Overdrive Transmission for 1976 K20 crew cab

[red EOD]

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?

Best to check with ORD or some of the other vendors but thought I remembered seeing all the parts to convert a typical married np205 to a divorced unit. New input shaft and front seal retainer should be about it.

Think the ranger unit is limited to stick shift applications only, has good reviews.

Gear vendors is one of the options I'm considering in my 3500hd tow rig to help the nv4500. It's either that or go to a bigger trans with smaller gearing gaps

 

[6.2Blazer]

Not sure the intended use of the truck but have you considered a TH700R4 along with a 241 transfer case? If you are not towing heavy or running big HP I would consider it. Way better crawl ratio than just about anything listed above, no computers needed, easy to to come by, no special adapters needed. I ran the stock setup in my '90 K5 for years with 1-tons and 40" tires. Had the trans rebuilt at around 120k but it had developed a leak and was ran low on fluid. Another off-road friend ran this combo for many years on his 1-ton 40" tire K1500 that was fully tubed out with good luck. On his new build he's using a 4L60E mated to a 205 doubler setup. Reason for the 205 was that he physically broke the housing on a 241, plus wanted to the front wheel drive/dig ability.

 

[k10klub]

Not sure the intended use of the truck but have you considered a TH700R4 along with a 241 transfer case? If you are not towing heavy or running big HP I would consider it. Way better crawl ratio than just about anything listed above, no computers needed, easy to to come by, no special adapters needed. I ran the stock setup in my '90 K5 for years with 1-tons and 40" tires. Had the trans rebuilt at around 120k but it had developed a leak and was ran low on fluid. Another off-road friend ran this combo for many years on his 1-ton 40" tire K1500 that was fully tubed out with good luck. On his new build he's using a 4L60E mated to a 205 doubler setup. Reason for the 205 was that he physically broke the housing on a 241, plus wanted to the front wheel drive/dig ability.

Daily driver, take the kids to get a donut, road tripper, cruising, camping, etc...
Future plans would not exceed 33" tires, 400 HP, or any heavy towing.
700R4 might be worth another look.

 

[Wes Harden]

Do you know what gear ratio you have?

Gonna guess and say you have a 78, and a small block. Probably came with 4.10s. The 700r4 is going to put you @ 1500rpms 55 mphand 1900 at 65 mph. With 33" (32.5). Just under the power band.
I didn't like this in my 77 k20 sbc.
I re geared to 4.88, which has me at 2500 rpm at 65. I see 35's in my future.
If you use a 700r4 31" tires are going to be the ticket with 4.10

 

[k10klub]

Do you know what gear ratio you have?

Gonna guess and say you have a 78, and a small block. Probably came with 4.10s. The 700r4 is going to put you @ 1500rpms 55 mphand 1900 at 65 mph. With 33" (32.5). Just under the power band.
I didn't like this in my 77 k20 sbc.
I re geared to 4.88, which has me at 2500 rpm at 65. I see 35's in my future.
If you use a 700r4 31" tires are going to be the ticket with 4.10

Good point. Currently have 31s & pretty sure I have 4.10s. I'm around 3000 rpms at 65 MPH +/-

Plan is to put in an OD trans and mash the gas until the tach reads 2500

 

[mpg7_400]

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.

View attachment 345660

Before rushing into an Overdrive transmission, I suggest if you have not already done so is to take a look at the gear ratio in the differentials you are running, along with your preferred tire size. From the picture it doesn't look like you are into monster tires. Good. So for instance, if you are running 4.11's well you shouldn't be. Even 3.73's may be a bit too short in the legs. How about 3.11's? Not enough low range? Well, that's why the transfer case has a low range on it so no worries. This would be al lot cheaper then a manual transmission swap out. Moreover, I agree with you about not wanting to get your truck involved with a computer! Off Road Designs can probably dig up a NP205 T-case. They also make a torque doubler dual transfer case adaption to the 205 giving you two shift levers, low range (2 to 1) and double down low range (4 to 1) and of course regular drive range (caution: not shiftable on the fly as you probably already know). I have this setup on my 78 K5 along with a THM 400 bullet proof automatic. A THM 400 which would take whatever beating you can give it in that CK 20 full size truck. So I would take a look at the diff case gear ratios and tire size first as that would be a cheaper and more reliable way to address the issue.

 

[Wes Harden]

Good point. Currently have 31s & pretty sure I have 4.10s. I'm around 3000 rpms at 65 MPH +/-

Plan is to put in an OD trans and mash the gas until the tach reads 2500

yes the number come out close to 4.10.

Even if you went to true 33" with current set up you would be 2713 at 65. True 31s and 3.73 ratio 65 is 2627, with 3.42 ratio 65 is 2409. To buy ring and pinion sets with install kits no labor $500-800 depending on brand and quality of bearings. Good new is the carrier in each axle will do those gear sets.
I will say putting a 700r4 in my 77 has been an adventure. I started having fuel delivery issues. I think it was a combination of low rpms and 15% ethanol. I tried several things believed solved several times. I think I have it solved now time will tell. It now has a Holley red label pump and fuel reg with return to tank. re jetted Qjet. I am in over 2k for the swap. Don't really want to add it up. I re-geared to move the rpms at cruise back into the torque curve of my 350 sbc. Even with the 4.88 ratio at 3000 rpm is 83 mph. 65 is @ 2250.

 

[6.2Blazer]

Before rushing into an Overdrive transmission, I suggest if you have not already done so is to take a look at the gear ratio in the differentials you are running, along with your preferred tire size. From the picture it doesn't look like you are into monster tires. Good. So for instance, if you are running 4.11's well you shouldn't be. Even 3.73's may be a bit too short in the legs. How about 3.11's? Not enough low range? Well, that's why the transfer case has a low range on it so no worries. This would be al lot cheaper then a manual transmission swap out. Moreover, I agree with you about not wanting to get your truck involved with a computer! Off Road Designs can probably dig up a NP205 T-case. They also make a torque doubler dual transfer case adaption to the 205 giving you two shift levers, low range (2 to 1) and double down low range (4 to 1) and of course regular drive range (caution: not shiftable on the fly as you probably already know). I have this setup on my 78 K5 along with a THM 400 bullet proof automatic. A THM 400 which would take whatever beating you can give it in that CK 20 full size truck. So I would take a look at the diff case gear ratios and tire size first as that would be a cheaper and more reliable way to address the issue.

The trouble with putting high axle gears in to reduce the RPM on the freeway is it has a big downside of making the truck a dog on acceleration. The factory put a lot of 3.08 gears in axles back in the late 70's and through the 80's prior to OD transmission to get the RPM's down on the road and improve fuel economy, and I can tell you that those trucks felt like dogs in every other driving situation except for cruising down a flat freeway at 65 mph. I've driven a bunch of TH350 (or C4 or C6 Fords) with around a 2.5:1 first gear and 3.08-ish gears and with 33" or larger tires they are horrible around town. Taking off from a stop was slow. Even on the freeway if you got on a grade where it struggled to pull 3rd you had to slow down to get 2nd without over revving the engine.

I would take a TH700R4 with 4.10 axle gears any day over a non-OD transmission and 3.08 gears. Let's look at the gear range in 2 high running around the streets.

TH700R4 w/ 4.10
1st gear = 12.5:1
4th (OD) gear = 2.87:1

TH350 w/ 3.08
1st gear = 7.7:1
3rd gear = 3.08

The 700R4 combo give a MUCH lower first gear/take of gear and a slightly higher (lower rpm) upper gear.

 

[Blue85]

Not enough low range? Well, that's why the transfer case has a low range on it so no worries. This would be al lot cheaper then a manual transmission swap out. Moreover, I agree with you about not wanting to get your truck involved with a computer! Off Road Designs can probably dig up a NP205 T-case. They also make a torque doubler dual transfer case adaption to the 205 giving you two shift levers, low range (2 to 1) and double down low range (4 to 1) and of course regular drive range (caution: not shiftable on the fly as you probably already know). I have this setup on my 78 K5 along with a THM 400 bullet proof automatic. A THM 400 which would take whatever beating you can give it in that CK 20 full size truck. So I would take a look at the diff case gear ratios and tire size first as that would be a cheaper and more reliable way to address the issue.

So we're talking about a dual transfer case, cutting the floor, buying a dual shifter setup, getting another crossmember and new driveshafts. This is to avoid adding a relatively simple transmission swap, just to get a 4th gear like most cars have had for the last 40 years? (and now they have 5, 6, 8 gears....)

The trouble with putting high axle gears in to reduce the RPM on the freeway is it has a big downside of making the truck a dog on acceleration. The factory put a lot of 3.08 gears in axles back in the late 70's and through the 80's prior to OD transmission to get the RPM's down on the road and improve fuel economy, and I can tell you that those trucks felt like dogs in every other driving situation except for cruising down a flat freeway at 65 mph. I've driven a bunch of TH350 (or C4 or C6 Fords) with around a 2.5:1 first gear and 3.08-ish gears and with 33" or larger tires they are horrible around town. Taking off from a stop was slow. Even on the freeway if you got on a grade where it struggled to pull 3rd you had to slow down to get 2nd without over revving the engine.

I would take a TH700R4 with 4.10 axle gears any day over a non-OD transmission and 3.08 gears. Let's look at the gear range in 2 high running around the streets.

TH700R4 w/ 4.10
1st gear = 12.5:1
4th (OD) gear = 2.87:1

TH350 w/ 3.08
1st gear = 7.7:1
3rd gear = 3.08

The 700R4 combo give a MUCH lower first gear/take of gear and a slightly higher (lower rpm) upper gear.

I tend to agree. A doubler is for a rig insisting on maximum brute strength drivetrain. These rigs have big tires with high differential gearing (relative to the tire size) to avoid an overdrive transmission (because those are weak and/or too complicated). In double low range, a TH400/203/205/4.10 gears = 45:1. A TH-700/NP241/5.13 gears in single low = 43:1. Highway cruising TH400/doubler/4.10 = 4.10:1 while TH-700/NP241/5.13 gears = 3.6:1.

 

[6.2Blazer]

In my opinion axle gears and doublers (or low geared transfer cases) are for two completely different reasons. Axle gears are mainly to optimize on-road driving while the doubler is if you want really low gearing for off-road use. Going from 4.10 to 5.13 axle gears is "only" a 25% difference where as going from a standard NP205 to a doubler (2:1 range box for example) is a 100% difference. Just shifting from high range to low range on a NP205 is a 196% reduction, or a 272% reduction with a NP241. Basically swapping axle gears can't come anywhere close to the overall gear reduction that any normal transfer case can.

Anyway to get back on subject, a doubler would serve no purpose on the OP's rig.

 

[sweetk30]

but you also need gears for tire size and load on the vehicle or it can be a super dog .

all real good points here .

 

[k10klub]

Well, I have pretty much made my mind up to go with a Ranger-SM465-NP205 swap. #1 is for simplicity of the finished product - no computer, no cooler, no TV to monkey up. #2 is similar to #1, I have an SM465 in my 72 K10 and absolutely love it. #3 builds on #2, I figure if I love my 4-speed in the K10, an few extra gears will be icing on the cake.

I found a guy in Mississippi with a 2wd 76 suburban who is swapping from a 350/SM465 to an LS/4L80, and I am going to swap pedal assemblies and steering columns with him, and get his clutch linkage, bellhousing, flywheel, clutch, etc...

Also going to look at an SM465/NP205 this Saturday with cash in my pocket.

 

[k10klub]

Now I'm looking at this picture again and it looks like this T-case has a slip yoke rear output. What years would that have been and is it worth the trouble to eliminate?

 

[max 02]

Now I'm looking at this picture again and it looks like this T-case has a slip yoke rear output. What years would that have been and is it worth the trouble to eliminate?

1980

Run it with the skip yoke if your not going wheel it hard.

 

[diesel4me]

Slip joint is fine for street & light duty off roading..you'd have to flex it pretty good or get it airborne to get the rear drive shaft to ever come out..

I like divorced transfer cases best...with one of those you can use any manual or automatic in front of it simply by making up a short drive shaft with the right u-joint at each end to mate the transmission to the transfer case..but a K5 doesn't really have enough wheelbase to use one easily..long bed pickup would..

I drove a '68 K10 a friend had ,that had a divorced transfer case (Rockwell T-221 I think ,it was a factory GM setup)--he put a Muncie M-22 in front of it instead of the factory SM-420 4 speed (which is a granny low type like the SM465),and the truck was much more streetable with a 4 speed that had a synchromesh first gear ,and ratios that made more sense for his uses,mostly it was just a daily driver and light duty chore truck..
The transmission came out of a corvette his dad crashed...an SM 465 is basically a 3 speed,has nearly the same ratios as a 3 speed in second,third and fourth--first is granny low,non synchro,only good for creeping slow or pulling stumps..
Truck went really good too,had a warmed up 350 and being able to use first gear and shift it as fast as you wanted made a big difference..the only bummer was the shifter handle was too short and close to the bench seat..but he could have made up a better one..

 

[shady]

Only bad I ever heard about the ranger box is it eats up your syncros faster. More rotating mass to slow down/speed up

 

[Wes Harden]

The CC long bed is going to use a 2 piece drive shaft. So slip yoke out put shouldn't be an issue

 

[k10klub]

I currently have a u-joint rear output on the NP203 & it seems like it would be easier to shorten the rear driveshaft if I stay with a similar configuration. Gotta check those u-joint sizes. Also, if I go with a yoke output I have the potential for interchangeable transmission and transfer case with my K10.
I located one with a yoke output yesterday, talked to the guy, he sounds like he is gonna throw a few things in on the deal too, also sounded more knowledgeable than the folks with the other case.

 

[max 02]

I currently have a u-joint rear output on the NP203 & it seems like it would be easier to shorten the rear driveshaft if I stay with a similar configuration. Gotta check those u-joint sizes. Also, if I go with a yoke output I have the potential for interchangeable transmission and transfer case with my K10.
I located one with a yoke output yesterday, talked to the guy, he sounds like he is gonna throw a few things in on the deal too, also sounded more knowledgeable than the folks with the other case.

View attachment 357352

View attachment 357353

View attachment 357354

Definitely a better way to go with that set up. The only thing better about the other with the slip yoke is that it would be a 32 spline shaft in the T/C.

That said the fixed yoke may have 32 spline too?

 

[k10klub]

Yesterday was a busy day. I rode over to Alabama to pickup a trans & t-case, spotted a 77 C30 on I-10 along the way. We pulled over and chatted, he has a turbo'd 6.0 LS & 4L80. Snapped some pics and I was on my way. 360 mile round trip and wishing I had that extra gear the entire way.