No oil pressure after an LS swap — what should I check?

Start with the parts that go missing when engines are pulled, cleaned or rebuilt:

1. The oil cooler block-off plate on the pan above the filter. the plate that bolts to the oil pan just above the filter is often missing from used pans. Junkyard buyers take them because the GM part with a threaded tap is expensive. Without it, the engine won't hold pressure.
2. The front oil gallery plug behind the timing cover, if the block was cleaned or rebuilt. Jeff Smith has seen this on fresh LS builds. A small (about 0.625 in) steel press-in plug sits in the front of the block, behind the timing cover, in the main oil gallery on the lower driver's side. Machine shops often remove it to clean the gallery. Without it, oil dumps straight back into the pan inside the front cover, and oil never reaches the pressure sensor. The fix: pull the balancer, water pump and front cover and press a new plug in flush. Iron and aluminum blocks use the same plug size.
3. The pickup O-ring if the pan came off — install it well oiled; a torn O-ring causes oil pressure problems.