Is long crack repair possible? Yes.
Is it cost effective compared to replacement? Most of the time.
Is it safe? Maybe.
A windshield is much more than a "shield" for "wind". It is a crucial safety component of any motor vehicle. So much so, that there are Federal Motor Vehicle Safety Standards (particularly 208 & 216 amongst others) that pertain to there proper installation and structural integrity. In fact, a windshield is a major component in passenger ejection prevention, air bag deployment and rollover crush protection. Here at GLASSWORKS, we recommend that six inches be the limit for repairing a crack versus replacing the windshield. Below we will address our reasons, as well as give recommended procedures for repairing cracks that fall within the acceptable guidelines.
There are four major reasons that we believe the acceptable limit of repairable crack length is 6 inches:
1. If a rock chip or short crack fails to bond properly, due to technician error, unknown contaminants, or other reasons, the windshield as a whole is not structurally compromised (until it begins to spread) to a significant degree to cause a safety concern. On the other hand, if a 15 inch crack is not bonded properly, is could allow for passenger ejection, airbag deployment malfunction and more.
2. As of yet, we feel there has not been enough independent data collected to support the safety of long crack repair.
3. The same new equipment (sensors, cameras, etc.) that is causing glass prices to rise and creating the demand for long crack repair, may have its ability to function properly inhibited by a crack, even if it has been repaired.
4. We feel that even 6 inch cracks need good judgement when deciding whether to Repair or Replace a glass. Liability is a true concern. Do we believe it's possible? Yes. However, until we can eliminate more variables and be positive that a long crack repair is completely safe for our customers, we will not recommend performing them. BUT WE'RE WORKING ON IT!
The use of Cured Resin to bond a crack is called a "Chemical Stop". Another method to keep a crack or long "leg" of a Star Break from spreading is a "Mechanical Stop". In fact, this is a critical component to successfully perform a repair on any crack of significant length. Although judgement must be used, a good rule would be any crack over 1.5 inches should have a mechanical stop. So, what is it?
A "Mechanical Stop" is a purposely made "bull's-eye" break that will terminate the crack and stop it from travelling further. This will perform two functions. It will stop the crack from spreading and allow another injection point for inserting resin into the break. This method is useful for performing re-repairs, as well, when the original Pit has been sealed. Here is how its done.
1. Use a Glass Probe to create a small pit about 1/4 inch directly in front of the end of the crack.
2. Use a Tapered Bur to drill half way through the first layer of glass. (The Pit you created will be a guide and keep the Bur from sliding and scratching the glass.)
3. Insert the tip of a straight pick or scribe into the hole and tap the top lightly with your Blade Holder until a Bull's-eye forms.
4. You will notice that the crack travelled to the Bull's-eye and is now both terminated and ready to accept resin through the pit!
There are procedures for inserting resin into the middle of the crack itself and creating an airtight seal for creating a vacuum. These are addressed elsewhere. For this section, just realize that you now have the original Pit and the one that you have created that can accept Pedestals for injecting.