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Expert Q&A: Cause of Star Crack to Spread

Question: 

What causes a star crack (leg) to spread while injecting resin? And if it does grow how do you stop it & fix it? I use a B-250 bridge. Also, I am about to purchase the B-150 bridge. How many times do you turn the level screw and the & resin chamber once the B-150 Bridge it attached to the windshield?

Answer:

Could be a couple of answers:

To much pressure from the injector, heat, cold or stress. Some legs on star breaks are actually longer than they appear. You should always probe the repair site prior to find the tight areas of the break.

As the legs gets further away from he impact they get get smaller and so tight that you can't see them until you start the injection process. The injection pressure, even a slight amount, will cause the leg to open to make it more visible, in reality it was already there but you just could not see it. If the leg is longer than about 2 to 3 inches you might consider drilling at the end of the leg, popping a mini bulls-eye to terminate the leg from spreading, but now you have two repairs to complete. Be sure to check that the leg doesn't come to the surface, (resin seeping out of the leg) or you can gently drag a razor blade across the leg and feel for a catch. If it does come to the surface you will have to repair it as a surface crack.

The B-150, first the resin chamber, touch the glass and add 1/4 of a turn (90 degrees) and the same for the leveling leg. Different than the B-250. Hope that helps, or you can call anytime for free tech assistance.