Timing covers (upper and lower) are on the front of the engine, covering the timing chain. The upper cover fixes to the head and the lower cover to the block. If the head is skimmed without the upper timing cover then the surface will have a step in it, causing issues
The compression difference is interesting. Low compression on adjacent cylinders can mean CH gasket failure
More testing, including a leak down test, required I think. This will tell you if piston rings, cyl walls, etc are indeed your problem
The compression difference is interesting. Low compression on adjacent cylinders can mean CH gasket failure
More testing, including a leak down test, required I think. This will tell you if piston rings, cyl walls, etc are indeed your problem