Moparts
Broke 727 Tailshaft... Why?
Posted By: Mopar_Mike
Broke 727 Tailshaft... Why? - 06/27/09 03:03 PM
Went to check out a small trans leak and low and behold see the crack around tailshaft. Starts at the speedo cable and up and around..
What could have caused this?
67 Coronet 3800#, 500hp 440, 727 with 3300-3500 stall. I put about 15 runs on the car so far this season. I have solid engine mounts and the poly trans mount.
This Sux..
Posted By: Dodgem
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:10 PM
Your rear bushing in the tail shaft may have been worn and allowed vibration and or your yoke may be a bit to far out (too short of a drive shaft) I broke a couple before I went and got a new drive shaft made and not a problem since. only about 3/4" to 1" longer but made a huge difference.
Changing some things in your rear suspension can affect drive shaft length
Posted By: MR_P_BODY
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:14 PM
If your car has alot of flex in it the poly mount
could be too stiff, 3/4" max for the drive shaft
movement... they tend to break where your did... thats
the weak point in the tail shaft... also make sure
the drive shaft isnt bottoming out in the trans...
do you have engine limiters
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:29 PM
The only vibration I feel is just before I hit the traps. But I am thinking it is just suspension/wheel balance... ?
I am about to pull the driveshaft, I will check to see if it bottoms out, but I am not sure I understand the play?
...Also, I was foot braking the car and held it back to around 3200-3300 rpm and then would launch the car. Think maybe I torqued it too much and it broke that way?
Posted By: MR_P_BODY
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:39 PM
Quote:
The only vibration I feel is just before I hit the traps. But I am thinking it is just suspension/wheel balance... ?
I am about to pull the driveshaft, I will check to see if it bottoms out, but I am not sure I understand the play?
Bottom out the shaft in the trans, then when you pull
it back to mount it on the diff you shouldnt have
more than 3/4" movement.... but if its too long you
could be bottoming out in the trans(as the rearend
gets closer to the same level the drive shaft is
getting pushed into the trans)
Posted By: Dodgem
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:44 PM
The vibration I felt was breaking mine new longer driveshaft fixed that.
with solid mounts it should not move front to back like on elephant ears?
hears a dimensions picture
Posted By: Dodgem
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:44 PM
never took?
Posted By: 68roadrunner
Re: Broke 727 Tailshaft... Why? - 06/27/09 03:50 PM
i broke one when i switched to a dana and the shaft was bottoming out. i was told that when tieing one down not to pull the rachet strap against the parking brake, it could crack it, im not for sure on that one. good luck
Posted By: mcat4321
Re: Broke 727 Tailshaft... Why? - 06/27/09 04:43 PM
my dad had a problem when he would lift after the stripe it would break the trans . he tried mounts , shafts, shocks and a ton of other things. he went through 4 housing . new leaf springs fixed the problem. its getting to the point where age HAS to be considered on parts, if they are beat on or not
Posted By: Dale
Re: Broke 727 Tailshaft... Why? - 06/27/09 05:45 PM
I had a bent output Shaft do the same thing so chech it for run out.
Posted By: 65dragnet
Re: Broke 727 Tailshaft... Why? - 06/27/09 06:56 PM
Quote:
my dad had a problem when he would lift after the stripe it would break the trans . he tried mounts , shafts, shocks and a ton of other things. he went through 4 housing . new leaf springs fixed the problem. its getting to the point where age HAS to be considered on parts, if they are beat on or not
That is usually caused by by pinion angle being wrong 
Posted By: mcat4321
Re: Broke 727 Tailshaft... Why? - 06/27/09 07:00 PM
Quote:
Quote:
my dad had a problem when he would lift after the stripe it would break the trans . he tried mounts , shafts, shocks and a ton of other things. he went through 4 housing . new leaf springs fixed the problem. its getting to the point where age HAS to be considered on parts, if they are beat on or not
That is usually caused by by pinion angle being wrong
the set pinion angle had nothing to do with it.
going from under heavy load to relaxed had the pinion moving because of the front of the spring being shot , it would let the rearend yolk move up and down too much.
Posted By: Quicktree
Re: Broke 727 Tailshaft... Why? - 06/27/09 07:19 PM
Quote:
Quote:
Quote:
my dad had a problem when he would lift after the stripe it would break the trans . he tried mounts , shafts, shocks and a ton of other things. he went through 4 housing . new leaf springs fixed the problem. its getting to the point where age HAS to be considered on parts, if they are beat on or not
That is usually caused by by pinion angle being wrong
the set pinion angle had nothing to do with it.
going from under heavy load to relaxed had the pinion moving because of the front of the spring being shot , it would let the rearend yolk move up and down too much.
pinion angle has everything to do with it 
what the heck do you think happens when the rearend yoke moves up and down to much?
Posted By: mcat4321
Re: Broke 727 Tailshaft... Why? - 06/27/09 07:56 PM
Quote:
Quote:
Quote:
Quote:
my dad had a problem when he would lift after the stripe it would break the trans . he tried mounts , shafts, shocks and a ton of other things. he went through 4 housing . new leaf springs fixed the problem. its getting to the point where age HAS to be considered on parts, if they are beat on or not
That is usually caused by by pinion angle being wrong
the set pinion angle had nothing to do with it.
going from under heavy load to relaxed had the pinion moving because of the front of the spring being shot , it would let the rearend yolk move up and down too much.
pinion angle has everything to do with it what the heck do you think happens when the rearend yoke moves up and down to much?
thats why i put the "set" pinion angle.. changing it with shims or rewelding the perch would not stop the problem. the leafs was the problem.. thats like saying running over a chunck of steel didnt cause the wreck, the flat tire did..
Posted By: 7sec.cuda
Re: Broke 727 Tailshaft... Why? - 06/27/09 09:19 PM
wrong pinion angle or drive shaft is to long
Posted By: dvw
Re: Broke 727 Tailshaft... Why? - 06/28/09 02:35 AM
I agree with not enough nose down on the pinion. All the stock perches I've seeen never have enough. Your lucky if you have 1 degree down. My buddys 10.80 GTX and another friends 65 Belvedere both broke a few tail housings. Cut the perches off and rewelded them 5 degrees down, problem solved. My 11 seconnd Volare has never broken one. It's been at 5 degrees since its been raced starting in 1990. The drive shaft bottoming out will break it as well and should be verified for clearance.
Doug
Posted By: goldmember
Re: Broke 727 Tailshaft... Why? - 06/28/09 02:45 AM
I saw one cracked a few months ago,the govenor lost a clip allowing it to slide out of it's bore and breaking the tailshaft housing.
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/28/09 10:08 AM
Quote:
I agree with not enough nose down on the pinion. All the stock perches I've seeen never have enough. Your lucky if you have 1 degree down. My buddys 10.80 GTX and another friends 65 Belvedere both broke a few tail housings. Cut the perches off and rewelded them 5 degrees down, problem solved. My 11 seconnd Volare has never broken one. It's been at 5 degrees since its been raced starting in 1990. The drive shaft bottoming out will break it as well and should be verified for clearance.
Doug
It is a stock 8 3/4 axle housing with stock perches. It is a 742 case with MW spool. When I bought the car it had a 7290 differential joint. I am not sure what brand it was, could have been stock. I did purchase a different (lighter/stronger) 7290 joint when I had the center rebuilt. I suppose it is possible the length of the joints was different...?
Posted By: turner
Re: Broke 727 Tailshaft... Why? - 06/28/09 12:12 PM
Quote:
wrong pinion angle or drive shaft is to long
I broke my tail shaft housing, due to an incorrect pinion angle, mine broke on the finish line after I got out of the throttle, I believe my driveshaft was too short and pulled out of the trans too far.
Posted By: Jimi_Vignogna
Re: Broke 727 Tailshaft... Why? - 06/28/09 01:16 PM
it is the poly rear mount that caused it you cannot do that with solid engine mounts you need a solid rear trans mount.
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/29/09 12:07 AM
Quote:
it is the poly rear mount that caused it you cannot do that with solid engine mounts you need a solid rear trans mount.
I have heard this before too..
Does anyone know where to get a solid transmission mount for a 727?
Posted By: Dodgem
Re: Broke 727 Tailshaft... Why? - 06/29/09 12:35 AM
I've heard the opposite the solid trans mount will brake tail shafts. I've run solid motor mounts for years with a factory trans mount or better MP but only broke a tail shaft when my drive shaft was too short after I put different rear shackles on which moved rear end back. New drive shaft and never another problem
Posted By: 440Jim
Re: Broke 727 Tailshaft... Why? - 06/29/09 12:35 AM
Quote:
it is the poly rear mount that caused it you cannot do that with solid engine mounts you need a solid rear trans mount.
I don't know his problem, but that isn't it, IMO. I have been running a front motor plate, with a fore-aft "limiter" (no movement) with a stock (new) rubber trans mount. 7-8 years, ladder bar rear suspension and minimal pinion angle (correct for ladder bars). And this car launches hard...
Posted By: VernMotor
Re: Broke 727 Tailshaft... Why? - 06/29/09 12:38 AM
Do not run a solid trans mount with solid engine mounts !!! It will break parts ! Car flexs something has to give...
Posted By: barracudabob
Re: Broke 727 Tailshaft... Why? - 06/29/09 03:23 AM
Are there different strength tail shafts? I was told somewhere, maybe here, that the ones with the opening for the circlip on the side were stronger than the bottom opening ones? Apparently they're a truck application. Any truth to this? Thanks, Bob
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/29/09 10:39 AM
Quote:
Do not run a solid trans mount with solid engine mounts !!! It will break parts ! Car flexs something has to give...
Thanks Guys.. I appreciate the help with this.
Posted By: dvw
Re: Broke 727 Tailshaft... Why? - 06/29/09 11:44 AM
Any tail housing will work when properly set-up. Look at a 727 compared to other transmisions. The tail is very long. There's not any support way out at the end. When the drive shaft fits correctly,the pinion angle doesn't pull over center at launch, and the shaft is balanced with tight u-joints there won't be a problem. The only other cause is chassis flex. The stock crossmember and mount don't seem to be an issue. Unless the car has no frame connectors or cage and it is in the power range that it should have them. Tube chassis could be a different issue as I have no experiance with them. If I had a car with a tube chassis and it flexed more than a stocker I'd be worried about more than trans cracking.
Doug
Posted By: an8sec70cuda
Re: Broke 727 Tailshaft... Why? - 06/29/09 11:57 AM
The poly trans mount is not the problem. I've been running a motorplate w/ a poly mount and haven't had a problem breaking tailshaft housings.
In the past I broke two tailshaft housings, both times it was due to a twisted driveshaft. Had them checked at my local driveshaft shop and they were twisted. I twisted one stock driveshaft, broke the housing...replaced it with another stock shaft thinking it was a fluke. 3 passes later it twisted that shaft and broke another housing. I then had a custom 3" thickwall shaft built and never had another problem. This was with stock motor mounts (with a torque strap) and a poly trans mount. I later switched to the motorplate and installed a motor that makes 250 hp more and haven't had any problems.
CHIP
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/29/09 12:41 PM
Thanks, this is looking more and more like it is a driveshaft problem. I do have frame ties and as mentioned the rear housing is stock with stock perches so it should be stock pinion angle.. right?
I know it is a custom shaft (was this way when I bought the car) and is xtra thick, 3"+
As mentioned above I did replace the differential yoke so length might be the issue there.
The trans is a 71 so it has only the rear yoke.
Posted By: JohnRR
Re: Broke 727 Tailshaft... Why? - 06/29/09 02:05 PM
Quote:
Thanks, this is looking more and more like it is a driveshaft problem. I do have frame ties and as mentioned the rear housing is stock with stock perches so it should be stock pinion angle.. right?
I know it is a custom shaft (was this way when I bought the car) and is xtra thick, 3"+
As mentioned above I did replace the differential yoke so length might be the issue there.
The trans is a 71 so it has only the rear yoke.
Mike what rear springs are you using ?
I'd ditch the poly mount for now and run a rubber mount .
Posted By: Al_Alguire
Re: Broke 727 Tailshaft... Why? - 06/29/09 02:14 PM
While these are all great suggestions my suggestion is have the driveshaft checked. EVERY broken tailshaft I have ever dealt with was caused by an out of balance driveshaft. Find a good shop that can check it that you can trust. Just my
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/29/09 06:32 PM
John,
I am using new (less than a year) cal-tracs 1" over ride height mono springs with the bars and rancho shocks.
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/29/09 06:33 PM
Quote:
While these are all great suggestions my suggestion is have the driveshaft checked. EVERY broken tailshaft I have ever dealt with was caused by an out of balance driveshaft. Find a good shop that can check it that you can trust. Just my
Since I have never done anything with the shaft, I think I should get it checked too. Thanks for your help.
Posted By: JohnRR
Re: Broke 727 Tailshaft... Why? - 06/29/09 07:16 PM
Quote:
John,
I am using new (less than a year) cal-tracs 1" over ride height mono springs with the bars and rancho shocks.
Then you need to look at your pinion angle also as the spring can have an effect on it. The stock housing is ok with STOCK springs and their just good enough factory tolerance .
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/30/09 01:41 PM
Quote:
Quote:
John,
I am using new (less than a year) cal-tracs 1" over ride height mono springs with the bars and rancho shocks.
Then you need to look at your pinion angle also as the spring can have an effect on it. The stock housing is ok with STOCK springs and their just good enough factory tolerance .
Whats the best way to measure pinion angle? I have been reading up on it, so I understand what it is and the need for a negative angle, but I am not understanding the best way to measure it.
If I need more of a negative angle what do you guys think of using wedges on the perches?
Posted By: J_BODY
Re: Broke 727 Tailshaft... Why? - 06/30/09 01:49 PM
Things I changed back when I was "using" tailshafts.
Added bars from the cage to the front.
Added engine fore/aft limiter
Stock mount to poly mount
Pinion angle was ok, the drive shaft had been checked prior the dana install and was "ok". Had the shaft shortened when the dana was installed. Was still breaking tails. Ordered an aluminum drive shaft.....been trouble free since.
Posted By: MR_P_BODY
Re: Broke 727 Tailshaft... Why? - 06/30/09 01:53 PM
Quote:
Quote:
it is the poly rear mount that caused it you cannot do that with solid engine mounts you need a solid rear trans mount.
I don't know his problem, but that isn't it, IMO. I have been running a front motor plate, with a fore-aft "limiter" (no movement) with a stock (new) rubber trans mount. 7-8 years, ladder bar rear suspension and minimal pinion angle (correct for ladder bars). And this car launches hard...
I'm with Jim..... I would NEVER run a solid trans
mount... if the chassis twists it will break the
tail shaft
For you pinion angle Caltracs calls for a 2*-4* angle
(less than stock type springs due to less wind up)
Posted By: 65dragnet
Re: Broke 727 Tailshaft... Why? - 06/30/09 01:55 PM
Wedges will work fine if you only need to move a few degrees. I agree with some of the others Mike have your driveshaft checked for balance first.As far as the trans mount I would keep it rubber if your using a motor plate or elephant ears , and yes use a fore and aft limiter on the motor. Hope this helps !
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/30/09 01:58 PM
Quote:
Wedges will work fine if you only need to move a few degrees. I agree with some of the others Mike have your driveshaft checked for balance first.As far as the trans mount I would keep it rubber if your using a motor plate or elephant ears , and yes use a fore and aft limiter on the motor. Hope this helps !
Thanks Guys, I am just trying to get all the info I can on this fix. What are "for and aft limiter on the motor" ??
Posted By: 65dragnet
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:07 PM
It's a fabricated means of keeping the engine from moving back and forth in the chassis from the torque of launching when the engine is mounted on a motor plate or elephant ears. Wow that the best way I know how to describe it !LOL I think they are all pretty much home made ,I put mine from the trans ear on the pass. side of trans to the trans crossmember. It's adjustable like a turnbuckle , alot of them I've seen are mounted from engine to frame on the front
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:11 PM
Quote:
It's a fabricated means of keeping the engine from moving back and forth in the chassis from the torque of launching when the engine is mounted on a motor plate or elephant ears. Wow that the best way I know how to describe it !LOL I think they are all pretty much home made ,I put mine from the trans ear on the pass. side of trans to the trans crossmember. It's adjustable like a turnbuckle , alot of them I've seen are mounted from engine to frame on the front
Thanks, good description.
I don't have anything like that unless a torque strap helps.
Posted By: 65dragnet
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:15 PM
Thanks ,I get a little windy. The torque strap is just limiting movement from side to side and not fore and aft.I don't have a pic of mine or I would show ya maybe some one else will chime in.
Posted By: MR_P_BODY
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:43 PM
Mike didnt you say you have solid engine mounts, if
so you dont need the limiters
Posted By: Mr.Yuck
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:45 PM
mine broke because of slightly bent drive shaft.
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:45 PM
Quote:
Mike didnt you say you have solid engine mounts, if
so you dont need the limiters
\
Yes, I have solid mounts. I thought about that after I posted my last post.. hahaha.
Posted By: Jimi_Vignogna
Re: Broke 727 Tailshaft... Why? - 06/30/09 02:56 PM
mike did you brake the tail housing like the pic turner posted or did you break it by where it bolts onto the trans case?
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/30/09 03:01 PM
Quote:
mike did you brake the tail housing like the pic turner posted or did you break it by where it bolts onto the trans case?
Its actually a small hairline crack that starts at the speedometer opening and up over and around housing. After a couple runs I was seeing smoke from the trans fluid dripping on the exhaust. Thought it was a bad O ring for the speedo cable. It was not.. I actually made a couple more runs too.. ... 
Posted By: DoctorDiff
Re: Broke 727 Tailshaft... Why? - 06/30/09 03:33 PM
Make sure your pinion angle is derived from the TRANSMISSION CENTERLINE, not the driveshaft.
In other words, make your pinion parallel to the transmission as a base line, then roll the pinion downward (ie. 4) degrees. The purpose of pinion angle is to insure the pinion centerline is parallel to the transmission centerline when the vehicle is under power.
Too much negative pinion angle can cause the U-joints to bind on deceleration. Not enough negative pinion angle can cause the U-joints to bind on acceleration.
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 06/30/09 05:05 PM
Quote:
Make sure your pinion angle is derived from the TRANSMISSION CENTERLINE, not the driveshaft.
In other words, make your pinion parallel to the transmission as a base line, then roll the pinion downward (ie. 4) degrees. The purpose of pinion angle is to insure the pinion centerline is parallel to the transmission centerline when the vehicle is under power.
Too much negative pinion angle can cause the U-joints to bind on deceleration. Not enough negative pinion angle can cause the U-joints to bind on acceleration.
Yes, thanks Doc. That was my next question. How to set it up. I changed my response, I see what you mean now. Thanks for your help!
But just to reiterate:
If my transmission angle is 3 degrees + I should adjust my pinion angle to the same 3 degrees. And in this case, at this point, I should turn my pinion down 3 degrees to get the optimal 3 degrees - angle.
Question: When I see the angles are different, as I am sure they will be.. How do I get the pinion to match the transmission angle to start with a baseline.
In the case above, If the trans is 3 degrees + should I just set the pinion to 6 degrees - ?
Posted By: Mr.Yuck
Re: Broke 727 Tailshaft... Why? - 06/30/09 05:43 PM
Quote:
Quote:
mike did you brake the tail housing like the pic turner posted or did you break it by where it bolts onto the trans case?
Its actually a small hairline crack that starts at the speedometer opening and up over and around housing. After a couple runs I was seeing smoke from the trans fluid dripping on the exhaust. Thought it was a bad O ring for the speedo cable. It was not.. I actually made a couple more runs too.. ...
When mine let it was on a 12.12 pass. I think it went when I hit 3rd because that's when I smelled smoke. Got back to the pits and I thought the seal had broke to so I checked the level it was only a tad low so I drove it home. The only time I notced leakage was when I nailed it. I got under there w/ a light and noticed it wasn't the seal and it was "bleeding" from the case.
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 07/01/09 05:38 PM
I measured my driveshaft.. I have 1" 1/16" play from the shaft being bottomed out to where it bolts to the rear yoke. Is this a big enough distance that I should have it shortened?
Thanks.
Posted By: JohnRR
Re: Broke 727 Tailshaft... Why? - 07/01/09 06:07 PM
shortened ?? it's not long enough , you want 3/4" , but get your pinion angle right before dinking with the driveshaft length
Posted By: Mopar_Mike
Re: Broke 727 Tailshaft... Why? - 07/01/09 07:23 PM
Quote:
shortened ?? it's not long enough , you want 3/4" , but get your pinion angle right before dinking with the driveshaft length
Yes, sorry, I knew that..