I'm looking at buying a '72 C3 and most things have checked out. In terms of the under body the 2nd owner back replaced the entire suspension and cleaned up the sides and bottom of the frame and rust treated the visible parts. Tapping the rails has yielded no odd sounds, there is no significant externally visible rust and the birdcage seems okay. However, there is rust inside the rails, and the car was not "refreshed" frame off so the condition of the top of the frame is unknown.

For the folks who have done frame up restorations or repairs. How worried should I be? Does the rust typically eat up the car from the inside out or is it mostly from the outside in? Now that I know about the rust it will be on the back of mind until I clean it up. I think that would mean an inevitable frame off down the road....

 

From my understanding the frame rust is mostly from the inside out as it is boxed & once dirt & moisture get in there it just sits & eats away. From my reading it would seem the biggest issue can be found just in front of the rear tires where the frame goes from horizontal to vertical, there are some internal gussets that can hold dirt cause issues as well. FYI, I am in no way an expert & basically in the same position as you looking to buy & asking for help from the forum..

 

No disrespect, but I'm wondering if you might be trying to convince yourself you are going to need a frame off restoration.

What is the actual extent of the rust you've seen and are concerned about? Consider that it took forty plus years to get to the point where it is today. It might take another forty years before you have a genuine problem.

 

It's worth being concerned about, one of my old vehicles (not a vette) was kept totally rust free for some 20-25 years by the previous owner (a family member who owned it from new) who kept it under a car port and the car still had it's original paint. She then felt it was looking a bit faded, so got it resprayed at some point, but the respray wasn't as good protection as the original paint, so it looked good for 2-3 years then started rusting at paint chips and scratches. Most the rust on the car started in the last 10 years, and only in the past 2-3 years has it really gotten bad and actually rusted through in spots. The car is now 34 years old, but all the serious rust on it can be traced to the last decade or less.

I seriously regret not dealing with the rust earlier, as it only took a few years to go from "that looks passable" to "oh crap, what do I do now".

 

The most damaging rust often comes from the inside, dirt holding moisture, untreated metal. I had the body off my 70 many years ago. Sure, there may be rust on the top surfaces which can't be accessed with the body on but this is mainly surface rust. A frame off won't necessarily help unless you plan to have the frame dipped and inspected afterward. I'd say if you give the usual rust prone areas a good rap with a hammer and they ring out, not a thud, the frame is probably OK.

There was a posting from GD70 a while back which contained photos of frame rust issues. The shop went over the frame with some sort of improvised air tool which gave the frame a good shock. Interesting to see what they found.

 

Check out Post # 6
http://forums.corvetteforum.com/c3-g...r-project.html

This hi tech scratch tool aka piece of wire, took off more internal rust than I expected. I also used metal bristle brushes with extension wands to get the entire length of the rails cleaned, followed up everywhere with internal frame coating.

With the rust inside its very hard to tell what the condition is like, sometimes you will only get small skinny flakes and sometimes a big thick piece will detach itself with a little more persuasion. No real accurate way to check for the healthy metal thickness that is still left behind except for it sounding different....

 

Looked at that post, nice!! You did all that frame on? Looks like you had a lot of the mechanicals off. To get to inside the rails like you did what is the bare minimum that needs to come off the vehicle?

 

I'm not disagreeing; all I'm saying is that we do not know what has been found or the extent of it.

 

Yep, frame on but all of the suspension, steering, and exhaust were off.

Bare minimum would be to have the rail openings exposed. You can then snake the brushes through those openings.

 

I would take into consideration the climate that the car lived its life in. Although my 72 has lived its life in the desert of AZ and CA, it had some surface rust. Even the insides of the frame had minor surface rust. Car is 41 yrs old, made in an era when corrosion protection really was not high priority. When I pulled the body, the top surface of the frame had much of its black paint intact. Areas that trap water and dirt are probably the areas to really look at. These areas are well documented on the thread about buying your first C3.

 

Why not invest in one of the cameras that are available now that have a two - three foot long "snake" on them with the lens and some LEDS for lighting at the end? You can put this into the frame rails and have a proper look inside. I have even poked one into a spark plug hole to check the condition of the cylinder bore. These cameras have come down considerably in price in the last year or two.

 

Thats a cool idea. I was considering the camera for other uses but my only concern with using it for investigating internal rust is... you will only be able to tell whether there is rust on the surface (and 99% chance there is).... but will not know how deep it goes into the metal, unless you alternate scratching the rust off with camera views.

 

You simply wire a scratching/poking device to the end where the camera can just see it. Also, you can attach a hook to the end for pulling things you can't get to. Great tool to have.

 

The frame on these cars and birdcage rust from the inside out in some places and on the outside in others.

The frame rails along the door sill just ahead of the back tire usually rot from the inside out because dirt, water salt etc works its way in there from the tire splash and the end cap on the rail just holds it in and the little drain usualy clogs right up. Also mice will take your carpet and seat foam and pack it into the side rails for nests. I have seen so many rotted side frames. I have also seen plenty of side rails with surface rot as well.

The kickups will rot from the inside out where the trailing arm pocket is located and attaches to the #3 cross member due to the overlapping metal especially where the trainling arm bolt goes through and again the road spray and dirt that gets packed in there. You will see the metal swelling in those spots but sometimes its hard to see in there with the trailing arm in the pocket.

You can inspect the #3 mount in the rear wheel well by removing the access plate to see what shape the mount is in. If its crunchy most likely the frame is too.

Also check in the #2 mount locations by removing the kick panels see what kind of debris is in there. If its packed with crap and really ugly then your birdcage is rotted and dumping water in there. Most guys know this and will clean them up prior to a sale so as to hide the damage above that you cant see.

The #4 mount is pretty easy to see and if its rotted or the frame mount itself thats another bad indication.

Look at the shape of the seat belt mount plates, seat track mount plates, the battery hold down mount plates in the fiberglass floor. If they are crunchy its a bad indication. You need to pick and scratch at these things to see what flakes off. Someone can easily cover them with under coating to hide this and with fiberglass floors you will think the car is ok when it may not be.

Also, the windshield header, corners and uprights are real prone to rot holes and much harder to see. Take of the upper wind shield frame pad if you can and look at the screw holes and surface of the frame if you see water trails or rot there its most likely rotted. these pad hold water against the metal and this metal had almost no paint on it at all from the factory.

Get it up in the air (preferably on a lift) and really look it over. If you tab with the round end of a ball pean hammer you will hear the difference and if you leave dimples in things like the #3 crossmember RUNAWAY!.

 

Thanks for that great information.

We did get the car on the hoist and tapped around from the bottom and didn't hear anything obvious. Seat belt/battery mounts seemed okay. The previous owner the coat the bottom with rust treatment so it's hard to tell but there is nothing obvious when viewed from underneath.

However the kick panels were not removed, neither was the windshield trim (it's brand new). I'm most likely going to need to hire someone to specifically inspect the frame (I don't live close to the car). Here was another thing that worried me. This picture shows an alignment gap between the windshield and fender that is eerily reminiscent of this thread http://forums.corvetteforum.com/c3-g...-aligning.html. Could be totally unrelated but....

Also the car is a Pennsylvania car with an unknown history. No idea if it was driven in the winters. I'd be less concerned if it was a southern/southwestern car.