Put the timing cover on, but do not tighten it down.
Install the harmonic balancer.
Tighten the timing cover bolts.
Install oil pan.
This sequence allows the front oil seal in the timing cover to "find it's center" on the harmonic balancer- there's not a lot of wiggle room, but it can be enough to cause premature wear of the seal. Oil pan is last, because the end seals can push the timing cover up, or not even allow the bolts to start on the timing cover if done first. It can be done, but not preferred.
