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Discussion Starter #1
Folks

My 67 GTO Sports Coupe, 400, 4 speed has developed an interesting problem. When the car hits a bump, the driveshaft knuckle at the back of the transmission strikes the top of the driveshaft tunnel. I recently replaced all of the frame to body bushings and as of this morning this is my prime theory of the problem. Last weekend was the first long drive since replacing the bushings. I drove the Goat across Denver to a local show without issue. After the show as I left the show parking lot, I heard a loud thump as if something hit the underside of the car. As I drove further, every time the car hit a ripple in the road, there was a distinctly driveshaft sounding thuds as the forward U-joint knuckle as it struct the top of the tunnel just to the rear of the cutout for the gear shifter. The problem became more severe as I drove getting worse if I happened to be braking while hitting a bump. All of the bushings were the same size, appearing very close to the stock bushings removed. Most bushing bolts were re-installed into the same location, and all torqued to 60 foot pounds. There is no apparent movement of the body and all gaps (doors, bumpers etc) remain unchanged. There is no movement of the transmission when I push up or pull with the car on the lift. Has anyone else had and solved this problem?

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My best guess would be your rubber transmission mount has separated or something.....but you seem to have checked that possibility although the forces acting on the driveshaft may be stronger than that of a human pushing and pulling. I would be tempted to pull the trans mount and check out its condition.

If that's not the problem, I'm hoping some of the more expert members will chime in soon. Best of luck getting this resolved.
 

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Beautiful car! I love the hubcaps and redlines, very nice!

That said.....I would check the pinion angle of the rear and put your angle meter on the trans also. Car should be sitting on all four wheels when you do this.

John
 

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Discussion Starter #4
ALKYGTO - Thanks for the positive comments on the car. I too like the hubcaps on steel rims. Do you know the angle spec on the trans and differential yoke? One other possibility is too much air in my shocks?


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Nice work!! Looks awesome!

Your rubber inserts on the sides of your transmission crossmember look odd? The crossmember should fit inside a slot located in the rubber insulators wrapped with the metal brackets. Might be worth double checking there fitment as they could raise that section as well.
 

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Discussion Starter #8
Pinion Angle Measurment

Pinion Angle Measurement

How To Set Pinion Angle - Hot Rod Network

Basically what you don't want is the rear end pinion pointed upwards, this would raise the driveshaft and may be what is contributing to your problem. Are you experiencing any vibrations in the drivetrain?
ALKYGTO

That seems straight forward. This would be much easier checking on a parking lift where the underside is easily accessible with the car sitting on its wheels. May be time to buy one...

There is no driveshaft vibration.

I've visually inspected both the center transmission rubber mount and the cross member rubber mounts. Both of these were replaced last year when I refreshed the car after storing it for 24 years. This was a visual inspection and a check to ensure all 8 bolts were tight. I did notice a slight upward tilt to the drivers side crossmember mounting bracket. It is possible that I've slightly bent the lower side frame upwards when I used a 2x4 and jack to raise the body to replace the body to frame bushings. This would account to the fact that this problem only surfaced after the body bushings were replaced. What is odd is that I drove the car 30 miles to the local show with no noise. Then the noise starts as I am leaving the show. The only variable was the outside air temperature. Much hotter on the drive home.
 

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Nice work!! Looks awesome!

Your rubber inserts on the sides of your transmission crossmember look odd? The crossmember should fit inside a slot located in the rubber insulators wrapped with the metal brackets. Might be worth double checking there fitment as they could raise that section as well.

Gotta say x2 on this one. That left trans crossmember mount looks odd in the photo. What's up with the large bolt and nut?

Here is what the rubber insulator should look like in pic #1 . The metal brackets that hold it in place and bolt to the frame are seen in pic #2 . The flat ear found on the end of the trans crossmember slides into the rubber insulator and then the metal bracket goes over the top and bolts to the frame rails.

The mount under the transmission case could be thicker than original as aftermarket pieces can be incorrect. I assume it has been replaced? If you had an original to measure by, you would know. Also the trans looks in the photo to be off center to the left. It may be the photo. The 2 bolts that attach the trans mount to the transmission go through the crossmember. These holes are slotted/elongated so you can center the transmission. You may want to check this.

My 1968 Lemans (3-speed manual trans) front U-joint used to hit my floor every once in a while. It seemed to do it mostly when I put a passenger in the seat. My car was jacked up a little in the rear and I ran 14" tires up front and 15" out back. I had new U-joints and driveshaft/rear was original. It is possible that the bushing were worn out in the rear/control arms allowing the rear end to twist upward or move around when extra weight in the form of a passenger(s) was added and changed the driveshaft angle which in turn may have been the cause. I also went with a polyurethane trans mount which could have been thicker than factory.

Did not do it enough to worry about as I rarely had a passenger. I looked a couple of times and never saw anything evident that stood out, but I also never checked the rear bushings as I knew they were original and worn, nor did I check pinion/driveshaft angles. Drove it this way for about 7-8 years.
 

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Had the same problem years ago, after a service on my 70 GTO.
Seems they slightly bent the cross member up. I uses a 2x4 as a lever and
bent it down a little and all was well. I was surprised how easy it was
to bend.

Larry
 

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Discussion Starter #11
Thank you all for the great posts. I couldn't get Pontiac Jim's pictures to resolve. I've got a couple local GTO club members with unrestored original cars. I'll take a look next time we're together.

An update on mine. I removed the transmission brace and noticed that the slot on my new rubber mounts were not centered. One side was slightly thicker than the other. It also looked like the frame was slightly bent upwards. Using a block of wood and a long prybar I bent the bottom lip of the frame about an 1/8" lower. Assembled it as it was, made sure the trans was centered and took it for a drive. All is good. The shaft was totally quiet and smooth.


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