Questions about secondary door locks

Both the 82-040 and 82-051 are designed to install using #8 screws, but the length is variable. That depends more on your door than the lock. You have to be sure the screw isn't so long that it hits the glass, but still long enough to secure it. You really need to use one of the old screws as a reference.

The diameter of the bolt for both the 82-040 and 82-051 is approximately 9/32".

I'm not sure that I understand your third question. The 82-040 and 82-051 are designed to install on the door itself, near the bottom of the frame.

While I can't be certain, if there's room on the lower face of the active door you might be able to install either the 82-040 or 82-051. You'd then need to also install one of our keepers: the 82-052, 82-053, or 82-054. There's no way to know for sure if this could work for you, but we've done it before on other jobs. Note that a simpler, old school solution would be to use a security bar lock instead like our 80-006.