Ok, I am in the process of a frame off restoration on my 75 (been doing so for a few years now). I am a week or two away from finally dropping the body back on the frame. I figured I would call a corvette shop and get an alignment before I mount the body (so I wouldn't have to trailer the entire car to their shop).

I called a local shop and he asked if I checked the frame measurements before I reassembled. As his experience is that when C3s have the body pulled, 75% of the time, the dog leg section sags once it is removed from the body? Thus, causing a large space at body mount #4 during reassembly.

My frame had some rust and pitting but no rot. Has anyone else experienced rear frame sag during a body off? I am wondering if this guy is referring to rotted out frames....

Thanks all

Kurt

 

Hi Kurt,
The only way the rear of the frame at the kick-up and beyond can sag is if the rust-out at the connection between the kick-up and the frame rails is so severe that the parts have broken loose.
If you stripped and re-finished the frame you'd know if you had frame rust problems by now.
While there are cars with very rusty frames around I wouldn't take much stock in his 75 %.
By 'alignment', do you mean a suspension, steering, and wheel alignment?
If so, that really can't be done until the car is completed.
If it's done with just the chassis and running gear it involves pulling and chaining the frame down to simulate the load of the complete car.
Regards,
Alan

 

I am not a mechanic but that sounds ridiculous. If it was going to sag why would it not sag until you disconnected it from the body?

 

Alan,

Yes, steering and wheel alignment. That makes sense now, I wasn't thinking about the load of the body...
Also, the entire frame was sandblasted so I know I was working with good metal.

He concerned me a little bit. Then, I went out to the garage, stared at the frame, and was trying to figure out what he was saying?!

This was an all corvette shop in MD.

Kurt

 

To sag after the body is removed suggests the body was holding up the rear of the frame rather than the other way around, frame holding up the rear of the body? This sounds like a bad frame, could be rot at the dog leg. I have noted some sag or deflection if the car is jacked in front of the rear wheels leaving the weight of the gas tank, spare tire, differential, suspension and rear wheels unsupported. Not a lot of change. In this case some may sag more than others even if there are no holes depending on how much metal was lost to rust, thinning the metal.

No reason I can think of a bare frame should sag at all. You should be able to confirm it's within spec.

 

Hi Kurt,
You do see frames that are severely rusted and are certainly sagging; a few rusted so badly that the body is holding things together.
But that's certainly a small percent of the total 68-82 Corvettes still around.
Good Luck as you continue!!!
Regards,
Alan

 

setup the frame on jack stands,
use the body mount #position for reference,
and measure frame mount height to floor.
post the measurements. we can compare to a couple know good frames.
someone might have frame dimensions already, maybe they will post.
if you have no frame rot, I doubt your frame is sagging.

 

Run away.....

 

75myway,

I have pulled my fair share of bodies and I can not grasp what this Corvette shop is trying to say. I understand the physics of how the center section of the frame could sag down...but that is putting an awful lot of faith in the rear body of the car in holding the rear section where it need to be. I CAN NOT grasp that part. Because how much weight is actually on top of the transmission...which is basically where the weight would need to be in order to get the frame to sag down and allow the rear to kick up. And yes...if the frame is a rust bucket...then it could collapse.

My one question to you is this. When you removed the upper control arms....did you pay careful attention to the thickness of the shim packs at the upper control arm mounting points???? The reason is this...knowing that the cradle collapses over time....and in order to align the car...shims are removed in order to set the camber/caster. And when you run out of shims.....there are TWO choices...actually THREE.

1.) Do nothing at all.
2.) Have the cradle stretched back out so shims can be re-installed.
3.) Install the off-set upper control arm shafts and hope for the best that they will work...depending on how far the cradle has collapsed.

DUB

 

Sounds to me the shop is suggesting the frame from the dog leg back is sagging downward. This would create a larger gap between the #4 mount and body. I don't know what could cause this. Bending in the other direction would close the gap between #4 mount and the body or possibly increase the gap at #2 or #3 mounts? No idea why this would change.

 

I agree with Allen. The car needs to be completely assembled and weight distributed before you can do a proper alignment.

 

For what ever it's worth....on the horizontal part of the frame between the #2 and #3 body mounts, you will see ..on each side two 9/16 inch frame holes (best I remember they are 9/16)....these holes match corresponding holes in the birdcage. They are factory drop alignment holes. For factory alignment, 9/16 inch steel dowel rods where used to drop the body on the frame. Interesting...the holes on the passenger's side of the frame are a little different spacing than the driver's side. OK, so you want to buy some 9/16 inch steel rods to perfectly align the birdcage to the body......you can't buy 9/16 inch dowel rods from, say, Home Depot. Their 9/16 inch steel dowel rods are not 9/16 of an inch..they are less. It's like if you buy a 2X4 piece of lumber from Home Depot..if you buy a Home Depot 2X4, it's going to be 1 7/8" X 3 7/8." Similarly, if you want to buy an EXACT 9/16 inch steel dowel rod, you have to go to a machinist's shop an order 9/16 (if they don't have in stock). Machinest's supply shops carry steel dowel rods in exact dimensions. Machinest's supply shops sell steel rods in exact diameters. lf you're not that fussy about alignment, you can just use the standard hardware store "9/16" inch rod.

 

Great write up. I wish they would all work with dowels like you described...

But honestly..my alignment dowels are not perfectly machined for a reason I have come across so many Corvettes that this concept would NOT be able to be used. So...the alignment of the body has to be altered slightly due to an impact area that the owner does not want to repair. In a perfect world....maybe...but this car was never built to exacting standards of current cars of today...so a few thousandths off in diameter is not going to effect anything at all....or at least for me...I have never had a Corvette that I put back on the frame cause me to re-adjust it a few thousandths. It all depends on how the alignment dowel is in the hole.

DUB

 

100%.

And with a full tank of fuel also....or some feel that a half a tank of fuel is better.

And if you are super picky....use weights and put your weight in the drivers seat to simulate you being in the car.

DUB