Correct Driveshaft Length for 700-R4/NP208/14bolt FF

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,090
Reaction score
5,978
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
@AuroraGirl it seems like all the slip yoke style T-cases used a yoke roughly the same length. They are longer than your average car type TH400 yoke, but some newer 2wd trucks and vans use the same long style slip yoke.

My '88 Burb was originally a 700/208/10bolt rig. When I lifted it and put the 14ff under it, the shaft was a bit short and I just ran a conversion u-joint at the rear and kept all the yokes as the 1330 or 3R or whatever they were (1350 on the differential, obviously). I ended up gathering a long slip yoke from a '00 4L80e van (1350), a section of C30 shaft with two yokes and same tube diameter as my original shaft, then took it all to my local driveline shop. Using a new tube, they built a "new" shaft using all the parts I brought them. The same shaft is in use now with the 400/208 combo in there now.

Another interesting bit of swap info. That original 1/2 ton np208 slip yoke came in real handy when I swapped the long tail th350 for a short tail th400 in my C10. It killed two birds with one stone- lengthened the shaft and converted it to 32 spline.
Nice.
You must be registered for see images attach
The next step is to rescue this driveshaft and find a life for it LOL I’m pretty sure that’s a race car shaft..
 

scrap--metal

Full Access Member
Joined
Sep 15, 2021
Posts
673
Reaction score
1,255
Location
MN
First Name
Calvin
Truck Year
'85, '81
Truck Model
K10, K20
Engine Size
305, 350
My '88 Burb was originally a 700/208/10bolt rig. When I lifted it and put the 14ff under it, the shaft was a bit short and I just ran a conversion u-joint at the rear and kept all the yokes as the 1330 or 3R or whatever they were (1350 on the differential, obviously). I ended up gathering a long slip yoke from a '00 4L80e van (1350), a section of C30 shaft with two yokes and same tube diameter as my original shaft, then took it all to my local driveline shop. Using a new tube, they built a "new" shaft using all the parts I brought them. The same shaft is in use now with the 400/208 combo in there now.

Conversion U-joint!!! Why didn't I think of that... I have a spare 54.25" K10 drive shaft that's 1.5" longer than my K20 drive shaft, which is 52.75". I still think it's shorter than what my K20 needs, but that may be my solution. At least a temporary solution until I can source the pieces to have a new driveshaft built.

@bucket, any issues or things to be aware of with the conversion U-joints? Also, how much did you pay to have your driveshaft parts cut, welded, and balanced? There's a couple shops up here around the Twin Cities, but I have no idea what a job like that should cost.
 

AuroraGirl

Full Access Member
Joined
Sep 8, 2019
Posts
9,090
Reaction score
5,978
Location
Northern Wisconsin
First Name
Taylor
Truck Year
1978, 1980
Truck Model
K10, K25
Engine Size
400(?), 350
Conversion U-joint!!! Why didn't I think of that... I have a spare 54.25" K10 drive shaft that's 1.5" longer than my K20 drive shaft, which is 52.75". I still think it's shorter than what my K20 needs, but that may be my solution. At least a temporary solution until I can source the pieces to have a new driveshaft built.

@bucket, any issues or things to be aware of with the conversion U-joints? Also, how much did you pay to have your driveshaft parts cut, welded, and balanced? There's a couple shops up here around the Twin Cities, but I have no idea what a job like that should cost.
Know any racing friends? You can ask them they tend to be distributed through the country and there is no way they all travel long distances to find one. And then of course, shops that do builds or work on heavy equipment and machinery. Like construction and agriculture have a lot of PTO and driveshafts. I’d think they have the tools and wizzardry.off-roading friends too.

What’s a conversion u joint
 

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
28,910
Reaction score
23,314
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Conversion U-joint!!! Why didn't I think of that... I have a spare 54.25" K10 drive shaft that's 1.5" longer than my K20 drive shaft, which is 52.75". I still think it's shorter than what my K20 needs, but that may be my solution. At least a temporary solution until I can source the pieces to have a new driveshaft built.

@bucket, any issues or things to be aware of with the conversion U-joints? Also, how much did you pay to have your driveshaft parts cut, welded, and balanced? There's a couple shops up here around the Twin Cities, but I have no idea what a job like that should cost.
If your K10 shaft is from a 10 bolt truck, Precision #447 should be the conversion joint you need. I burned through a couple conversion joints, but that was due to my rear yoke being pinched. It required the shop press to get both caps in far enough to install the clips and the joint was always tight.

I don't recall what I spent for all that. I had them install new quality joints too. I'm wanting to say it was somewhere around $200. Might have been less.
 

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
4,107
Reaction score
4,942
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
Know any racing friends? You can ask them they tend to be distributed through the country and there is no way they all travel long distances to find one. And then of course, shops that do builds or work on heavy equipment and machinery. Like construction and agriculture have a lot of PTO and driveshafts. I’d think they have the tools and wizzardry.off-roading friends too.

What’s a conversion u joint
Conversion U-joints have one style of joint on one side and another style on the other side. For instance, one pair might be 1310 while the other set is 1350.

FWIW, the slip yolk on the 1997 K2500 454/4L80e/NP241 donor for my swap was identical to the original 350/TH400/NP205 driveshaft slip yolk. I would put your old driveshaft up as close as possible, then measure how much longer it needs to be. Then go to the junkyard and look at newer trucks and SUV's for the correct length complete driveshaft. They should all have the same 32 spline slip-yolk that your 208 takes and any 1/2 ton will have the 1310 u-joints. The 3/4 and 1 tons will have either 1350's or 1410's
 

Frankenchevy

Proverbs 16:18
Joined
Jan 3, 2018
Posts
5,891
Reaction score
7,228
Location
USA
First Name
Jeremy
Truck Year
Square
Truck Model
CUCV
Engine Size
Small
@scrap--metal I found your post over on NastyZ28 and responded there too. I forgot all about the M1008, a shaft from one of them should work for you.
Fwiw, my DS was a bit on the short side from the factory. It seems GM used what was close enough for it, but not optimal with a little over 3” of yoke exposed.

Edit: I live in a pretty expensive area to get shop work done. That being said, having my DS re-tubed, new spicer u-joints installed, balanced and painted was about $200. Not too bad to have it dialed to your application. In the end, I have about 2” of yoke travel inward.
 
Last edited:

scrap--metal

Full Access Member
Joined
Sep 15, 2021
Posts
673
Reaction score
1,255
Location
MN
First Name
Calvin
Truck Year
'85, '81
Truck Model
K10, K20
Engine Size
305, 350
If your K10 shaft is from a 10 bolt truck, Precision #447 should be the conversion joint you need.

Thanks for the part number. My K10 driveshaft is from a 10 bolt truck, so that should be the U joint I need. I'll try to order one up tonight.
 

scrap--metal

Full Access Member
Joined
Sep 15, 2021
Posts
673
Reaction score
1,255
Location
MN
First Name
Calvin
Truck Year
'85, '81
Truck Model
K10, K20
Engine Size
305, 350
After calling one shop, they quoted me $240 - $280 JUST IN LABOR to cut, shorten/lengthen, and balance a driveshaft. No U-joints, no slip yoke, and no new tube itself. Sounds ridiculous to me...

I'll keep calling around, and looking for an M1008 driveshaft as well. I'm really thinking that a conversion U-joint will work for me in the mean time.
 

scrap--metal

Full Access Member
Joined
Sep 15, 2021
Posts
673
Reaction score
1,255
Location
MN
First Name
Calvin
Truck Year
'85, '81
Truck Model
K10, K20
Engine Size
305, 350
Fwiw, my DS was a bit on the short side from the factory. It seems GM used what was close enough for it, but not optimal with a little over 3” of yoke exposed.
I have a buddy preparing to part out a square plow truck with a huge hole in the frame, and a couple previous frame patches. I checked the driveshafts in that truck yesterday evening, and the other truck he's going to move the plow to. Both driveshafts were like yours, Frankenchevy, with around 3" of yoke exposed. The shafts fit nice and snug in the transfer cases though.

The additional 1.5" on my K10 driveshaft compared to my K20 driveshaft should get me right around that 3" of exposed yoke since there's around 4.5" exposed right now.

Thanks for the 3" confidence measurement ;)
 

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
28,910
Reaction score
23,314
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
After calling one shop, they quoted me $240 - $280 JUST IN LABOR to cut, shorten/lengthen, and balance a driveshaft. No U-joints, no slip yoke, and no new tube itself. Sounds ridiculous to me...

I'll keep calling around, and looking for an M1008 driveshaft as well. I'm really thinking that a conversion U-joint will work for me in the mean time.
Shooooweee, that's a lot of money.

Just in case, here's a chart to positively identify the current yoke on the K10 shaft.
You must be registered for see images attach
 

scrap--metal

Full Access Member
Joined
Sep 15, 2021
Posts
673
Reaction score
1,255
Location
MN
First Name
Calvin
Truck Year
'85, '81
Truck Model
K10, K20
Engine Size
305, 350
@bucket, thanks for the chart. I now see what I've been calling 1310 U-joints are not 1310s. My K10 shaft has the inside snap rings, so this chart will certainly come in handy. Thank you.
 

rpcraft

Full Access Member
Joined
May 31, 2016
Posts
1,329
Reaction score
507
Location
Texas
First Name
Robert
Truck Year
1985
Truck Model
Jimmy
Engine Size
LS 6.0 364 CID
Sounds like a recipe for future broken parts. Hammer and chisel mechanics can make a heck of a mess.
 

scrap--metal

Full Access Member
Joined
Sep 15, 2021
Posts
673
Reaction score
1,255
Location
MN
First Name
Calvin
Truck Year
'85, '81
Truck Model
K10, K20
Engine Size
305, 350
Sounds like a recipe for future broken parts. Hammer and chisel mechanics can make a heck of a mess.

There are some M1008 driveshafts currently on eBay. I'm considering buying one of those instead of the conversion U-joint. That'd avoid some of the future mess. Now I just need to make up my mind...
 

QBuff02

I like Big Blocks and I cannot lie
Supporting Member
Joined
Feb 28, 2017
Posts
867
Reaction score
1,591
Location
Central IL
First Name
Quincy
Truck Year
1982
Truck Model
K30
Engine Size
454
Does anyone have a link for the "proper" way to measure driveshaft length? I've heard too many different methods, so I'm looking for a source of truth.
Late to the party and didn't read through all the replies.. So maybe it's mentioned, but have had a couple driveshafts built from scratch and the shop has always said vehicle on the ground at ride height, then bottom the slip yoke out in the transmission or transfer case and measure center to center of the u-joint cups on the yokes and subtract one inch off the measurement. Buddy just replaced the two piece with carrier bearing driveshaft on his '78 GMC 1 ton with a one piece that the same shop made for him. except this had Fixed yokes on both ends, so they put the slip in the shaft, all new assembly ready to bolt in was about $650 out the door. They told him to measure center to center of the u joint cups on the yokes and they'd take it from there. Also, I had a friend that had a custom shaft made by a company in California and in that instance I know they told him measure from the output seal to the center of the u-joint on the yoke on the rear axle and it turned out just as fine. That was a custom aluminum driveshaft for a crew cab long box behind a duramax with a hand shaker and they assured him it would hold up to the power he's making, and so far so good! Lol My experience is the former example though.
 
Last edited:

Bextreme04

Full Access Member
Joined
May 13, 2019
Posts
4,107
Reaction score
4,942
Location
Oregon
First Name
Eric
Truck Year
1980
Truck Model
K25
Engine Size
350-4bbl
@bucket, thanks for the chart. I now see what I've been calling 1310 U-joints are not 1310s. My K10 shaft has the inside snap rings, so this chart will certainly come in handy. Thank you.
That shouldn't be the case. The only inside U-joint universals I have ever seen on a squarebody are the double cardan universals. All chevrolet rear driveshafts I have ever seen use outside snap rings. A 14BFF should be a 1410 yoke. My stock K25 rear driveshaft that was a slipyoke into a NP205 and with a 14BFF had 1410's on both the axle yolk and the slip joint yoke side.
 

Latest posts

Forum statistics

Threads
41,750
Posts
901,126
Members
33,283
Latest member
dadsc10
Top