repacking boot.img

[imrija]

Hi, I'm trying to modify my android ROM. I have working scripts to unpack android image and then unpack boot.img (for apk and initramfs modifications). I wonder if someone did this before, because I'm having trouble repacking boot.img back (script for repacking whole ROM works fine). When I unpack and repack initramfs using cpio, it has few hundred bytes less than before.
Does anyone have working boot.img (initramfs) repacking script ? (preferably bash)
Thanks !