inspect starter
- check for cracks in starter nose​
- verify gear engagement / shim spacing​
- verify starter bolts w/ knurled shoulders (replace if worn)​
- Replace flexplate bolts w/ARP

## Transfer Case Work
- Install Tom Wood's SYE in NP241C T-Case
- install speedo correction adapter from texasindustrialelectric.com (Thanks Wes!!)

## 700R4 Trans work
- Inspect around VB for debris (no VB teardown)
- Replace TCC Solenoid
- Re-Torque VB bolts
- Weld in a 3/8" NPT Drain bung & Plug to the pan
- Weld in a 1/8" NPT Temp Sensor Bung to the Pan
- Replace Dipstick Tube gasket (GM Genuine Parts 1259475 takes 30-60 days to arrive)
- Replace Filter & pan gasket
- Replace the Front Pump Seal and retainer
- Install torque converter

## Reinstall trans & T-Case
- Attach torque converter to flexplate w/new ARP bolts

## Install Tom Wood's DC/CV driveshaft

## Install rear leaf spring shims (~10°) & replace rear axle U bolts
- Rear pinion should be 2° lower than driveshaft to account for axle wrap

## Refill trans ATF (DEXRON III/MERCON) & engine oil
 
Last edited:
clutch2k3 said:
I'm thinking about swapping a 4L80E into my ’89 K5 (currently 700R4). Looking for feedback before I start parts-hunting.

Truck / Use Case
  • ’89 K5, 4WD, 5.7 TBI
  • 700R4 (rebuilt ~9k miles ago — TCC clutch already glazed + shuddering)
  • Derale 17 row trans cooler (DER33604), temp gauge, AN-6 SS braided cooler lines
  • NP241C (rebuilt 9k miles ago)
  • 33×12.5 KO2s (<10k miles)
  • 10-bolt axles w/new bearings & seals
  • 4.10 gears just installed, <500 miles
  • 2" lift, Bilstein shocks
  • Driveshaft angles:
    • T-case output: –5.1°
    • Rear pinion: +6.1°
    • Rear shaft: +19.6°
  • Purpose: long highway trips in FL + towing a <3,000 lb Casita for 100s–1000s of miles
  • Mall crawler until the kids get older

My 4L80E Swap Checklist — please sanity-check this

Transmission / Drivetrain Parts
  • 4L80E built with the correct 4WD output shaft (mandatory)
  • 32-spline NP241C input gear *or* full 4L80E→241 adapter kit
  • Standalone controller (leaning US Shift)
  • 4L80E dipstick & tube
  • Cooler line adapters or custom -6AN lines (4L80E ports are further back)
  • Shifter rod/bracket adjustment
  • Move crossmember rearward (existing crossmember reused)
  • Front driveshaft longer, rear shorter
  • Possible floor/tunnel tapping near cooler line bosses
  • Verify flexplate spacing & converter bolt pattern
  • Verify starter alignment/engagement
TPS & MAP to controller
  • Simply tap existing TBI TPS sensor signal wire
  • Use a T-fitting on an existing vacuum nipple and a second GM MAP sensor - only needed if the controller requires it

VSS

Driveshaft / Angle Stuff
  • With the longer 4L80E + 2" lift, rear shaft angle gets steeper
  • May Will need a SYE + double-Cardan rear shaft depending on final angles
  • Front shaft length + slip travel needs to be checked too

Gear Ratio Note
  • 700R4 1st = 3.06:1
  • 4L80E 1st = 2.48:1
  • I’m totally fine with this — towing reliability > off-the-line punch

Questions for the CK5 experts
  • My understanding is the stock crossmember is reused — I just drill new holes to move it back. Correct?
  • Do the common 4L80E→NP241 Adapter kits include:
    • the adapter housing and
    • the 32-spline input gear?
    • Or does it vary by vendor?
  • If I order a fresh 4L80E, can a shop like Monster or Certified just build it with the correct 4WD output shaft from the start?
  • Given my lift + angles + the longer trans:
    • Is a SYE + double-Cardan rear shaft basically mandatory?
    • Anyone running the stock slip yoke with a 4L80E successfully?
    • Any issues with front shaft clearance after the swap?

If I’m missing something obvious, feel free to roast me before I spend money.
Click to expand...
It's a good idea BUT that two different types of transmission housing! Because eventually im going to put and Allison transmissions in my K5. So there alot of reworking the drive train.
 
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