Here’s the short version for folks who don’t like to read a thorough description about my modification. I installed a longer transmission in my 1963 C10 short bed. The newer T5 transmission was 3.25 inches longer than the factory transmission, so I needed to get a shorter driveshaft and change the slip yoke. I found one at the junkyard that is 4.875 inches shorter measuring from the front U joint to the rear U joint. The swap worked great after changing the rear U-joint to a conversion U-joint. I did some math and found that the original 3° of driveshaft angle changed to 3.2866°. I would have thought the angular change would be greater than that and that I would need to install a shim, or somehow adjust the angle of the rear axle pinion, but I did the math and everything looks great and works great. Read the rest of this post if it interests you.
What I will attempt to describe in words, is much easier to show with diagrams and visual aids. Unfortunately, I’m not that good of an artist.
My 1963 Chevy C10 short bed came from the factory, with a Muncie three speed transmission. When I changed to a T5 transmission, I needed to shorten the original driveshaft because the Muncie transmission is only 22 inches long and the T5 transmission is 25.25 inches long (measuring from the front of the transmission case to the very tip of the output shaft).
A 1963 Chevy C10 short bed has a one piece open driveshaft. The engine crankshaft centerline points towards the earth at 3° front to back. The pinion of the rear axle points upwards at 3° These two lines are parallel to each other and the center line of the pinion is actually above the centerline of the engine crankshaft/transmission shaft.
I read somewhere that a drive shaft from an Chevy Astro van was the correct length for a Chevy truck after doing a T5 swap. This intrigued me. I made some measurements and went to the salvage yard and found a really nice driveshaft laying in the gravel. It was the right length, (approximately 4 inches shorter than a factory driveshaft), and it had the correct slip yoke. I did some research online using the identification sticker and found that it came from a late 1990s Chevy 2WD Blazer. The driveshaft was ready to bolt in place after exchanging the rear universal joint for a conversion universal joint. My total cost was about $60.
The whole purpose of this discussion is to tell you that I did the math to determine what angle change occurs to the driveshaft at the universal joints when you shorten a driveshaft by 4.375”. Those of you who are engineers or who just like trigonometry can confirm my measurements. A factory short bed C10 driveshaft is 56.25 inches measuring center of the front U-joint to the center of the rear U-joint. The salvage yard driveshaft is 51.875 inches. The angular change at the U-joint goes from 3° to 3.2866 degrees.
The new driveshaft has a slip yoke that is approximately 1 + 5/16” longer than the original driveshaft. That 1 + 5/16” inch plus the 3.25” of extra transmission length make up a total of 4 + 9/16” but of course the entire length of the slip yoke is not used, so there is a bit of forgiveness there. That’s how I can get away with a new driveshaft that is 4.375 inches shorter.
If you haven't changed the angle of the engine or the rear the angles will be correct. In order to the correct drive shaft measure from the center of the front U-joint to the center of the rear U-joint and a driveshaft company can build you a driveshaft from these dimensions. I have driveshafts built quite often. Trying to find a used driveshaft that will work isn't worth the effort. Put the slip joint in the transmission, measure from center of joint to center of joint. Take the slip joint and the rear joint to a drive shaft shop and they will build a shaft. I took trig in high school and college. But you are trying to make something complicated out of something very simple.
If you haven't changed the angle of the engine or the rear the angles will be correct. In order to the correct drive shaft measure from the center of the front U-joint to the center of the rear U-joint and a driveshaft company can build you a driveshaft from these dimensions. I have driveshafts built quite often. Trying to find a used driveshaft that will work isn't worth the effort. Put the slip joint in the transmission, measure from center of joint to center of joint. Take the slip joint and the rear joint to a drive shaft shop and they will build a shaft. I took trig in high school and college. But you are trying to make something complicated out of something very simple.
You are giving sound advice. However, my salvage yard driveshaft has been working fine for several thousand miles. I posted to basically report that the DS angle changed less than 1/2 degree even though the driveshaft was shortened more than 4”.
Doing what you recommended above is how I did my first conversion when I needed a shorter driveshaft.
