72 Convertible: When I tore the car down it was obvious someone had done some work to the front from an accident by the misalignment of the front clip. Inspected for obvious damage, creasing, buckling and found none. Had the frame powder coated and put the engine and transmission in. The transmission mount was off by about an inch, had to pry it over to make the holes in the crossmember line up with the transmission mount. In addition when I mocked up the side pipes they were obviously at a slight angle and not parallel to the frame. I took the car to two different body shops and both told me the the frame frame was straight. This was eight years ago and the project has sat since.

I am ready to go full steam and get it done to include a 454LSX and T56. However I do not want to put the body on frame and order the engine and transmission if I have a frame problem.

Has anyone else experienced an issue with the transmission mount not lining up? Possible causes other than a bent frame, maybe motor mounts bent? Are the two body shops I took it to wrong and do I need to take it to a Corvette specialist?

 

I suspected correctly that my frame was out, so I took to my local Fix Auto. Using Forum provided frame specs they straightened the issue and gusseted it to reinforce and with that piece of mind prevails!

Dave

 

Dave,
Already taken it to two different shops and both say it is good. Don’t understand why transmission is not lining up if frame is good.

 

I would guess that you would have to provide the frame spec sheets to your frame guy.
I had to do that with mine.
They did not have anything that old in their database.
And the printed specs still do not have a dimension for the motor mounts, so yes those could be bent even though everything else is good. Mine seem to be perpendicular to the frame centerline.
Here are 2 charts. one for a 69 and one for a 79. The only part of a 79 that is different is the very front and rear where the bumpers attached. At least it has diagonals, that the 69 does not.

69 frame specs from GM

79 frame specs from GM

I also do not know if you have a manual or auto frame and what kind of crossmember adapter you are using for the T56. IIRC the entire engine & trans & diff pinion is offset 1" to the right anyway for weight distribution.

Keep digging! Either you or they will find the issue.

 

Try to contact David Howard who's screen name is/was "AllVettesforme"(close). He is very knowledgeable about frames, has done a number of them. He has a good shop to straighten and realign them. Believe he's in PA.

 

Leigh is spot on. You need a shop that knows how to use a tram gauge and has the full specs. It takes time to properly measure a frame. A small deviation in one area can have a huge impact on another. Producing a report that shows all of the actual dimensions vs spec is a far cry from calling it “good”. What is “good” to a shop may be nothing more than “close enough” and “close” only counts in horseshoes.

 

Both shops had tram gauges and I know the second shop had the full specs. Not sure if this matters for the transmission mount but the car is originally a four speed 454 car. The motor and transmission in the car is a big block with solid motor mounts and an turbo 350 automatic transmission going to the the fixed four speed crossmember. I will take a picture and post but doubt that is the issue. I suppose someone could have messed up when they put the mounting plate for the automatic transmission on the crossmember but doubt it as the engine seems crooked with the alignment of the headers to the frame.

 

Lets say your frame is straight.
Do you know that the big block and transmission do not sit perfectly centered between the frame rails?
If you already swapped engines, double check your engine mounts to insure they are correct.
Double check that you have the correct transmission mount.
Auto to manual transmission swap?
Was the crossmember swapped?

 

If it were me before starting a major project on the car I'd get the frame stripped bare,put it on a level surface on jack stands and go from there.

 

Verifying the integrity of the frame horns may be tricky since there are no book specs.
My left frame horn is almost exactly 2" longer than the right one, so the engine is significantly offset to the right by 1.0"
The trickier part would be to ensure they are lined up exactly 90 degrees perpendicular to the center of the trans mount.
That too is not exactly centered. The chevy diagram shows one of those bolt holes being on the centerline, not the middle of the trans mount.
I will go measure mine now, I need to confirm that anyway.

 

Engine mount measurements
I wanted to document this to help others in the future. It is not hard to measure these frames at home. The factory specs are all plus or minus 1/8" and you can easily measure closer than that.

There are no book specs on the frame horns on the GM frame dimension sheets. So I measured mine. I have a 72 BB manual frame that the engine fit well in.

Front view with frame centerline marked. You can see how much longer the driver's side motor mount is. It is 2.0" longer. So the engine is offset 1.0" to the pass side.


Frame front crossmember centerline according to GM. Center of large hole. Measures to true center.


Trans crossmember centerline according to GM. Center of drivers bolt hole. Measures to center, 28.0" both ways. It means the engine is offset 1.0" but the trans is offset 0.75" So the engine is actually on an angle in the car.


Centerline of frame pinion crossmember according to GM. Left side of large hole. Measures to center, 28.0" both sides. It means the driveshaft and pinion is offset 0.5" at this location.


Centerline of frame on diff, and laser level I used for other frame measurements. Pinion is offset 0.5" to pass side.


First attempt at triangulating motor mounts to check for squareness.


Closeup of right side measurement, to measuring slot in frame. 43 1/8" to back of slot.


Left side (driver) measurement. 43 1/4". Slight discrepancy due to how the angle on tape changes. Not happy with these results. But later proved correct.


For second attempt I established an "engine centerline" mark on trans crossmember exactly 1" to pass side of frame centerline mark.


Measured engine mounts to this mark and checked both diagonals this way.


I got exactly 44 1/2 " from both motor mounts to this mark. Diagonals are an exact match! Success! My motor mounts are aligned.

 

Thanks Leigh that is awesome will get out and measure this weekend!

 

What sidepipes....? Factory? The N14 side exhaust option included a trans spacer that many forget to use......

Jebby

 

Glad to have been of some help!

Now anybody want to help me move a lower a-arm mount???

 

If you were closer I would!

 

Leigh, looking at those spec pics the 69 and 79 motor mounts look completely different. could you post a straight-down shot of your 2 mounts and the square they live in? and Pat, it is time to buy or fab a removable crossmember. the engine is offset to the right, but i think the trans is pointed somewhat inboard so the u-joints have less angle to compensate for to line up with the rear, which is NOT offset...

 

Derek I think that is just a not to scale drawing.
My left motor mount measures 2.0" longer than the right side one.
So engine is offset to the right 1.0"
Trans mount is offset 0.75"
Pinion is offset 0.5"

 

I’ve always wondered why the engine, trans and rear diff were offset in the original design?
Was this just in the early C3 years?
Was it changed in the later C3’s?
What about C1’s and C2’s?

 

It was offset so there is more room in the driver side foot area. That way you weren't stepping on two pedals or the wrong pedal easily..
I had to modify and tighten up clearances so my pedals would fit on my drivers side.

 

I read Duntov did it in the original design to balance the car right to left for optimum traction.
I saw a four wheel scale results somewhere (can't find it now) and the front wheel weights were amazingly close, even with the driver sitting on the left.