There have been a few threads running recently regarding Corvette frames, how to clean them up, inspect for rust, possibly replace. I found this video put out by a vendor which I found interesting. My question is how much flex in a C3 frame, specifically 68 - 72 is typical? What is considered to be bad, requriing repairs or replacement? A frame doesn't need to be perforated to be weak, rust over time may thin the metal causing the frame to flex, showing up as a door gap which widens when the car is jacked up. I narrowed my question to 68 - 72, solid mounts, taking the effects of rubber mounts out of the equation. Other variables are a full tank of gas and the spare being present, adding maximum weight to the rear.

Has anybody ever measured how much flex they get when the car is on jack stands? I see reports of none to over 1/8" when looking at the door gaps, one over 1 inch which is obviously bad. One thought I had was to put the car on jack stands, measure the distance from the floor to some point on the rear of the frame, then jack up the rear until the frame lifts from one jack stand. This should tell me how much the frame is flexing. What may be typical or dangerous I don't know. This could be a way to evaluate a car being considered for purchase? Some sort of standard good/bad indication?


It would be nice to have some standard to judge the condition of the frame other than cosmetics. It may be hard to justify buying a replacement without knowing how much improvement we'd expect to see in terms of how much flex.

Is it true the frames cannot be repaired? Anyone had any experience with this? Thoughts?

 

you have an early 70 frame (like the 69's) that doesn't have the buttresses welded from the crossmember to the kick-up, so it's going to flex more than a reinforced frame. these can be added for strength. my 70 also does not have the supports, and i never jack the car from the gusset in front of the rear wheel-i always use a jack (with a 2x4 as a cushion) under the diffy. if i have to remove the diffy jack, i place jacks in the gusset area and use tall jacks on the rear frames placed before i remove the diffy support.

 

The frame of my '72 coupe has no rust. When I put the car on four stands, the doors get tight. They still open and close, but you can feel the change in the clearance. I suspect this is probably in the striker. I've never measured the door gap clearance pre and post jacking but the change isn't visually striking.

 

My doors open and close freely, no noticeable difference when I jack the car up. The car handles well, no unusual squeaks, etc.

I'm in the middle of cleaning up, refreshing some restoration work I did twenty years ago. Some surface rust had started up again, road debris being kicked up by the wheels had partially sandblasted the paint. While I'm in there I'm doing a close inspection of what I've got. What I noticed is the door gap at the top of the door, rear, opens slightly when the car is on jack stands placed in front of the rear wheels. It's been like this for some time, never gave me any problem, doors operate just fine as I said before. I'm curious how this compares to others, how much is typical. I have no plans to do anything with this at this time since it hasn't been a problem. For now, I'm keeping this in mind as I consider what to do with this car in the future.

I do have the reinforcements welded to the kick up to the crossmember. We're referring the the area where the brake junction blocks are located? The reinforcements make access to the crossmember difficult. Wasn't this added in 69? Were there other changes after early 1970? My car was built some time around March 3, 1970.

Problem I have with the video is many C3s on the road today may need new frames if we go by the criteria stated in the video. I'm wondering what's typical? So far I'm hearing no noticeable change in the door gap at the top rear?

 

Pssst. The guy in the video is selling his restoration services. You're supposed to want to buy what he offers.

 

Yep, the thought crossed my mind hence my general question to forum members, what's typical, what's abnormal?

 

I believe mid 68 is when the gussets were added for the kick-up. A 70 frame should have them.
http://www.71corvette.com/frames.html

 

Maybe I should ask a different question, when you jack your Corvette up just in front of the rear wheels, do you see any change in the door gap at the top rear? If you do, how much? So far I've heard anything from no change at all to 1/4" or more. Related to the video, it makes sense to me that frames will weaken over time, rusting from the inside out, cracked welds, etc, probably more of an issue for cars which have seen northern winters. I'm wondering what's typical.

 

I'm betting there is no "typical."

 

You may be right.

 

Hi BBC,
On my 71 coupe there isn't any difference in the door gaps, or the way they close, when the frame is raised at one corner.
I think this is more typical on coupes than convertibles.
Regards,
Alan

 

Same here as Alan experiences on my '71 coupe. I can jack up the right or left side of the car with the jack point just behind the front wheel where the frame starts to bend and the whole side lifts. Doors still open and close easily. Fairly original southern car, body never off frame. Agree with Alan as you may not find this true of a convertible.

 

69 vert here and this last winter did more work on it. Had it on wheel dollies and jacked it up at the rear jack point high enough to remove both front and back wheel dollies. Both doors opened fine and saw very little change in door gap at top rear of door. Probably about 1/32" at most. This has always been a rust belt car (Minnesota) and never a frame off restoration. However when I first did this car in 1989 I did spray rust neutralizer inside and outside the frame rails. It still has rust blisters, but no rust through. Also keep her out of the rain and small oil leaks over the years have probably kept the old girl from rusting down below.

Mark Smith