I’m on my fourth C3 and so far none have had issues with stress cracking of the body panels. Yet I see many very nice cars with cracks everywhere. Headlight surrounds, 68-73 tail light panels, and fender crowns seem to be most common spots. Besides poorly executed collision repairs what, if anything else, causes some cars to crack and others not? A friend of mine recently sold his 70 LT1 Vert because a painter told him “they all crack sooner or later” so he bailed on a nice car out of paranoia.
So, what gives? Have I just been lucky? 🤷🏻‍♂️

Ken

 

My aftermarket L88 hood has spider webs adn cracks, the rest of the 68 looks great crack wise. Has a lot of chips up front from hard driving and few door dings from car shows..... MIght give her a paint job just before Iretire so I can pay it off before im on a fixed income.. I've seen some of the old urethane bumpers cracking from age and sun, most replace them with fiberglass. Too bad your buddy panicked.....guess he never hitchhiked the universe...old reference

 

Leading causes of stress cracks:

1) Many coats of paint. There are a number of corvettes (mine included when I bought it) that by now have been painted and repainted, then sprayed with a thick layer of polyester (sprayable) filler .....and repainted again a time or two! My car had like 15 layers of primers, paint and clear on it (not a joke). It had cracks all over the place! That's because it's expensive to strip and repaint a corvette, so guys just keep repainting them ....and then find another guy they know who will do yet anothe 'cheap' paint job and put yet another coat or two on it! The paint on mine for example was almost 1/8" thick! ha ha. But the cracks were in the actual paint, not the fiberglass. Soooo....sometimes the cracks are in the paint, not the fiberglass and it's difficult to tell which it is until ya strip it all down to the GM black (or the bare fiberglass)

2) Cracks in Fiberglass Gel Coat. Cracks can occur in the actual fiberglass too, usually in the top gel coat. These are usually caused by age, continued expansion, or stress (rough driving, close-calls ....or light collisions). Usually it's just in the gel coat or top coat. It's less common on SMC-type fiberglass. The cracks often occur in areas where stress (movement) is greatest, like the thin headlight areas. Fiberglass has a high degree of expansion/contraction compared to steel, so a dark car sitting the hot sun for years can lead to cracks. And 30-50 years of age can lead to the gel coat becoming brittle. Bad front end mounts that allow front end flexing can also lead to a stress event around the headlights.

SMC is very resistant and quite tough and flexible. But if there is a traumatic event where the panel flexes a lot (like a light collision), it can cause stress cracks.

Corvettes tend to be cars where at some point in time guys will 'horse-around' and show their buddy "what she'll do" ...which sometimes leads to a close call and maybe running off the road bouncing all over in the ditch. Such a 'close call' may not have been a collision, but all that bouncing can cause stress cracks.

Stress cracks are easy to repair. But they take a little extra time during the bodywork process, depending how many there are.

 

Mark G, this is kinda what I’d imagine when I see lots of cracks. Like an indicator of abuse rather than neglect, if these things could talk we’d hear some wild tales. Thx for your response.

Ken

 

It's a trade-off of materials: a 50-year old metal body will have rust areas while a 50 year-old fiberglass body will have stress-cracks. On my '72, very little, if any, stress cracking but you can see all of the bonding lines telegraphing through the original paint...

 

If you can back the crack up with glass then you can V the area out and DON't use a local filler. Use slow JB weld first, then a local filler.
Now if you cannot get behind the crack the, again V it out and lay some cloth with JB weld and then finish with a filler.
Done this for years and NO more crack in that area!!

Dom

 

No one's going to comment on the subject line? I'm 75 and it caught my attention.

 

And, we can only guess what "caught your attention" means.

 

Just step away from the CRACK!

Yeah they all DO Crack.

1. The number one thing for MAKING your car crack is lifting it up from ONE WHEEL/ONE CORNER...lift up the whole side, or the front or back, NEVER a corner.
2. Rough roads and bump-stop abuse....(yeah right! lower the car until it's hitting the bump stops at every pavement change.) When you hit the bump-stops the suspension goes 'infinite'.
3. Undo application of poly bushings. (I love the guys who put solid body mounts!)
4. Boeing did a study of fiberglas longevity on RADOMES , Basically 80% strength in 20 years...okay we're all 40 now!
5. Very few cars have the original paint...even fewer have had really good bodywork re-bonding the bonding strips and checking the fiberglass to metal body re-inforcements at the body mount stiffeners.
6. God made guns for those people that sit and lean on Corvettes...just takes one second!
7. You saw the Iron Resurrection show when they crawled over the fender and had to re-do the entire nose? This applies to owners...it's not STEEL...stay offa it!
8. Moisture entering the nose/headlight metal mounts...in concrete that's called SPALLING....same principle applies.
9. One tap on your oh so coveted chrome bumper(s)...its all over....it's not a bumper...it's TRIM!
10. Yeah! keep slamming that door/ hood/ hatch that'll help!

Nature of the beast!

Unkahal

 

Yess I agree, but in the mid west, they filled a hole in the road and made a bump out of it!! I installed softer springs in my 67 435 coupe and finally did the body work to stop all the cracks. New ones did show up in other places and I used the same method to fix them. Lots are caused by the rough roads.

Dom

 

Same with my 72, you can see every bonding line where the paint has separated, but it still shines very well. As the paint is 100% original it will never be repainted as long as I own it, coming up on 50 years.

 

We are NOT delving into Plumber's "details"....

 

"They all crack sooner or later" Mine must be later. This is my crack free 45-year old St. Louis lacquer....


IMG 0165
All urethane doesn't dry out & crack either....

 

Kind of like concrete, if you don't want cracked concrete don't use concrete.

My 73 was a re-paint 10 years ago and not a single crack anywhere. I think it comes down to how you use the car, don't let fat chicks sit on the fenders.

 

For me, a chopppy solid lifter 700hp dynoed Big Block certainly expedited the process.

 

My 77 spent most it's life indoors in storage in Michigan and has plenty of cracks, only 34k when I got it with cracks, not really noticed any on my 86 from Arizona. Doesn't matter tho my 77 was my dads car and I'm gonna mod, race n enjoy it as he would have.

 

This is the kind of driving that could lead to a few stress cracks! -ha ha.

You can actually see the top hood/fender surround panel come apart at the headlight area if you look closely, which indicates there's a lot of stress in that area. The region around the headlights where the panel is thinnest is 'weak' just by the nature of the design (In all fairness, some of that damage could have occurred from previous stunts for the movie, we don't know). But what the video illustrates is that area around the headlights is one of the first to come apart under major stress (even when the cars were new). So obviously it wouldn't take a lot of trauma to cause mild stress cracks over years of normal or abusive use. I actually think the bodies hold up remarkably well given all things considered. Note: If you watch the longer version of this video, you'll get a sense where the bodies are weak and tend to come apart first. Good to be aware of if you're restoring an old C3.

Another thing to think about is the Bonding in the front fender and surround areas gets weak from age. Mine was like that. You see a lot of cars with the rear fenders raised mis-aligned door gaps ...loose around the hood sides area. Weak or delaminated bonding weakens the overall strength of the front clip, causing even more flexing than originally intended.

[note: I know some of you have seen this video before, but I hope it illustrates my point...]

 

About any 'bump' with a chromed front bumper (no damage on chrome or on contacted surface) could cause minor cracks to form in the corners of the headlamp openings. Such cracking was common when the cars weren't that old. I suspect that those having been repainted got some reinforcement prior to paint. That is likely to be most of them in the present.

An original paint car with no paint damage or cracking of any kind is simply AMAZING ! That car has led a charmed life with a very careful and watchful owner, to be sure.

 

It's possible if you....
Buy it new! I was also lucky enough to have a garage from day one. And I never got stupid. Even at age 18 when I bought it.
And I never drove it in the winter. I borrowed Dad's car during the winter of '76-'77 & I had a beater after that.
Plus the last time it was on a wet street was 1985. A charmed life indeed...

 

I lifted mine this weekend to change wheels. Placed the jack in the middle of the frame (under the tranny crossmember) to lift the whole drivers side.
BAM! Cracked in front of the door with nice loud sound. (Front clip to firewall). Paint job is 5 months old. Oh well, it's not that visible from the outside...