I replaced the plastic timing cover on my ZZ crate motor with a steel cover from GM when doing a cam change. A seal was installed in the cover when I got it. It leaked. I pried it out, and found the rubber on the seal to be hard as a rock an un-pliable. Installed a new seal - problem solved. My guess is that it was old.
It is also possible for oil to leak past the keyway in the balancer. A small dab of silicone in the keyway when installing the balancer is advisable.
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