Do-It-Yourself Driveshaft Shortening

There are some times of automotive work that most Shade Tree Howley Monkeys avoid. Driveshaft modifications are one such example and there's good reason for that. Driveshafts are fairly heavy, they can be quite long, and they turn at high speeds. Because of this, they require precise construction techniques and proper balancing. Having the proper tools and skills is crucial to successfully modifying a driveshaft. But like most things, there are exceptions to this rule. For your average 4x4, the rear driveshaft must be straight and balanced. The front driveshaft, however, can get by with looser tolerances. In non-full-time 4WD systems, the front driveshaft only turns when the vehicle is in 4x4 mode. And 4x4 is usually only used when off-road at relatively slow speeds. In this case, your average, competent, backyard mechanic is capable of performing some types of driveshaft modifications. That brings us to the topic of this article. If you have access to a lathe and some other basic tools, you can shorten your own driveshaft. In the example below, I'll be building a front driveshaft to replace the bent one on Project YJ. I obtained a stock YJ's front driveshaft. Because I'm running a Dana 300 transfer case, I knew it would be too long. Remember, it's always easier and cheaper to shorten a driveshaft than it is to lengthen one. I need to shorten the stock driveshaft by about 3 inches. The basic idea is to cut the last 3 inches of tubing off the splined end of the driveshaft. Then, using the lathe, we remove the 3" of tubing from the splined end, and re-attach the splined piece to the now shortened driveshaft. Here's how I did it:

The first step is to separate the driveshaft into its two pieces. The red line indicates approximately where I'll be cutting.	I used a chop saw to cut the shaft but a bandsaw or similar tool would also have worked just fine. The key concern is to make the cut at 90 deg to the length of the shaft.	We'll use the lathe to cut through the weld holding the driveshaft tubing to the splined shaft.
I used the lathe to slowing cut through the weld and tube wall until I could see the splined shaft's material. I've circled it in red for easier identification.	Here, I've separated the piece of tubing we'll be discarding from the splined shaft.	Before inserting the splined shaft back into the driveshaft, I had to use a die grinder to remove the inner seam to provide the proper clearance.
It took a lot of pounding (dead blow hammer) and heat to persuade the splined end to fully seat itself into the driveshaft. The next time I do this, I'll ever-so-slightly shave some material off the splined shaft to make the job easier.	This was the tedious part: I put the driveshaft on the lathe with the splined shaft clamped in the chuck. The goal here is to tap the far end of the tube (right hand side of this photo) until it doesn't wobble when I turn the driveshaft. Note: I turned the chuck and driveshaft by hand, I did NOT use the lathe motor to turn it.	By placing the cutter close to the tube, I was able to make very shallow scratches in the tube, indicating where offset too far in one direction. Wherever the line was, I tapped it with the hammer to knock it closer to alignment. Alignment would be reached when the line on the shaft went right around the surface, indicating that it wasn't wobbling at all.
After a lot of tapping, I got it down to around 1/16" out of round which was good enough for me. The last step was to make short welds to permanently attach the splined end to the shaft. I altered sides and let the welds cool down in order to minimize warping.

The tapping with the hammer style of alignment can only take you so far. The perfectionists among you may take it one step further than I did by using a torch and water to heat and cool the tube to bring it into perfect alignment. Some day, I'll give that a shot but for now, it's good enough for this monkey. By the way, it took me about 4 hours to do this. For many of you, it may be more worthwhile to simply get a shop to do the job. For the time involved, I may go that route next time. But it's nice to be able to say that I shortened my own driveshaft, and it's an ability I now have in case I ever need it again.

A sincere tip 'o' the hat to Rick "Tiny" Reimer to teaching me how to shorten a driveshaft, and a big THANKS to Chris Olson for providing some help with this article.

...lars