My '73 is a big block with a 4-speed & A/C. When I put it on 4 jack stands placed far apart on the frame, but between the wheels, there is noticable droop front to back. It's so bad that it's hard to open the passenger door and then it cannot be closed again. I'm unsure if I want to go down the slippery slope of a frame off restoration. On the other hand, I don't want to invest a lot of time and money on a car with a poor foundation.
Any advice is sincerely appreciated.

Thanks,
John

 

You have a weak frame. BTDT,GTTS.

 

Thanks for the quick reply Mike, but I don't understand what BTDT,GTTS means???

 

I had to look that up too.... BTDT Been There Done That ... GTTS Got The Tee Shirt.

How far apart are your stands on the frame? Do you have any pictures of what your working with as far as the frame? Noticeable Rust rot etc.

These frames do flex if you don't support them well enough. I replaced 3/4's of my frame but still will not hang the car up in the air only by the 4 stands only on the side frame rails. I always support the front cross member, especially with a big block. There is just no point in stressing it. I have a 1/4" plate with a pull tab for a trailer on my front cross member (someone must have raced my car back in a day) so I can jack on the front member with no problems.

With your doors rubbing when jacked up you maybe looking at some tell tale signs as mike indicated.

Some tell tale signs of a soft frame are the #3 cross member. You can tap it with a ball peen hammer and if it just thuds and dimples the frame its soft. Also the end caps on the frame rails were they end by the rear tire. If they are rotted out or if the gusset plates (triangles) where the frame rails and #3 crossmember meet are rotted. Also, if your kickups openings (where the trailing arm bolt slots are) are rotted out or really bad looking you maybe in trouble. Look at your front cross member where the lower control arms bolt to it. If its real crunchy your in trouble. Ive seen lower control arms rip off from the front cross member due to rust.

You have to get under there and look real close. Do some searches on frame rust and rot and you will see what others have come across and it will give you places to look. Just be careful your not supporting the frame poorly and causing it to flex. My father 55 belair has a clean frame but if you don't support it properly the doors will hang up on the lock plates in the door sill.

 

Mike P,

Thanks for all the great information. You gave me lots of things to check out. This will help me decide if I want to pull the body off or not.

John

 

Where did you place you jack stands? Are you using the factory jacking points?

It makes a difference.

 

I placed the rear jackstand as far to the rear as possible. The front jackstand is as far forward as possible; just before the frame begins to rise up.
When I put the front wheels on Rhino Ramps and leave the rear jacks on the frame, I can open and close both doors easily. This makes me think that the problem is coming from the heavy front end.

John

 

My doors would bind no matter where I put the stands, including in the factory position. With a replacement frame it makes no difference and both doors open and close freely.

 

Mike Ward,
Did you replace your frame with another stock frame or what?

Were you able to identify any specific problem areas on your original frame. I guess your original frame was beyond repair...

Thanks,
John

 

To give you an idea of how bad it can be, I recently pulled the body off and with the frame on jack stands I stood on the rear bumper and bounced a bit and it split the frame about 3" long where the kick up meets the rest of the frame.

 

Yes, I got hold of a pristine donor frame from a car that had burnt to the ground when almost new.

The old frame had rotted and collapsed under the drivers side pillar post but surprisingly the kick ups were in reasonable condition. The birdcage was also in poor condition.

 

My 73/454/4spd conv. with ac ( owned since 77) has always been garage kept . Frame solid as a rock . On jack stands has always flexed some . Doors open and close but a little resistance , pass. rear a little light on the jack stand .

 

Hello
My 68 had bad frame rot behind the drivers side door- side rail and kick up completly rusted away but the door did not bind.
The car is undergoing a full chassie restoration with new frame from front door post all the way back pluse new front end and a few other things. Not to bad cost wise at about $ 7,500.00

Good luck

 

all these 35+ year old frames are weak. they can be sand blasted externally, flushed out internally, por 15'd, powder coated, blessed by the pope, whatever-if you drill a hole in one, you will find rust between the layers of metal.
i always jack using the front cross member and put stands under the frame horns in front, and jack the differential and put stands under the gusset area by #3 bolt and stands under the frame rails by the gas tank support before removing the jack. just my experience after doing three of these cars.

 

A picture is worth a thousand words here. If I jacked my car the way I am picturing in my head from your description, I would expect the doors to open and close essentially the same as if the car was on the ground, but I could be wrong here for the following reason: If I want to lift my car safely on 4 jack points, ideally the center of mass of the car would be somewhere near the center of the 4 jack points. On my BB car, the car itself is front heavy, and the front jack point is further back from the front wheel than the rear jack point is forward of the rear wheel. That might sound weird, but the result is that the center of mass of the car is well forward of the halfway point between the rear and front jack points.

This has lots of implications, including but not limited to the inadvisability of working under the front of your car with it jacked up in this manner. Another is that a big block car is going to have more flex in the frame with more "droop" in the front than a small block car, due to the greater weight in the front, and having AC will only exacerbate the situation. This is really a long winded way of saying that if you check the frame as described above, and it is solid, I wouldn't worry too much about some front end droop when lifted in that manner, but I would also make a mental note that that isn't a great way to get your car up in the air. If you find soft metal in your frame, that is a different story.

 

Big blocks with air are nose heavy and will droop on jack stands with good frames. I have 4 73 big block convertibles with good to bad frames they all droop some. I think convertibles vs coupes have less support and will sag more. You may feel better with front and back crossmeber support along with side frame rail support. The door problem is more because of it being a convertible than a weak frame. My $.02.

 

Thanks to all who responded. I've given my frame a good screwdriver poking and haven't found any soft or rotted metal. Of course, I could only check the areas I could see. Now I'm convinced to always support the front crossmember in addition to the four jackstands.
On the positive side, this advice is encouraging me to install some aluminum parts on engine

Thanks again, I love the Corvette Forum!
Merry Christmas,
John

 

You should have some access to the side rails of the frame more or less from front to back, and most areas of the cross members. The bird cage is much less accessible, but bird cage flex will be limited by frame flex (since it is bolted to the frame), so your main concern is frame strength.

If I wanted all four wheels up, I would add two more jack stands to the front of the side rails. I've never felt comfortable using cross members for my main support.

In '70 (my car is a '70) the LS5 engine was only about 150 pounds heavier than a base engine, that may not sound like much, but it is significant. I forget what AC adds, but I would guess about another 100-150. Power steering adds pounds, power brakes adds a few, it's not hard for me to imagine a fully loaded BB car having 400-500 pounds more weight in the front than a low-option base engine. A substantial amount of the engine weight difference can be undone with aluminum heads (I've read as much as 70 pounds of savings) and intake (another 15?), after that things get expensive.

The bottom line is that a car like mine ('70 BB AC PS PB), and it sounds like yours is similar, is really a cruiser with a healthy amount of rip to it. If you want a real screamer, it would be best to take away some of the creature comforts. I'll think about that if I ever want to start track racing my car. Until then, I'll leave her just the way she is

 

is there any way to stiffen the frame up other than putting a roll cage into an already cramped interior?

 

Perimeter weld the frame (weld between the gaps in the factory welds).