SBC oil leak puzzle
I've got a very challenging (infuriating) oil leak on a fully rebuilt 350. Basic specs: '69 iron block; hot tanked, magnafluxed, bored to +0.060" (last go-round for this block), various other machining operations but both decks were machined and confirmed flat. Edelbrock E-street heads on Cometic 0.036" gaskets. Edelbrock intake manifold on Felpro intake gaskets with ultra-black sealed ends. One brass fitting on rear shelf for oil sender; threads sealed with permatex thread sealant. Distributor gasket is felpro with a light coating of permatex gasket sealer on the block side only.
This engine has 40 minutes of cam break-in time and the oil has been changed once. Break-in went smooth, engine runs beautifully, no evidence of any failures, abnormalities or other issues with engine. It has since seen only several sub 5-minute runs for moving around the shop and loading/unloading on the trailer (engine is part of a chevelle restoration that isn't finished yet).
So I notice a few drops of oil under bellhousing dust cover. Traced to outer rear corners of head gasket between head and block, trickling down various routes along rear of block, starter, etc.; eventually down the outside of the dustcover and dripping onto the floor. I added an ounce of dye to the oil and with the UV light cannot detect any evidence of oil running down to the rear outer corners of the head gasket area from anywhere higher than that location. This is happening on both banks. A small ribbon of oil, from the rearmost corner of the head gasket, extending maybe 2" forward.
So the first thing anyone; including me, would think is that it's definitely the rear corners of the valve cover leaking and running down the back corner of the head. I started the build using the silicone molded over steel core valve cover gaskets under cast aluminum valve covers with the gasket surfaces free from casting defects, flashing etc. (I've been down the leaking valve cover gasket route before). The valve covers are retained with ARP studs, nuts and washers torqued to 3lb-ft. There wasn't any evidence of oil running down the head from the rear corner of the valve cover gaskets, but since "it had to be coming from there", I swapped the gaskets for 7/32" cork gaskets, sealed to the underside of the valve covers with permatex gasket sealer.
Oil leak persisted, still no evidence of oil running down back of the head gasket from the rear end seal of the intake or from the oil sender or distributor. Again, no evidence of oil originating from rear corners of the valve cover, either on intake or exhaust side.
Oh, between each solution attempt, all the places oil had traveled were cleaned thoroughly with rags/solvent.
Another puzzling aspect of this is that no oil is actively observable anywhere while the engine is running; I've tested this a few times. After shut down, I'll check with the UV light and various little mirrors and over hours/day or two, oil will collect at the rear outer corners of the head gasket seam and eventually start to make it's way south. The maximum amount of oil observed on the floor was a quarter sized puddle after about a week sitting perfectly still after a ~5 minute engine run.
So now I'm stumped and could use some of your thoughts. Conventional wisdom suggests that the oil "can't" be leaking from the actual rear corner of the head gasket on this engine and certainly not when the engine isn't running, but I'll be damned if I can find the answer to this.
Oh, the head bolts are still at 65lb-ft torque with ARP bolts and ARP thread lube on the ones that get it and permatex thread sealant on the ones in the water jackets.