someone on facebook asked "can someone tell me the driveshaft length when swapping trans" he went from a th350 to sm465. an idea has been in my head for awhile now about using the pythagorean theorem, since we know two of the measurements (trans length and old driveshaft length. now we cant use the same info for all trucks because somethings will be different (i.e. short bed vs long, one vs two piece shafts and fixed yokes vs slip)

what i need from you guys is your opinion on this and if you could measure your set-up to verify this would actually work. once i get a base line of info i could theoretically make a formula and you could just simply look up the trans and it would show you what size shaft you need with your wheelbase

http://forum.73-87chevytrucks.com/smforum/index.php?topic=32506.0heres what i wrote on facebook:

you have to use the pythagorean theorem (one of these days ill make a write up with this info).

you have to measure your old driveshaft (yoke to yoke) however you measure it the first time keep doing it that way. Most do center to center and to do this accurately measure from the left outside edge of one of your u joint holes (in the shaft) to the left outside edge of your other u joint hole, this will give you an “on center” Measurement, you'll use this as the "hypotenuse" or "H" on the formula.

ill give you one of the "legs: or the "G" on the formula. now in reality the "G" would be measure from the tailshaft of the trans to the 90° angle right above the pinion yoke. but if you did it this way you'll have to measure the space between the tailshaft and the 90° and we can simplify this process just by saying the "G" is the length of the trans.

the "F" is the height between the yoke and an straight up to the trans yoke. Since we want a constant number we’ll use one formula to find that.

Now for the measurements of our one leg, this is going to depend on your tailshaft, I think the 9” is most common so ill use.

TH350 2wd 6” tailshaft 27.68"

TH350 2wd 9” tailshaft 30.68"

TH350 2wd 12” tailshaft 33.84"

SM465 2wd (bell to yoke): 24"

There will be a couple steps because we don’t know what leg “F” is and we have to figure that out. First and to do that we’ll have to use the third image.

F=(length of your driveshaft)²-(trans length 30.68)²

So whatever your length of your shaft times itself - 941.2624

And that gives you “F²”. now unsquare “F” by dividing it by itself and with finding the “F” we can move on to finding your new shaft length (from yoke to yoke) and to do that we’ll use the first formula in the image

Take the “F” from the first step and square that then add the sm465 length (24”) and square that (576)

So “F”+576= H² (just divide “H” by “H” and youll have the length of your new shaft yoke to yoke