I have checked all 3 of my small block engines (2 350"s and a 383) and they all have a square plug in this small passage that runs from the passenger side of the block just above the timing cover into the main oil gallery. My question is what is this passage for? Is it an alternate site for an oil pressure gauge or ???? Why would the engineers who designed this motor include this passage just to plug it ? Very curious.

 

If you still want to know, reaching way back in my memory and at my age memory is a fragile thing, the original '55 265 block was cast without an oil filter. There was an optional bypass oil filter available and I believe that they tapped the oil off of the galley line at that port. It makes sense as that port is the very end of the main galley line that runs through the block and the filter would not be robbing oil from more important places, mainly crankshaft oiling. Chevy never did remove anything from the 1st gen small block but just kept adding. When they cast the '56 and subsequent engines with a full flow oil filter that port no longer had any real use but it was never removed. I did not see many '55's with an oil filter but I seem to remember that the filter was in the vicinity of the front of the rh cylinder head.

 

What he said, The filter was the same as they used on the older 6 cylinder models. Not very effective but better than nothing! you can use that for a auxiliary oil pressure sending unit or Gage.