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Drill the hole about 1/4" longer than the length of the bolt. This is called OVERDRILLING and allows you to easily erase a badly placed bolt by hammering it deep inside the hole . Drill at a right angle to the surface of the rock to ensure that the hanger sits flush and there is no torque on the bolt. Hammer away small irregularities in the rock surface before hand to ensure flushness (if need be). Pull the bit out every so often 1-2 min. to let it cool down. Use a Blow Out Bulb or piece of tubing to blow the dust out of the hole. You may also want to alternate brushing and blowing. 3" x 3/8" stainless steel wedge anchor bolts are the norm and Hilti is the industry standard. Following the UIAA standard for rock anchors, in-hole bolt depths must be at least 5 times the hole diameter (not including the external bolt length assigned for the hanger, washer and nut). This means a min. bolt length of 2" in hard rock like quartzite (if no washer is used and the bolt extends beyond the nut no more than 3/16"). Longer than 3" bolts may be required in aerated limestone. Note that mixing alloys (i.e. stainless hanger with galvanized bolt) results in a chemical reaction that promotes rapid Galvanic Corrosion. Expansion bolts are standard, but you can epoxy a regular bolt in. Most expansion bolts require hammering to get them in. Hammer on the nut not the bolt itself. Tighten with a torque of 20 ft-lb. (for Hilti bolts). This is not a lot of force. Basically what you can muster with one hand on a 6 inch long wrench. You may have to tap the expansion bolt with a hammer to get the rubber centering ring past the hole in the hanger and into the hole in the rock. Stainless Steel bolt hanger, Expansion bolt (bolt sleeve anchor) Typically 3/8" diameter but also come in 1/2" See also Which Bolt is Best Links:
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