Overdrive Transmission for 1976 K20 crew cab

[k10klub]

Morning all,
I am trying to do a little planning for putting an OD trans in my K20. The truck currently has a TH350 and an NP203 that previous owner converted to part-time.
I am leaning pretty hard towards an automatic, 4L80E & an NP205, neither of which I have on hand.
I am curious about NV4500 options & what passenger-drop transfer case(s) bolt up. I am a little hesitant to do the 3-pedal 2-step dance in a 21 ft long truck, however the ability to have OD without a computer is appealing.
1. What passenger-drop T-case options do I have to go behind a 4L80E?
2. What passenger-drop T-case options do I have to go behind an NV4500?
2a. Can I bolt a 2wd NV4500 to a tranfer case? How?
3. Are there any other manual trans options out there that would have a passenger-drop T-case?
Thanks for the input.

 

[Bent77]

There are lots of trans options, but the 4L80 and NV4500 are more obtainable due to cost and availability

Having said that, they both use a transfer case that is 32 spline input

All will be 6 bolt round pattern
205 - fairly hot commodity due to use in doubler applications
208
241 - make sure it’s passenger, not driver drop.

 

[ZooMad75]

Keep in mind the 4500 will require a little more work than just adding the clutch/brake pedal set. The common route is to use an Advanced Adapters bellhousing kit. It can be used with mechanical linkage, but they have brackets to allow the use of later squarebody hydraulic slave and master cylinder parts. Only issue there is if you go that way, you need a later hydraulic version of the clutch pedal and pedal box since it has the added support struts to support the master cylinder on the firewall.

Or if you don't go with the AA bellhousing, you can use the later GM 4500 bellhousing for the '96 and up trucks that had the integral slave/throwout bearing. A couple of adapters and an AN line and it hooks right up to the Squarebody master cylinder. Still need the hydraulic version of the clutch pedal due to the different pivot point vs mechanical.

The 4L80e will require a harness and computer to run it since it's electronically controlled. Length is more than your existing th350 so the driveshafts are going to change lengths too.

Like Bent said any round pattern GM passenger drop transfer case with a 32 spline input will fit. 205/208/241. The 32 spline versions of the 208 and 241 are a little harder to come by but you can find the 32 spline inputs available separately if you found a more common 27 spline input version.

AA has an adapter kit to allow the install of an earlier 205 with the figure 8 style mounting to a 4500. It's AA silly expensive. It also requires the replacement of the rear extension housing with a AA part and for that to fit you have to chop off 5" or so off of the stock 4500 output shaft and change the 5th gear retaining nut to a clamshell-style device. Those are major modifications to a transmission that is getting harder to find replacement parts for. I'd avoid it and look harder for a round pattern t-case.

 

[6872xtc]

That's a clean truck! What are your plans for future use?
If you don't need a bomb proof transfer case and you wouldn't use a twin stick control on a 205, a 208 or 241 would be what I would look into. They have a better low range ratio, use a single lever to control and will bolt up to an NV4500. If you get the AA bellhousing, which is expensive, then you can find the mechanical pedal setup to use instead of the expensive hydraulic stuff.
The 4L80E would also need a TPS on the carburetor if you keep it automatically shifted, on top of what Zoomad posted.

 

[red EOD]

Another option is to run a 2wd trans with a divorced tcase. NP205 can be easily setup as a divorced unit.

 

[shady]

Can't he just do a 4l80 with a full manual valve body. Still shift every time like a 4500 but no 3rd pedal....

 

[red EOD]

Can't he just do a 4l80 with a full manual valve body. Still shift every time like a 4500 but no 3rd pedal....

Not aware of any daily driver reliable options for that with the 4L80E. There is a manual valve body and its been done, but it shifts at max line pressure every, single, time.

 

[shady]

Let off the pedal, shift, get back on pedal.
That would smooth it out a lot.

 

[Bent77]

Let off the pedal, shift, get back on pedal.
That would smooth it out a lot.

That’s just like how it’s done with 3 pedals

 

[shady]

Except no pushing another pedal lol

 

[shady]

I had. Aheavy shift kit in my old 2wd burb. Instead of letting it slam every time I just let off when I thought it should shift. It would shift while I was off it and then I'd just push pedal again. I kind of figured with a full manual it would work the same

 

[k10klub]

There are lots of trans options, but the 4L80 and NV4500 are more obtainable due to cost and availability

Having said that, they both use a transfer case that is 32 spline input

All will be 6 bolt round pattern
205 - fairly hot commodity due to use in doubler applications
208
241 - make sure it’s passenger, not driver drop.

ORD has some stuff on their site that looks like it would allow use of a 10, 23, or 27 spline input case behind a 4wd 4L80, but it ends up being about $500 worth of machining...

 

[k10klub]

Keep in mind the 4500 will require a little more work than just adding the clutch/brake pedal set. The common route is to use an Advanced Adapters bellhousing kit. It can be used with mechanical linkage, but they have brackets to allow the use of later squarebody hydraulic slave and master cylinder parts. Only issue there is if you go that way, you need a later hydraulic version of the clutch pedal and pedal box since it has the added support struts to support the master cylinder on the firewall.

Or if you don't go with the AA bellhousing, you can use the later GM 4500 bellhousing for the '96 and up trucks that had the integral slave/throwout bearing. A couple of adapters and an AN line and it hooks right up to the Squarebody master cylinder. Still need the hydraulic version of the clutch pedal due to the different pivot point vs mechanical.

The 4L80e will require a harness and computer to run it since it's electronically controlled. Length is more than your existing th350 so the driveshafts are going to change lengths too.

Like Bent said any round pattern GM passenger drop transfer case with a 32 spline input will fit. 205/208/241. The 32 spline versions of the 208 and 241 are a little harder to come by but you can find the 32 spline inputs available separately if you found a more common 27 spline input version.

AA has an adapter kit to allow the install of an earlier 205 with the figure 8 style mounting to a 4500. It's AA silly expensive. It also requires the replacement of the rear extension housing with a AA part and for that to fit you have to chop off 5" or so off of the stock 4500 output shaft and change the 5th gear retaining nut to a clamshell-style device. Those are major modifications to a transmission that is getting harder to find replacement parts for. I'd avoid it and look harder for a round pattern t-case.

Thanks Rob, I was not aware of all the bellhousing and slave cylinder compatibility requirements, you may have scared me away from the 4500, I'll need to think it over some more.

 

[k10klub]

That's a clean truck! What are your plans for future use?
If you don't need a bomb proof transfer case and you wouldn't use a twin stick control on a 205, a 208 or 241 would be what I would look into. They have a better low range ratio, use a single lever to control and will bolt up to an NV4500. If you get the AA bellhousing, which is expensive, then you can find the mechanical pedal setup to use instead of the expensive hydraulic stuff.
The 4L80E would also need a TPS on the carburetor if you keep it automatically shifted, on top of what Zoomad posted.

Thanks! Daily driver, road-tripper, some very mild wheeling... I forgot about the TPS

 

[k10klub]

Another option is to run a 2wd trans with a divorced tcase. NP205 can be easily setup as a divorced unit.

I have though about that, I definitely have the room on the crew cab long bed, but I guess I subconsciously decided that the price/availability of a divorced t-case and then the expense of the tranny to t-case driveshaft would make it as expensive, if not more, than the married setup. I'll have to revisit that option, can you actually convert a figure 8 or 6 bolt married NP205 to have a u-joint yoke on the input like a divorced case?

 

[k10klub]

Can't he just do a 4l80 with a full manual valve body. Still shift every time like a 4500 but no 3rd pedal....

Not aware of any daily driver reliable options for that with the 4L80E. There is a manual valve body and its been done, but it shifts at max line pressure every, single, time.

I had. Aheavy shift kit in my old 2wd burb. Instead of letting it slam every time I just let off when I thought it should shift. It would shift while I was off it and then I'd just push pedal again. I kind of figured with a full manual it would work the same

Apparently you can rig up 3 switches and shift a 4L80 with no computer or TPS or VSS needed, and I am honestly considering this option at the moment. One video I watched the guy does mention it shift at FULL LINE PRESSURE, so yeah, drive it like you have an imaginary 3rd pedal.

 

[k10klub]

I have about 810 things to do at work this week, so I'll have to come back later, but here are the options I'll be researching:

Option 1: 4L80 + NP205
Option 2: NV4500 + NP205, 208, or 241
Option 3: SM465 + NP205 or 208 with a Ranger Torque Splitter ($1600-1700 new) gives me 0.73 overdrive (AND another gear between 3rd & 4th)

 

[dyeager535]

Stupid question. Why not a Ranger or Gear Vendors with your current setup?

Dont know what a GV unit runs nowadays, but I assume they can be mated to a 203? Pretty simple setup, relatively.

 

[6872xtc]

Stupid question. Why not a Ranger or Gear Vendors with your current setup?

Dont know what a GV unit runs nowadays, but I assume they can be mated to a 203? Pretty simple setup, relatively.

While I haven't looked specifically for a GV unit for a 203, I don't remember one being listed, just for a 205. And the one that I saw for a 205 was for a slip yoke t-case. But, again I haven't looked at applications for a long time.
A Ranger with the SM465 would be neat to me.

 

[k10klub]

Stupid question. Why not a Ranger or Gear Vendors with your current setup?

Dont know what a GV unit runs nowadays, but I assume they can be mated to a 203? Pretty simple setup, relatively.

While I haven't looked specifically for a GV unit for a 203, I don't remember one being listed, just for a 205. And the one that I saw for a 205 was for a slip yoke t-case. But, again I haven't looked at applications for a long time.
A Ranger with the SM465 would be neat to me.

Good question, I'm not sure you can put a Ranger in front of a TH350, but I will look into it.

Gear Vendors is pretty pricey, $3000+ ballpark, not including a new or shortened rear driveshaft, for that money I could probably put in a 4L80 + NP205 and have a HD transmisison with lockup.