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73 K5 needs a new transmission, no idea what to ask shop for
- Thread starter rc73
- Start date
If you don’t drive it alot I would just get the current trans rebuilt if it needs to be…. Honestly it’s been in it for 50 +years. Not like your driving and classic Blazer on 35s for fuel mileage.
Oh yeah… we need pics of the rig! 
From the shop point of view most won't want to deal with the idea of a wholesale change in the drivetrain unless they are really familiar with it. Robert's Custom Trucks is there in southern CA somewhere that is a member here that could facilitate that type of swap.
Main reason being is since nobody makes a true "kit" to make a 700r4 swap idiot proof that means the shop/tech would have to do the research to figure out the nuances of making the swap happen. Yes, a 700r4 swap is truly a bolt in affair, but you need to know what parts you need with the trans to make it bolt in. Trans cross-member, T-case to match the trans and the other smaller items to make it work. Most won't be willing to go through the effort to figure it out much less know how to adjust the TV cable to avoid smoking the fresh 700r4 due to low line pressure.
They won't give you a hard estimate because they won't know what it may be. They will probably say the job is going for straight time and material. Meaning however many hours they spend to complete the job is going to multiplied by the hourly rate. Which I'm guessing in SoCal is probably $180/hr and up. That adds up fast.
I'm betting most production type shops will avoid the job like the plague as it's way outside the norm of yanking a trans, rebuilding and replacing. Don't be surprised if they give you an astronomical price to scare you away. That's known as the "F_U" price as it's so high they know you'll back off and go somewhere else so they don't have to deal with it. Consider it a polite way to say to take off without being upfront and jerks about it.
Here's the thing like others have indicated making the switch to OD and not changing axle ratios you might find the engine may not have enough power to maintain OD on hills or driving into a headwind. Making the whole change of the drivetrain kind of a waste of time/money. You can find many examples here with stock or slightly modded K5's with 35" tires, stock crappy gearing like 3.08's, 700r4 trans, and a weak stock 305 or 350 and find that the engine lacks the torque needed not to have to downshift all the time on inclines or into a headwind. I'd be curious what the axle ratios are to see if it's even worth going through the effort of swapping in the OD trans.
Main reason being is since nobody makes a true "kit" to make a 700r4 swap idiot proof that means the shop/tech would have to do the research to figure out the nuances of making the swap happen. Yes, a 700r4 swap is truly a bolt in affair, but you need to know what parts you need with the trans to make it bolt in. Trans cross-member, T-case to match the trans and the other smaller items to make it work. Most won't be willing to go through the effort to figure it out much less know how to adjust the TV cable to avoid smoking the fresh 700r4 due to low line pressure.
They won't give you a hard estimate because they won't know what it may be. They will probably say the job is going for straight time and material. Meaning however many hours they spend to complete the job is going to multiplied by the hourly rate. Which I'm guessing in SoCal is probably $180/hr and up. That adds up fast.
I'm betting most production type shops will avoid the job like the plague as it's way outside the norm of yanking a trans, rebuilding and replacing. Don't be surprised if they give you an astronomical price to scare you away. That's known as the "F_U" price as it's so high they know you'll back off and go somewhere else so they don't have to deal with it. Consider it a polite way to say to take off without being upfront and jerks about it.
Here's the thing like others have indicated making the switch to OD and not changing axle ratios you might find the engine may not have enough power to maintain OD on hills or driving into a headwind. Making the whole change of the drivetrain kind of a waste of time/money. You can find many examples here with stock or slightly modded K5's with 35" tires, stock crappy gearing like 3.08's, 700r4 trans, and a weak stock 305 or 350 and find that the engine lacks the torque needed not to have to downshift all the time on inclines or into a headwind. I'd be curious what the axle ratios are to see if it's even worth going through the effort of swapping in the OD trans.
You could post in the "Parts Wanted" section here to see if anybody is selling these parts or even a whole truck. It would be nice to have somebody who can vouch for stuff, rather than buy parts from a junkyard and pray. But if you can find a running, driving 80's rig with overdrive, it should have everything you need. Then when you're done, sell off all the extra parts and re-coup a lot. A shop doesn't want the homework of finding all of these parts.
Find out what gear ratio you have. With O/D you want something like 4.56 on 35's for best performance. If you find a donor vehicle and it has good gears, that saves $3-$4k on having a shop setup new gears. I mean, if you're diving into this, you want everything working well when you're done.
Next up, somebody will suggest upgrading to a stronger rear axle. The snowball effect continues... Also, there's a point where swapping the body is simpler...
Find out what gear ratio you have. With O/D you want something like 4.56 on 35's for best performance. If you find a donor vehicle and it has good gears, that saves $3-$4k on having a shop setup new gears. I mean, if you're diving into this, you want everything working well when you're done.
Next up, somebody will suggest upgrading to a stronger rear axle. The snowball effect continues... Also, there's a point where swapping the body is simpler...
My bone stock engine, 5.7TBI, drops out of OD in the 700r4 when it looks at a slight grade. 35" tires and 4.56:1 gears. You probably have 3.07:1 or 3.73:1 gears, so it would be worse. Rebuild what you have and enjoy the ride into town. If you are in a hurry to get there, drive something else. Just my $0.02.
I second speaking to https://www.robertscustomtrucks.com/ in Ontario, near Pomona/Montclair boarder.
Let me tell you about my 77 k20 conversion. In 2010 I had Bowtie Overdrives, Hesperia CA, build a 1991 K case 700r4. Swapped the working th350 out for this new trans, I needed to shorten the rear drive shaft 1.75". I used a NP205 from a 72 k5, my truck had been an np203, and I didn't want heavy full time t case. If you want you could still use a np203.
Shortly after the swap my truck didn't run well in 4th, I blamed the trans and quit driving it. fast forward to late 2018 I have the Toy Store, in Pomona CA, Go through my 700r4 a second time, there was a a bad band. My problem from before was still there. Turned out to fuel related. For some reason I still haven't gotten to the bottom of I could not get enough fuel at the carb in high gear, sometimes in 3rd. I put an electric pump and have 20k plus miles now.
It is very important to get a good tv cable. The aftermarket is selling an "adjust" sheath, with a sliding cable lock. I did not like this. After a few adjust of the slider the cable gets rough and chewed. I sourced a TV cable from mid 90's Caprice, fit perfectly. Then I had an issue with getting full throttle, I had tried a TV cable corrector that is supposed to put the in right place to travel the full arch, on a quadrajet carburetor. I could not get it to work. Sunex makes a little push spring with a plastic center. This works well, it keeps the cable adjust taught, and allows wide open throttle, using the factory detent cable location on a Qjet.
now you have to decide what transfer case, I went with np205 using the th350 adapter, and the Transdapt conversion output shaft, which is shorter, similar to the th350. This gets the the tcase adapter right up to the 700r4 case, which I think is stronger. After all the are only 4 bolts, and should have a support rod holding all this together. This all sounds great until you try to install a front drive shaft. There is very little room to clear the right rear corner of the pan. I was able to use a stock pan with the early style 3R cv drive shaft, not the strongest out there. I was able to use this drive shaft be cause I had a 10 spline front output, which uses the correct yoke. a later 30 spline output might not have a yoke to match.
Second option is to use the normal 700r4 output shaft and add a spacer between the trans case. this adds @4". Better clearance and angles for front shaft, but more severe angles and shorter rear shaft, no big deal for a burb, but a K5 might be an issue.
So then if you choose a later t case Np208 or np241c you could use the stock output, have good trans pan clearance, and get stronger drive shafts, with ok angles at @4" lift for 35" tires. You will not be able to use the flat tcase cross member with either of those 2 tcases.
Exhaust might and issue might need to be rerouted.
With more driving and traveling after I solved my fuel deliver issue, I didn't like where the engine rpm ran at So Cal freeway speeds ie 50 to 70, at 50 mph in 4th the engine was taching 1550 rpm. I could not accelerate to pass anything and the drop into 3rd took longer than I liked. 70 mph plus the truck was an animal but the rpm was still at the bottom of the torque curve, 80+ was easy. I decide to regear the differentials to 4.88 from the stock 4.10. This moved my 50 mph to 2000 rpm and 65-70 into 2300-2600 range. 65 in 3rd is 3400 rpm, pulls hills with a load well here. If I were to choose gears again I think 5.38 would be more appropriate that works out 3.76 in 4th.
I wanted the 700r4 to get a lower first gear 3.0 compare to th350 of 2.52. would I do it again? No, previous i had a k20 burb with sm465 4sp and 33" 4.1 gears and drove that truck everywhere, towed heavy cars, on highways out side Kommiefornias 55 mph limit 70-75 no problem. Dumont Dunes sandy ass camping areas never even need 4wd, that granny low got me out on top with a trailer. Only reason I switched burbs this one had a/c.
I am getting average of 8mpg my best is 9.3
Let me tell you about my 77 k20 conversion. In 2010 I had Bowtie Overdrives, Hesperia CA, build a 1991 K case 700r4. Swapped the working th350 out for this new trans, I needed to shorten the rear drive shaft 1.75". I used a NP205 from a 72 k5, my truck had been an np203, and I didn't want heavy full time t case. If you want you could still use a np203.
Shortly after the swap my truck didn't run well in 4th, I blamed the trans and quit driving it. fast forward to late 2018 I have the Toy Store, in Pomona CA, Go through my 700r4 a second time, there was a a bad band. My problem from before was still there. Turned out to fuel related. For some reason I still haven't gotten to the bottom of I could not get enough fuel at the carb in high gear, sometimes in 3rd. I put an electric pump and have 20k plus miles now.
It is very important to get a good tv cable. The aftermarket is selling an "adjust" sheath, with a sliding cable lock. I did not like this. After a few adjust of the slider the cable gets rough and chewed. I sourced a TV cable from mid 90's Caprice, fit perfectly. Then I had an issue with getting full throttle, I had tried a TV cable corrector that is supposed to put the in right place to travel the full arch, on a quadrajet carburetor. I could not get it to work. Sunex makes a little push spring with a plastic center. This works well, it keeps the cable adjust taught, and allows wide open throttle, using the factory detent cable location on a Qjet.
now you have to decide what transfer case, I went with np205 using the th350 adapter, and the Transdapt conversion output shaft, which is shorter, similar to the th350. This gets the the tcase adapter right up to the 700r4 case, which I think is stronger. After all the are only 4 bolts, and should have a support rod holding all this together. This all sounds great until you try to install a front drive shaft. There is very little room to clear the right rear corner of the pan. I was able to use a stock pan with the early style 3R cv drive shaft, not the strongest out there. I was able to use this drive shaft be cause I had a 10 spline front output, which uses the correct yoke. a later 30 spline output might not have a yoke to match.
Second option is to use the normal 700r4 output shaft and add a spacer between the trans case. this adds @4". Better clearance and angles for front shaft, but more severe angles and shorter rear shaft, no big deal for a burb, but a K5 might be an issue.
So then if you choose a later t case Np208 or np241c you could use the stock output, have good trans pan clearance, and get stronger drive shafts, with ok angles at @4" lift for 35" tires. You will not be able to use the flat tcase cross member with either of those 2 tcases.
Exhaust might and issue might need to be rerouted.
With more driving and traveling after I solved my fuel deliver issue, I didn't like where the engine rpm ran at So Cal freeway speeds ie 50 to 70, at 50 mph in 4th the engine was taching 1550 rpm. I could not accelerate to pass anything and the drop into 3rd took longer than I liked. 70 mph plus the truck was an animal but the rpm was still at the bottom of the torque curve, 80+ was easy. I decide to regear the differentials to 4.88 from the stock 4.10. This moved my 50 mph to 2000 rpm and 65-70 into 2300-2600 range. 65 in 3rd is 3400 rpm, pulls hills with a load well here. If I were to choose gears again I think 5.38 would be more appropriate that works out 3.76 in 4th.
I wanted the 700r4 to get a lower first gear 3.0 compare to th350 of 2.52. would I do it again? No, previous i had a k20 burb with sm465 4sp and 33" 4.1 gears and drove that truck everywhere, towed heavy cars, on highways out side Kommiefornias 55 mph limit 70-75 no problem. Dumont Dunes sandy ass camping areas never even need 4wd, that granny low got me out on top with a trailer. Only reason I switched burbs this one had a/c.
I am getting average of 8mpg my best is 9.3
Last edited:
MetalMulishaMarcus
1 Ton Status
@jkroberts is roberts custom trucks.
