graduation project need help

[minawageh]

in my project i wanna assemble a pump manually so
i wanna use snaping tool to help in assembling it as if you pick an object and get close enough to onther object and with suitable position and rotation it will be oriented so i can slide it along onther part axis (mainly x-axis) so i wanna calculate to differance between there position taking in considration orientation differance
so if it can be done any way i wanna know very soon
and i was trying use matrix to calculate it but till now i can't so i need help as soon as possiable
THX
MinA