Lowered my 79, now it's leaking from the trans tailshaft

[Scott76]

5/7 mcgauhys static drop. 454/350 driveline.

I've been building the truck for a few months, it's been on jack stands in my garage almost the entire time. I'm just about ready for paint and put the truck back on the ground, and it's leaking pretty bad from the trans tailshaft. It does not leak when on the stands. I'm thinking I stupidly put the drop blocks for the flip in backwards, but I can't remember which direction I put them in (hole facing front or rear) since I did the drop almost 2 months ago. I really don't wanna pull it apart and do it again, but that's the only scenario that makes sense to me. Could it be that jacking it up and having the axle moving around could have damaged the output seal and it may have a nick in it that only gets exposed /leaks when it's on the ground? If I put the lowering blocks in backwards would it be very obvious?

I know I can pull it apart and check it, but I'd obviously rather not if I don't have to. Which direction should the hole in the lowering block for the spring pack face? Front or rear?

[Shady Rascal]

Hole towards the front of truck, which will offset the differential back.

Either way, since the truck has only been on jackstands and not driven at all, it sounds like the seal just gave out. I can't think of an easier seal to change on the entire truck though, so it's not anything to get worked up over.

[Scott76]

Originally Posted by Shady Rascal

Hole towards the front of truck, which will offset the differential back.

Either way, since the truck has only been on jackstands and not driven at all, it sounds like the seal just gave out. I can't think of an easier seal to change on the entire truck though, so it's not anything to get worked up over.

I'm 99% sure that's the way I did it.

[Scott76]

Okay this is weird. Went out and double checked some things and I have 1-3/4" of yoke showing. As far as I know that's 1/2-1" too much.

Should I flip the blocks so that it moves the diff forward, or will that be too much?

The truck has a BBC that is not original (bought it that way) but the motor mounts look to be in the original mounting location so I don't think it's set too far forward or any weird stuff like that.

[Shady Rascal]

Ideal yoke insertion isn't necessarily something you can see. The way it's supposed to be is the yoke fully bottomed into the trans then pulled back out 3/4 of an inch. This isn't directly correlated with how much of the yoke surface you can see when it's installed. For instance, they will often have a fair bit of yoke surface showing even when bottomed out, and certainly when pulled back the 3/4 of an inch.

The holes in the flip brackets are offset to pull the rearend back to compensate for the lowering. Since the leaf springs aren't level (when viewed from the side they are lower in the front eye than the rear eye) so putting the axle on top of the springs also brings it forward naturally. The offset hole compensates for that so that (1) the wheel is centered in the rear wheelwell when the truck is viewed from the side and (2) the driveshaft isn't shoved too deeply into the transmission.

Every truck is different though. For instance on my stepside, when I used the offset hole my rearend was noticeable too far to the rear when you looked at the truck. I actually had to remove the brackets and drill a hole centered in them. That fixed the problem. I do believe it relates to how thick of a spring pack your truck has (Heavy Half, Big 10, etc).

[Scott76]

Hmm. Maybe I need to start playing with these blocks then because I really would feel less than comfy riding around with that much yoke showing. Unless I'm just being worried for nothing?

[Scott76]

Eh, I should probably pull the shaft and bottom the yoke and see where it's at. It could have been bottoming bad when I bought it I suppose, this thing has had a world record level of absolute hack work on it.

[Shady Rascal]

Unbolt the driveshaft from the rear end and see how far you can slide it into the trans until it bottoms. 3/4 of an inch is ideal. Even 1 inch is okay. Beyond that you need to start looking at addressing the length issue.

But, don't jostle the rearend forward or back just based on the driveshaft. The first consideration is that the wheel should be centered in the wheelwell front to back when you look at the truck. You wouldn't want to for instance slide the rearend forward to make the driveshaft right and have to live with a wonky looking truck forever because of it. Get the rearend sitting right in the fenderwell and then worry about the driveshaft. Worst case, you might need to have it modified. Not the end of the world, it's typically a bit under 200 to have one professionally done.

[Scott76]

I looked at it, it does seem to be set back in the wheel well a little bit. Not a huge amount, but I can tell for sure. Jacked the thing back up to check it out and deal with some other issues, and now with the axle at full droop it has 1-1/8" of the yoke showing and it's not leaking. So weird.