How can I shorten a drive shaft?
The easiest way to shorten a drive shaft is by taking it to the shop and having them do the work for you, to include balancing. I take it this is not what you're asking for though, so here goes:
Are there any warnings before proceeding?
There are three big issues with shortening the drive shaft (DS):
If you are going to do it right, you can do the first two, but leave the balance to a shop (if you could balance the DS, you wouldn't be asking us about it, lol!)
What tools are required, and optimal?
I don't know what exactly is optimal, but things I would use are as follows:
- Tunable MIG welder - This probably wouldn't be your run of the mill welder from Harbor Freight, but a good one which gives you plenty of heat adjustment. A DS usually isn't very thick walled, so welding it is probably like somewhat like welding sheet metal of a body. You don't want a lot of heat build up.
- Angle iron - A length of straight angle iron can be used to keep the DS straight while you weld it.
- Cutoff saw - Typical friction material cutoff saw will do just fine. The more square you can make the cuts, the more square your finished product is going to be.
- Angle grinder - To clean up your welds after the fact.
- Other welding accessories as needed
Is there a "required" way to measure correctly?
Required? Probably not. It's the end product which needs to be right though. To do that you need to:
- Ensure when you measure your needed length, the car is flat/level with full weight on the suspension.
- Put the yoke into the transmission to where it will normally be riding, then measure from transmission center yoke to rear diff center yoke. If the yoke is a used one, you'll see where it normally has been riding by a ring around it. Then measure your drive shaft in the same manner to figure out how much to remove.
- When welding the DS, ensure the u-joint loops are aligned. If you don't, the u-joints will fight each other and you'll get all kinds of vibration. An easy way to do this is to take a straight edge (or the edge of the angle iron) and run a long line down the DS with a Sharpie marker. This is most easily done when measuring for the cuts. Use the line to align the DS before welding to ensure the loops are exactly in the right place ... no guessing.
How can I keep the metal square?
Use the angle iron to keep the drive shaft straight while welding it. Roll it as you weld. If you are an accomplished welder, cut the portion out of the middle of the DS. At least give yourself some length on either end so it will roll true in the angle iron. You will probably need to tack weld it in several places around the DS as you are welding. Treat it like you would sheet metal. You don't want to over heat any part of it and cause it to warp. When you get the entire circumference welded, grind it down flush to the body of the tube and check to ensure you don't have any holes. Fix them if you need to. Take that into the balance shop to get it balanced. Use the angle iron at first to keep it straight. Obviously once you get enough of it welded you won't need the angle iron to keep it straight. At that point, you can slide the weld so it's over the end, then just use the angle iron as an easy way to hold the DS. I would have the tube spot welded in at least three spots before you go back to do any serious welding on it. Once you have it spot welded, you can hold the weld over the end of the angle iron and roll the DS to ensure it's true.
Once it's welded up, you'll obviously want to take it to a DS shop to have it balanced (as I stated already). You may want to change out the u-joints while you are busy doing the rest.
EDIT: I found this webpage of a driveline company which has a ton of information on how to measure (their way), please u-joint identification, and a slew of other stuff. May be of some interest to someone.