Broken 727 Tail Housing Causes?

Cracked the tail housing on the first pass of the year. Looking for probable causes.
1968 Cuda tube chassis car 440/727
1/4" front motor plate, 0.090 mid plate, bolt through urethane trans mount
New 3.5" CM driveshaft, 1350 solid joints, 1" freeplay, same length as old 3" shaft
Ladder bars with coil overs, 1degree pinion angle, so there shouldn't be much movement in the front yoke
4200 two step chip on the trans brake.

Have been running this setup 4 years, last year it broke 2 rods in a violent explosion at the 1/4 mile finish line so I suppose the shock from that could have cracks it.
Just trying to avoid cracking another one as it wasn't a fun ride when the fluid got on the slicks at 135mph

The problem is you have too solid of a support (rear trans mount) and have enough flex in your chassis to break the tail housing. Maybe it was barelyholding up, and the engine failurepushed it over the limits. I run a mount that "floats" between rubber bisquits from bayonet end shocks. That gives it support, but i am carefull there is still a bit of give incase of severe stress. Running that deal since 1997, maybe i should check to see if i need new biscuits. 8.70s, no problems ever.

Those poly trans mounts are really stiff, I'd put something much softer in there.

👍

How far does the drive shaft set from being bottomed in the trans? If the motor is on a plate, does it have a fore/aft limiter? Other than a broken u-joint I've never seen another cause for a broken tail shaft. People say a solid mount will break them. But I've never run a solid mount. The only tail shaft I ever broke came from broken front u joint at speed. In that case of broken u-joints twice I've seen them bend the output shaft as well. Over a 15 degrees of bend.
Doug

I've broken two tailshaft housings w/ my cuda years ago. Both were due to twisted driveshafts. This was back when I had stock motor mounts w/ a torque strap and a poly trans mount. Car ran 10.40s then.

I broke two years ago both due to bad u-joints, mine broke behind the trans mount. It appears to me with a front and mid plate if you run a rear transmission mount it would have to be in almost perfect alignment, at least with a rear mount that was very rigid.

Even with the front and mid plate you need a limiter for front to back.

One degree or minus one degree. Nose up or nose down.

I would look very closely at our driveshaft....Every broken tailshaft housing I have ever worked on or seen was due to a bad driveshaft.

The faster I went with my previous ride the more this occurred. Ladder bar car. Did the fore/ aft deal, had the drive shaft “checked” twice. Finally replaced the driveshaft and no more breakage! Good luck!

(Artist formerly known as J-body)

1 degree nose down

So no fore/aft limiter?
Doug

What's the front u-joint angle? Also what front slip yoke? (I've seen many sloppy billet ones which can vibrate like hell)

No fore/aft limiter

That would be my smoking gun. Unless the mid plate has some tubing across its length tied to the cage. That would leave all the fore/aft movement taken up by the trans mount. That transfers all the load to the tail shaft housing.
Doug

Wouldn't hurt to also check that the u-joints on the driveshaft line up. New doesn't always mean good and that's a newly introduced variable.

My experience is the same as Al_Alguire. Broken tailshaft almost always due to the driveshaft. (specifically approaching critical speed or imbalance)

We had a u-joint retaining clip come out and a u-joint cap come out.

The driveshaft had been shortened, rebuilt and had been in the car for thousands of miles.

It happened on a backroad at the top of second gear. Sliding around in transmisison fluid at close to 100 mph.

It didn't come fully apart but just the one missing cap caused the tailshaft housing to break and the tailshaft to bend.

I use another driveshaft shop now...

You could have just changed your username ya know, rather than "starting over"

Not sure what happened….. one day I just couldn’t log in. Been lurking a couple weeks or better.

…..and I still have 3 727 tailshafts from “the good ole days” 😂