Why did my C4 (1984-1996) glass roof crack right down the middle? This is a question we get every so often. Yes, I understand there are no glass roof panels after 1982. The material is either acrylic or polycarbonate plastic, and of course the lowly painted fiberglass 😊. The use of the word glass is brought on by the car community due in part to lack of understanding or better wording. We have succumbed to its use in certain aspects to accommodate people searching for this common misnomer.
Onto the matter at hand. This article applies to all removable transparent roof panels from 1984 to present Corvettes. For discussion purposes, it will mostly apply to 1984-1996 C4’s since this design best represents what happens when transparent plastic roofs join into marriage with inferior replacement weatherstripping.
The C4 was a huge redesign for GM at the time, and the C4 platform was originally supposed to have the two-piece T-Tops as did its predecessor. At one point the decision makers at GM decided if world class Ferrari’s and Porsches had Targa™ tops, our Corvette needed one as well. So, to “stay classy”, last minute changes were made, and here we are. The problem they could not predict was excess chassis flex, so last ditch efforts to solve this dilemma resulted in the much-hated bolt on roof panel.
This “bolt on” method of attachment was more than fine when these cars were “young”. They were adjusted by technicians on the assembly line and sat on premium weatherstripping spec’d out for their purpose and design. The engineers understood how much the weatherstrips would compress and they created a miniscule torque spec of 11 ft lbs. for the roof’s attachment bolts. Just enough to keep the elements out, but not so much as to damage the roof panel’s lens.
Skipping ahead to present day. We now have weatherstripping sourced from all over the globe, constructed by workers that don’t know or understand their purpose, bought and paid for by large importers who care more about profit than quality. How does this all relate to you? Everything - if you are currently thinking about replacing your weatherstrips or roof panel.
Cheap weatherstrips are typically harder and heavier than their more expensive counterparts. Don’t ask me why, as I am not a weatherstrip manufacturer. What I do know is they account for more damaged roofs than the entire membership of the “flying roof club”. Get to the point! Let’s say we just got done installing a brand X windshield weatherstrip, and now it is time to install the roof. The roof sits directly on top of this weatherstrip. The roof
then gets or bolted down onto the car. As the roof gets tightened It compresses the gasket forming a seal that keeps out the elements. Good in theory but our new brand X weatherstrip being thicker, denser, and harder just doesn’t seem to compress like the OEM weatherstrip did. Hmmm, what to do? Being the ham-fisted folk that we are, we make it compress dammit, and we continue to tighten the roof panel until the roof attach bolts bottom out. What we have achieved is only compressing the gasket at the bolt attach points and have created a dangerous bowing effect (see illustration) with forces “A” pulling down and “B” pushing up on the front lip of the roof’s lens. Unbeknownst to the installer, they have just secured the roofs fate, and it is only a matter of time before it goes to great parts heaven in the sky ☹.
So, what can we do? For starters, if you are of the aforementioned ham-fisted group, go out and loosen up the front attach bolts enough to slide a dollar bill between the roof and the windshield weatherstrip. With currency in place, tighten the roof until you can barely move the bill with just two fingers. That should be tight enough to keep out wind and water. (Legal Disclaimer- This is not intended to be used while driving the vehicle. Follow the owner’s manual for correct fitment prior to driving). Moving forward you have two options - source a weatherstrip that fits, and/or follow the correct roof adjustment procedures in the service manual that will restore proper roof fitment. Also see “C4 Corvette Roof Installation” under instructions here on this site.
In the end no one is going to look out for your car like you will. It is your duty to learn what good quality weatherstrips should look and feel like. Good ones should be spongy and pliable, and able to form to their mating parts i.e. door windows and other gaskets. If you have original gaskets on your car go out and feel them. If your new replacement feels heavy or dense like a gummy bear, trust your instincts. Order another brand and compare the two before installation. Secondly, where you buy from matters! I know eBay is fast and easy, but it is also a known dumping ground for unwanted inventory without the risk of blowback on the seller’s actual business name. What I say next might sound archaic to some of you, but I will put it out there. Call an actual corvette shop and ask them what they prefer to use. Not a big catalog company with a phone jockey that just got off the morning shift at Starbucks, an actual Corvette repair shop, they do still exist.
Melrose T-Top 2-24-2025