I was wondering if it is bad (for the frame or the drive train) to leave my 4WD V6 Tacoma on 4 jack stands for several days while I work on the transmission. I could certainly lower it down after taking the tranny off and raise it again to put it back on but would rather not do it if it is safe to leave it on 4 jack stands for several days.
I have seen a lot of qualified mechanics leave the car on jack stands for months without any problems. It is more a question of where the car is being stored while on jack stands. In a garage where it is safe from accidentally being leaned on and possibly tipped over would be best (and out of sight from vandals/hooligans that might want to cause havoc, depending on your area).
Be sure the car is mounted on the proper jacking points and that your jack stands are rated for the appropriate weight and you should be fine.
If where you are putting the vehicle up at is level ground (driveway, for instance), there is plenty of support under the jack stands (like concrete pad), and you are putting the jack stands at secure locations under your vehicle extended the same amount (so vehicle is level), you will have no issues what-so-ever. Vehicles which are suspended like this are very solid. I've done this many times without worry.
Some words of note:
- If you have to support your vehicle off of a frame rail, use a piece of 2x4 scrap between the jack stand and the frame rail to spread the load out. This will help prevent any damage to the frame from too much stress in a small space.
- Have no fear of putting your jack stand directly under an axle (especially in the rear). The axle tube is fully capable of supporting the load. Just place the jack stand as near as possible to the spring mount area as possible.
- In the front, it's a different story. I believe your truck has an independent suspension up front. You'll need to find a place which can stand up to the support. Again, using a piece of 2x4 scrap is a great way to disperse the load. Placing it near where the pivot for the lower control arm is at is a good place to look for such a point. If you can find a good perch point towards the outer part of the lower control arm (whee side) which will make it solid, this is a good place to look as well.
- When you get your vehicle up on all fours, give a good tug in all directions. You want to be safe about it, so be wary of what the vehicle is doing. If you can give it a good tug and it seems solid, it probably is. If it doesn't seem solid, ensure you go back and find new mounting points or at least discover why it doesn't seem solid. You may want to place your jack stands at different locations.
- Double check to ensure all of your jack stands are flat on the ground after getting the vehicle up in the air. You wouldn't believe how many times I've gotten one side up, then the other side, only to find the first side has tilted somewhat, leaving a gap between the landing leg and the ground.
- Ensure you give yourself enough room between the ground and the bottom of the vehicle to remove the transmission/transfer before you start working on it. There is nothing worse than having to go back and jack it up some more while in the middle of the removal. Just not fun at all. On the other hand, you don't want it too high. (Yes, you want the Goldielocks Zone! ... Just right!) The higher the vehicle is, the more stable it will need to be.
Hopefully this helps.
We have a few project cars and have left them on jack stands for long stretches of time while we were rebuilding stuff. I'll second Tariq that if the vehicle is held up at proper jack points you should have no problems. It's also a good point to have the vehicle somewhere secure, if only to keep curious people, pets, etc from crawling underneath and potentially getting hurt.
In general, anectodal evidence would show that there should be no issues, but there is one caveat which I didn't see addressed in the other answers:
With age, some vehicles can have shock absorber seals fail if they are raised with the load of the wheels still present, rendering them useless (e.g. '07 Camry). If the vehicle is known to suffer from such a failure it would be prudent to tease off the wheels with the vehicle raised slightly raised before going gung-ho with the jack and jack stands.
That said, this precautionary measure is not a guarantee that ageing shock seals will not fail.
I can't find jack stands that let you use them for this. it says not to lift the front and back at the same time as well as not to lift one side only. you can lift the front or the back and use both stands. that's it. I think it's because the pieces are cast/forged that are actually jacked up and touching your car.
... and here I'm referring to the legal issues also you don't want to be underneath the car with Jack's that aren't rated save your life
This is a late answer, but one might consider it. First, I tend to use jack stands which are rated for more than the car. So for a VW Jetta, I use 3 ton stands. Using a floor jack I get the car up off the floor and the four stands situated. I do this only on concrete, not asphalt or any soft surface. Then when the car is up, I slide 6x6 or 8x8 lumber ends which I use for a variety of purposes under low and beefy points, like axles, suspension points. The idea is that if a stand fell, there would be a backup piece of wood.
Years ago I knew a guy who did something similar but used cheap sheet metal jack stands. It turns out that he caught a breaker bar on the release handle of his main jack stand, and the car fell maybe three inches, but with enough of an impact to split apart the secondary backup jack stand. It split and the car came down. Fortunately his buddies were there and they found out that four of them could lift the car off of him enough for him to be pulled out. One of them was a materials guy, and suggested the lumber usage, rather than a cheaper jack stand as a backup, because the lumber could handle a shock of a dropping car better.
To this date I use lumber as a backup, and have used a couple of bottle jacks adjusted to a snug fit as backups as well. Someday I may buy a lift.
And keep in mind that your motion, or that of your tools, could disengage a jack stand under certain circumstances.