Which Bur for Windshield Repair?

The differences between bur styles and sizes and how they affect your windshield repairs

April 2011

This article has previously appeared in The Wise Crack and has been updated to reflect the most current information.

By: Brent Deines

When do I drill? Which bur should I use? Are carbide burs really better than diamond burs? While the answer may not make a difference in whether you land a customer or not, the questions are worthy of consideration. By the end of this article, I hope to have cleared up any confusion in regards to bur selection.

“When do I need to drill a break?”

We at Delta Kits recommend drilling during the windshield repair process in three situations: (1) those where you are unable to get the resin to flow into the break, (2) to anchor a crack longer than a couple of inches in length, and (3) when a pit must be capped for the injector to seal properly.

In most cases resin will be able to fill the break if you are properly using Delta Kits equipment, even when there is very little glass missing from the impact point. Many technicians drill the break to create a large channel so the resin will flow more easily. This does speed up the repair, but it also affects the repair cosmetically. Each time you drill a break there will be a visible drill hole. It is up to you to decide if the cosmetic difference is worth the extra couple minutes. To me it is definitely worth the wait if the end result is a better cosmetic appearance.

Every once in a while you come across a chip that doesn’t have enough glass missing from the impact point to inject the resin. In those instances, just drill through the center of the impact point or into an existing air pocket.

You may also want to drill the end of long cracks to help stop them from running as they are being filled with resin and add additional strength to the completed repair. After drilling the end of the crack (actually about 1/32” past the end of the crack), follow your slide hammer or spring hammer instructions to create a mini bull’s-eye and fill it with resin. Do not drill past the first layer of glass. The spring hammer includes a handy depth gauge that helps you drill to the perfect depth for making a mini bull’s-eye.

Finally, if you have encounter an impact pit that is large enough to prevent your injector end seal from sealing properly, you will need to cap the pit and drill a hole through it to allow the break to be filled. You can see this process online in the video titled “How to ‘Cap’ a Pit”.

“What is the best drill bur for windshield repair?”

Now here is something to argue about. You can go a couple different ways on this one. Many technicians stick to one all-purpose bur, and others take the fly fisherman’s approach, carrying several styles and us the one best suited for the situation. Let’s take a look at your choices.

FG701 (hole size .047in) - The FG701 is “Delta’s Choice” because we feel it is the best all around bur. It is a fast cutting, extremely durable, tapered bur that can be used in just about any situation that might come up. The FG701 is the perfect size for drilling the ends of cracks because the tip of the slide hammer fits neatly in the hole, and “pops” a perfect bull’s-eye nearly every time. Smaller holes will not accommodate the slide hammer, and too tight of a fit can also cause damage to the glass. THIS IS THE ONLY BUR RECOMMENDED BY DELTA KITS FOR USE WITH THE SLIDE HAMMER AND SPRING HAMMER.

FG2 (hole size .039in) - The FG2 is popular with many technicians because the round head makes it less apt to skip or travel across the glass when you start drilling.  The FG2 also drills a smaller hole, which is preferable to many technicians. Although not as durable as the FG701, the FG2 still out performs most other burs on the market for most break types. It is not however recommended for stopping long cracks. 

FG170L (hole size .039in) - The FG170L is great for the technician that desires the benefits of a tapered bur, but with a long narrow point that drills the same size hole as the FG2.

FG169L (hole size .031in) - The FG169L also features a long tapered point, but drills an even smaller hole.  The FG169L is a great bur for very small breaks where a larger drill hole may be very apparent.   

FG329 (hole size .024in) - The FG329 features a unique pear shaped head that drills the smallest hole of any Delta Kits bur. Many technicians feel the FG329 drills faster and lasts longer than the FG2, yet provides the skip free qualities that make the FG2 so popular. Testing by Delta Kits indicates the FG329 is less durable than the FG2 but if a the smallest possible hole size is desired the FG329 is a good choice.

Key points: 

As a rule, the smaller the hole size, the less durable, and easier it is to break the bur. Size does matter when you intend to “pop” a bull’s-eye at the end of a crack and smaller is certainly not better in this instance.

Delta Kits does not sell diamond burs as carbide burs are far superior in durability and cutting speed. Carbide burs are also much less expensive, although that is not our main consideration.

My opinion is that if you only want to carry one bur in your tool box the FG701 should be that bur. That said it’s much like buying a car. Some people swear by a Chevy, and others will buy only Fords. The only way to know which bur will best suit your windshield repair needs is to try them.