I really find it hard to believe it is an oil pressure issue....there should be wear on all cam lobes if that were the case. I think the roller bearings are failing on the lifters.....no roll = lobe dig.
Got 2 be a root cause for the needle bearings though...oil related?
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OIL CONTAMINATION
An engine can never be “too clean” during assembly. While properly cleaning a block is a time-consuming activity that usually requires specialized equipment like a hot tank and a heated parts washer, there is no getting around the bottle brushes. Bearing failure can often be traced to a build-up of dislodged sludge BEHIND the oil opening left over from an improper cleaning.
Dirt and grime accumulates throughout the oil passages, especially in engines that suffered an internal failure. It hides in nooks and corners, and simply blowing the passages out with compressed air can compact the grime even more. They must be thoroughly rinsed and scrubbed until no trace remains. Any and all oil galley plugs must be removed (even if they have to be drilled and tapped for NPT plugs) and the passages brushed out, as they offer access to critical debris collection points. Floss all crank oiling holes and rinse out pushrods, even if they are new.
Cams, lifters and bearings rely on a microscopic film of lubricating oil to keep them from galling, and any debris or grit suspended in the oil becomes an abrasive that quickly destroys their surfaces or gets stuck within the close clearances. Block those life-giving oil passages and the bearing or lifter begins to fail. This is especially true of silicone contamination left over from gasket surfaces, or excess material pinched at installation that drops into the block.
Bottom line, if you want your cam and lifters to live, it has to be clean, clean, clean!
INSUFFICIENT OILING OR HIGH OILING TEMPS
Proper oiling is key to cam and lifter life.
The “Four Rs of Proper Oiling” are the Right oil, at the Right place, at the Right time in the Right amount. Under normal operating conditions, oil temperatures do not spike. A rise in oil temperature is usually the result of a mechanical bind or blockage causing excess heat, such as a lifter sticking in a bore or a clogged passage.
ZDDP wear additives need heat and pressure to release the zinc molecule and form the protective phosphate coating in an even and calculated amount. With excess heat, the chemical reaction that forms the film doesn’t occur and the additives then become antioxidants and instead get consumed. This translates into the oil “breaking down” or losing its ability to “pad” the metal contact surfaces, and damage will occur.
Low oil pressure doesn’t create enough “padding,” or oil depth, between the moving parts. This allows them to touch and generate heat and begin to fail. A good rule of thumb is 10 lbs of oil pressure for every 1,000 RPM. A high-volume oil pump is wise when increasing RPM.
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http://www.cpgnation.com/top-10-cam-failure-culprits/
