We perform ab initio no-core shell-model calculations for A = 18 and 19 nuclei in a 4 (h) over bar Omega, or N-max = 4, model space by using the effective JISP16 and chiral N3LO nucleon-nucleon potentials and transform the many-body effective Hamiltonians into the 0 (h) over bar Omega model space to construct the A-body effective Hamiltonians in the sd shell. We separate the A-body effective Hamiltonians with A = 18 and A = 19 into inert core, one-, and two-body components. Then we use these core, one-, and two-body components to perform standard shell-model calculations for the A = 18 and A = 19 systems with valence nucleons restricted to the sd shell. Finally, we compare the standard shell-model results in the 0 (h) over bar Omega model space with the exact no-core shell-model results in the 4 (h) over bar Omega model space for the A = 18 and A = 19 systems and find good agreement.