Our BoltHold asphalt anchors feature an internal thread. The thread extends just short of 1 inch inside the anchor. You can thread longer bolts than 1" because the anchors are hollow, and as long as your bolt does not reach the bottom of the anchor, all is well.


There is no advantage to using a longer thread than required. The maximum strength of a steel bolt and nut occur when the length of the engagement between the nut and the bolt is the diameter of the bolt. Thus for a ?"-16 bolt (SP10 or SP12), the max engagement is reached at ½". That bolt has 16 threads per inch, so ½" is about 6 turns of the bolt. Increasing the engagement beyond 6 turns will not strengthen the resistance to pull.


Likewise for a 7/16"-14 anchor (SP18), 7 turns will guaranty max strength. For our 5/8"-11 SP58 anchors, 7 turns are the minimum to obtain max strength.


There is a drawback to applying excessive length of thread into the anchor. One is the amount of time it takes to tighten the extra long bolt. Another is that the thread that is not engaged with the internal thread is vulnerable to rust, and will make it harder to remove the bolt when required. We therefore recommend not using more than 1" extra length when calculating the length of the bolt.


You calculate the length of the bolt by adding the length of the thread inside the anchor (½" to ?") to the length of the bolt above ground. If you are securing a ¼" plate and using our 1/16" thick washer under the bolt head, you need ?" bolt. Using our supplied 1" (SP10, SP12) or our 1.25" bolt (SP18) will be perfectly adequate.