Deployable bow spring centralizer

A centralizer sub, and system for enhanced access to subterranean zones from the surf ace as used in oil and gas wellbore installations to center a pipe or casing within a wellbore or previous casing string during run-in, installation, or cementing procedures. In under-reamed applications, casing strings and centralizers pass through a smaller casing string before opening up to a larger hole where repeated compressions and decompressions of bow strings of centralizers can compromise integrity and reliability. A centralizer sub, and system reduces and controls insertion and running forces to preserve centralizer integrity and down-bore surfaces and equipment.