Movement in the driveline
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
Movement in the driveline
I've noticed when I get on it the driveline moves a little bit because the shifter feels like it moves up maybe a half inch... when I installed the new boosted 383 I put in new motor mounts. The car is a 94 6 speed making about 500 ft lbs of torque to the wheels and close to the for horse power... is this normal or should I try a different set of motor mounts or possibly solid mount it if they make them?
Melting Slicks
Joined: May 2014
Posts: 2,891
Likes: 591
From: New Port Richey FL
It's fairly normal to have a bit of movement up and and even side to side especially on Hi HP use, but to take any normal to abnormal you did step 2 already step 1 is the Torque arm beam (C channel) is properly tightened.
There will be movement a bit because as the name implies Torque arm as power is put down the differential will want to twist which goes down the beam to the transmission.
There will be movement a bit because as the name implies Torque arm as power is put down the differential will want to twist which goes down the beam to the transmission.
Le Mans Master
Joined: Aug 2015
Posts: 6,090
Likes: 1,972
From: St. Charles MO
This is 100% due to your C-beam bolts not being tight enough at the transmission tail shaft side. I had the same thing happen. There should be no up-down movement of the shifter at all.
Race Director
Joined: Sep 2001
Posts: 14,668
Likes: 749
From: NEwhere Ohio
do it before the c beam bolt holes ovalize
Thread Starter
Melting Slicks
Joined: Feb 2005
Posts: 3,466
Likes: 43
From: Auburn MA
the Cbeam bolts felt tight when I put the car back together using the german torque method... what is the propper torque specs I'll give them a once over.
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Le Mans Master
Joined: Aug 2015
Posts: 6,090
Likes: 1,972
From: St. Charles MO
For my 96, the Factory Service Manual says the C-beam (they call it "Driveline Support") bolts to the transmission should be 37 lb/ft and the beam-to-diff should be 59 lb/ft. I'm going to tell you that's not enough. Keep in mind that on my car, at least, GM also put some goop in there to help keep things from shifting around. I don't recall how tight I made mine, but they were a lot tighter than that.
Racer
Joined: Jul 2023
Posts: 252
Likes: 85
Same here. Even after torquing to spec at 1st install, they ALL needed a retorque less than 500miles later. & Half of that was very casual driving. It's tough to access the bolts w/ factory exhaust in place, but worthwhile to loosen & lower enough to snug em back up 👍🏻 Def don't want to leave it loose and cranking around. Might be a recurring check on your build at those power levels 💪 🍻
Racer
Joined: Jul 2023
Posts: 252
Likes: 85
Yes, I would back them off a bit, not fully loose but partially. And then go to spec. This way you have a known baseline. I feel the rear torque spec felt adequate/ appropriate when I did mine, but the front seemed pretty low by comparison. I inquired with ZFDOC about it, cause I used his Beam Plates, and he cautioned against any extreme over tightening as it can distort the tail housing and cause binding. Made sense, the mounting is less beefy on the transmission. On my 2nd retorque I went a bit higher, maybe mid ~40's lb/ft. Good for now 👍🏻 Sadly it seems like a more frequent retorque is the safest policy for me, vs cranking it down possibly too much 🤷♂️ Not to challenge anything Matt Miller offered for his setup. I'm curious what #'s you ended up using to keep things locked down?
Le Mans Master
Joined: Aug 2015
Posts: 6,090
Likes: 1,972
From: St. Charles MO
Yes, I would back them off a bit, not fully loose but partially. And then go to spec. This way you have a known baseline. I feel the rear torque spec felt adequate/ appropriate when I did mine, but the front seemed pretty low by comparison. I inquired with ZFDOC about it, cause I used his Beam Plates, and he cautioned against any extreme over tightening as it can distort the tail housing and cause binding. Made sense, the mounting is less beefy on the transmission. On my 2nd retorque I went a bit higher, maybe mid ~40's lb/ft. Good for now 👍🏻 Sadly it seems like a more frequent retorque is the safest policy for me, vs cranking it down possibly too much 🤷♂️ Not to challenge anything Matt Miller offered for his setup. I'm curious what #'s you ended up using to keep things locked down?
Pro
Joined: May 2025
Posts: 594
Likes: 140
From: OK
With the TKX in my 86 and the shifter being rigidly bolted to the trans I noticed first off how the driveline flexes up/down.
It's a lot of weight and stress on a piece of aluminum, and it's not even when accelerating just with undulations in the road you can
feel the trans following the chassis a split second behind in sort of a springy/flexy motion. It's really kind of interesting to watch.
Never felt or noticed anything on hard acceleration but I don't have any power to speak of either so hey.
And no nothing is loose.
It's a lot of weight and stress on a piece of aluminum, and it's not even when accelerating just with undulations in the road you can
feel the trans following the chassis a split second behind in sort of a springy/flexy motion. It's really kind of interesting to watch.
Never felt or noticed anything on hard acceleration but I don't have any power to speak of either so hey.
And no nothing is loose.
Pro
Joined: May 2025
Posts: 594
Likes: 140
From: OK
Motor mounts are new and the batwing bushings appear to be hard as a rock, which no doubt indicates needing replacement but they aren't loose.
I would bet a significant amount it's the C beam flexing. I suppose it could be the motor mounts compression but it seems like that would exhibit some
other symptoms, and they are new. It is a very rubber-band feeling at the shifter and on only specific movements of the chassis.
You'd never feel it with the 4+3 since the shifter is body mounted and the later ZF cars may not exhibit it from all the differences vs an early one.
It's probably less than a quarter inch but there is a definite "bounce" there.
Also there does not seem to be any movement going hard and fast on and off the throttle, it's just when there's a big enough dip in the road
for the chassis as a whole to compress fairly evenly. It's really not especially objectionable but I thought it was interesting.
I would bet a significant amount it's the C beam flexing. I suppose it could be the motor mounts compression but it seems like that would exhibit some
other symptoms, and they are new. It is a very rubber-band feeling at the shifter and on only specific movements of the chassis.
You'd never feel it with the 4+3 since the shifter is body mounted and the later ZF cars may not exhibit it from all the differences vs an early one.
It's probably less than a quarter inch but there is a definite "bounce" there.
Also there does not seem to be any movement going hard and fast on and off the throttle, it's just when there's a big enough dip in the road
for the chassis as a whole to compress fairly evenly. It's really not especially objectionable but I thought it was interesting.
Le Mans Master
Joined: Aug 2015
Posts: 6,090
Likes: 1,972
From: St. Charles MO
Two things I can imagine: 1) The shifter could rock side to side a bit as the engine and diff twist in their mounts, and 2) It might move a bit over certain kinds of bumps. The C-beam is designed to allow some twist, but not any vertical bending at all. The beam is more than enough to prevent vertical movement of the tailshaft, even when I put over 400rwhp (on a pretty torquey engine) through it to 315 Hoosier A7 tires on 17x11 wheels...except for the time I didn't have the bolts tight enough.
Pro
Joined: May 2025
Posts: 594
Likes: 140
From: OK
No side to side movement that I can detect, and nothing I can feel with a hand on the shifter while accelerating or decelerating hard.
The shifter on these is extremely rigidly mounted and tight, way more so than any of the stock ones. It telegraphs every little noise and movement
of the driveline right up into your hand.
Only time it's noticeable is when just cruising and the weight of the whole car smoothly comes down evenly in a shallow gulley in the road, then
the shifter kinda springs down for a split second and back up. Sometimes it'll sort bobble up and down on the right type of crappy
road (we have major crappy roads here). It's not extreme but I noticed it after the trans swap. Nothing is moving, something is flexing.
The C beam bolts are not moving, if they were it wouldn't be springy like it is. Could be old age and mileage.
It's possible it's an early car thing, you'd never notice with the 4+3 shifter and the later ZF cars just don't do it for whatever reason.
The shifter on these is extremely rigidly mounted and tight, way more so than any of the stock ones. It telegraphs every little noise and movement
of the driveline right up into your hand.
Only time it's noticeable is when just cruising and the weight of the whole car smoothly comes down evenly in a shallow gulley in the road, then
the shifter kinda springs down for a split second and back up. Sometimes it'll sort bobble up and down on the right type of crappy
road (we have major crappy roads here). It's not extreme but I noticed it after the trans swap. Nothing is moving, something is flexing.
The C beam bolts are not moving, if they were it wouldn't be springy like it is. Could be old age and mileage.
It's possible it's an early car thing, you'd never notice with the 4+3 shifter and the later ZF cars just don't do it for whatever reason.