Curious if lengthening a driveshaft is possible?
After my Gen3 swap, engine moved forward, so the yoke engagement is less.
Wasn't optimal before the swap, but now it's even less, probably ~1.5" back from bottoming out.

Wondered what detrimental affect the less yoke engagement might cause?

How much engagement do you still have?

The place I use doesn't necessarily lengthen them, but will use your ends and put a new tube in.

Three things,
#1 as mentioned above (spline engagemnt)
#2 Overall length of the yoke and output bushing placement
#3 If in doubt get it lengthened.

how you drive the car & how much power come into play as do tires and others as well, #3 is Much cheaper that the potential damage it can cause /cost

With the small amount you're talking, it is likely a non-issue, but anytime you lengthen a driveshaft you reduce the critical speed and you can get into issues with driveline harmonics. The critical speed depends on the length, the diameter, and the material.

I don't think my local shop will lengthen one. It could be done I'm sure, but I wouldn't.

They just take your ends off of the existing tube, put in a longer tube and reweld and balance. Not a big deal. Take it to any truck shop that makes driveshafts. Now unless you have 1000 hp, I would get someone else to make it.

None of those places have the ability to balance at the speeds needed for a race/performance app.

Been there, done that. Was like riding a paint shaker. One race old. It came out and went in the scrap pile. But not before beating up the good yoke and tailshaft.

Few years ago had one done for a customer car. Pretty mild combo. Visible runout.

Driveshafts are no joke.Use a shop that does race shafts. Send them the yoke, if it's a good piece, give them a C-C and pay them. Sleep better.

Anyone have a measurement on the length of splined part on the transmission side?
I'm going to take some measurements to calculate the engagement.
Need to know how deep on the trans side has splines.

Interesting to see the first 1.75" on the yoke side is not splined?
That seems to indicate the amount of contact isn't critical?
Why would they leave that much length/area with no contact/overlap?

Well, all I can say is the place that did mine did a very good job of balancing it. I guess your results will vary with anything.

i'm wondering if not having the yolk splined for the first inch or so, is to allow ease of assembly. in other words, the unsplined section allows the yolk to be supported on the tailshaft, while the assembler then has it easier to rotate the shaft to engage the tailshaft and yolk splines.
however, the tailshaft only has a certain length of splines on it, so [in my opinion] the yolk must [or should] have the female spline length that would be completely engaged with the male spline length on the tailshaft during all suspension movements and articulations.
i also agree about getting the shaft trued and balanced for high speed operation, not for truck use. this is especially true if one has an overdrive transmission and deep rear gears.

The smooth area at the front of the splines was to accommodate a seal on the output shaft (seen on right in pic below) that was used for the first years of the slip yoke.

Thx for the pic, would it be possible to get a measurement of how long that splined section is?
My guess would be only 3"

Yes, retubing is a common deal. Not what I thought of when I see "lengthening a drive shaft.

When I installed the S-60 in my Dart I had a driveshaft built by a very highly recommended shop in Michigan. After the install the car had a vibration, I played with pinion angles, rebalanced tires, rechecked the driveline angles, nothing helped. Finally, I had the transmission out for a converter change and called my local driveshaft shop to have the balance checked. It was waaaaaaaaaay out! He rebalanced it and it's smooth as silk even at 135 mph.

BTW, the shop in Michigan charged almost $500 for a "race" driveshaft, snake oil.

It can be done, I watched my auto shop teacher lengthen a driveshaft 2" back in the 80's using some pipe and lathe.
The equipment that the modern driveshaft shops have now is so good and quick, I'm not sure that a backyard fix is worth the time though in messing around.

Yes, the splines on the shaft are 3" long. On a typical 727 factory slip yoke, the bore is 6 1/2" deep and the un-splined area is 1 1/2" deep, so there are 5" of splines internally.

Appreciate the info/confirmation.
My 95 518 yoke bore is 7" deep.
Last piece of the puzzle is how much of that 3" on the trans side protrudes beyond the end of the tailshaft.
Hope to get my trans back this week, will measure then

i learned something here. thanks John.

Just note that those slip yoke that used the seal JK mentions have a vent hole on the end.

Sure, but I doubt it would be worth it, and even if welded straight and true, the welds could be a stress point as the tube twists. Likely cheaper (labor wise) to just have a new tube of the correct length welded to the yokes.