To Re-torque or not
I have Mopar Performance aluminum covers that have no flex. After changing the gaskets twice I put a straight edge on the head surface and I could slide a penny under it. The OE stamped steel covers probably would flex and conform to the uneven surface but the aluminum can't.
I have been dealing with the same thing on the 383 in my Newport. It always leaked a bit at the rear with the factory steel valve covers. I switched to Crane aluminum covers a few years ago, but no help. I tried silicone gaskets a few months ago, since others claim the leaking is a heat issue. It then leaked terribly, dripping right on manifold and making my car a mosquito fogger. I found the gasket surface on the head wasn't flat, dropping about 1/8" at the rear (from a straight-edge). The rear is where the oil pools and there is flashing in the casting that blocks the drain valley. In trying tightening it more at the rear first, the rear corner snapped off when I tightened the front nuts.
After my 3rd or 4th pass this year (several in past years), the leak is tolerable with a rubber gasket and the aluminum cover (welded the corner back on). I still have the traditional slow seep at the rear but no more fogger. I found the real culprit (I think) in causing the fog was the oil fill cover. If you don't get it positioned just right, oil will seep by and run down the cover. I had to catch it in the act when I saw a few drops on top of the cover. That is an issue with after-market covers with no baffles to deflect the flinging oil.
For those who are rebuilding heads, I recommend asking the machine shop to mill the gasket surface flat, as in modern cars. Then you can use a stiff aluminum valve cover and silicone gasket. Also have them grind down the flashing in the casting so the oil can drain back freely. That is my plan.
Re can a head gasket cause an external oil leak? That was a major problem in the Chrysler 2.4L engine (Mitsubishi design?). Our Voyager started that just after the warranty at 39K miles and got unbearable by 95K. By then they had a fix in the Multi-Layer-Steel (MLS) gasket which became production ~2000. A strange head gasket leak. No cylinder or coolant problem, just oil from a passage to the outside.
