View Full Version : 1/2 ton to 3/4 rear axle conversion q's

05-22-2001, 10:53 PM
I just got a 14 bolt axle for free from an uncle. Problem is the yoke on the 14 is bigger than the 12 so the u-joint doesn't want to bolt up. The driveline off the 3/4 ton is a two piece one and it is from a 2wd so it's not going to work. The one on my 1/2 ton is 1 piece and is heavy duty so I want to keep it. Now, should I change the yoke on the axle, or the yoke on the driveshaft, or should I just get a whole different driveshaft? I heard that you can get the yokes changed on the driveshaft, but what I don't know is if you can get them changed on the axle. Also is the end which splines in the same size on all drivelines from 73 to 74? I mean the end without a u-joint you can just pull off.



05-22-2001, 11:16 PM
Garet: I don't know about the spline question, but I would definitely not get into changing the yoke on the axle. Have the driveshaft modified to suit - mixing parts this way often leads to a bit of "custom-fab" work.

Chuck Smith
05-22-2001, 11:41 PM
If the driveshaft from your truck, is the proper length, possibly you can find a U joint to make it work. You can get u joints with 2 different size bearing caps on them. Then again, you could also get your driveshaft modified for about $150, if the length is OK.


05-23-2001, 12:00 AM

Hey just a quick note. I have 8" of suspension lift on my shortbox. When I lifted it that high my rear D-shaft was a little too short. By switching to a 14 bolt axle which is a LOT larger and has a longer Pinion snout I gained back a few inches that I needed.

I used a conversion U-Joint (Like Chuck said) and kept my half ton driveshaft. The Joint was a NAPA part and when I get a chance I will go look in my tool box for the Part #. It was for a half ton FORD, '77 I think. The U-Joint has a larger cross and bearing caps on one side and the other fits into the stock driveshaft. The Joint is kind of expensive, (About $30) but it got me by untill I could get new larger shafts made up. Good luck,


05-23-2001, 12:18 AM
Thx Andy. I would appreciate you looking that up. I am concerned about the length. I am going 6" so it should be long enough with the extended 14-bolt bolt distance over the 12 bolt. By the way, if I do go with a special u-joint is it going to be strong enough to power the 14 bolt with 36's? The differnce n size isn't all that much but a little probably makes a big difference. My 350 doesn't have the torque the 454 has so it probably doesn't need a bigger u-joint. Which puts more stress on u-joints anyway, engine torque or tire size?


Power mad
05-23-2001, 12:49 AM
Hey all
There are 2 u-joints depending on what yoke is on it.
Napa # 348 or 447.
I think one is for a 1 1/16 cap and the other is 1 3/16 inch cap.
See ya

Chuck Smith
05-23-2001, 12:49 AM
Actually, 3 things are involved in you question Garet. The engine puts out X amount of torque. That is transferring through the ring and pinion. Depending upon the axle ratio, the amount of torque needed to turn the tires can vary, in a sense. The lower the gear ratio, the easier it is for the tires to turn, which = less resistance, which = less torque on the yolk. I have seen trucks (not pick ups) twist driveshafts into pretzels, due to high torque, and a heavy load on them. Amazingly, the u joints didn't blow, but there was enough torque to twist the driveshafts.

So in a nutshell, the bigger the tire, the more resistance, the more stress put on the U joints, and yolks. Match your gear ratio to your tire size, and that reduces the resistance.

Hope I didn't confuse you. Not sure what size tires you plan to run, but a set of 4.11 gears, and 36" tires is a good combination.


05-23-2001, 02:31 AM
I am going to be putting on 36's so that is a plus if they work well with 4.10 gearing. I now just need a th400, dana 60, and a good transfer case....:)

Power mad
05-23-2001, 05:01 AM
Thanx for the insight.
I have a 78 K5 355 plant and 465/205 drive train.
I also have a 73 C20 pick up with a .040 over 454.
I am going to swap the 14 bolt and 454 into the Blazer.
The 14 has a 3:73 same as the 12 bolt in it now.
I have 33's on it same as the 73, but I will be going to 36's on the next set.
After reading your post I realized that allthough I would be able to twist 36" tires with a BBC and a 3:73 ratio I would be winding the shaft like a pretzle.
I have a shaft from a 72 riviera(sp) that has a double cardon on it the same size as the one in the front. The Riv had a 455 in it and this is one beefy piece of pipe.
Now I just need to make another front shaft that will take the abuse of a BBC and 3:73 gears.
Which would be cheaper than a set of gears.
I have a buddy that has a full on machine shop at his house.
He's 58 and the neatest guy you ever met. You can make anything at Jims house. He's even got 2 engine boring machines that he hardly ever uses(10 years) Can you say 468 cubic inches YEAH
See ya

Chuck Smith
05-23-2001, 11:02 AM
Here's a link that explains a lot about driveshafts. This company makes driveshafts for lifted trucks, that have a problem with a high driveshaft angle.