I mean that machine shops always claim that they clean the dirt, rust, and metal chips out of the parts they machine. They'll maybe even charge you for that "service". And if you don't use HOT water, soap/detergent, white lint-free towels/rags, and an oil-gallery brush to verify EVERY FOOKIN' OIL PASSAGE in the block, and the cylinder bores, the main saddles, the oil drillings in the crank, the hollow pushrods, the spurt hole in the rocker arms...there's particles left behind by the machine shop (or by normal engine wear on other used parts) that will gouge oiled parts once the engine fires and runs and has oil flow.
Sludge in the lifter valley may get wet with oil...but sludge in the lifter valley isn't in the oil pan to be picked-up by the pump and circulated against close-tolerance, finely-machined parts.
The parts--including the "new" parts you buy--pistons, cam, timing set, etc. also need to be cleaned in hot, soapy water. They may have an oily film on them to prevent rust; the oil picks up dust at the manufacturer, or in your shop.
And for those of us that re-use hydraulic lifters, a hydraulic lifter has a decent-sized oil inlet hole--but microscopic clearance for the oil to get back out. Therefore, hydraulic lifters are like 16 tiny oil filters--dirt and sludge get in, but can't get back out. Every re-used lifter has to come apart ONE AT A TIME for cleaning and inspection. DO NOT mix the plunger from one lifter body into a different lifter body. Keep all lifter parts together on the same lifter.
You cannot trust the oil filter to "catch" the debris before it circulates, because the debris is already in the oil passages.
There's likely debris in the coolant passages, too--but nothing in the cooling system has close tolerances like the oiled parts. And the water pump bearings are sealed.
My favorite machine shop left this in my boat 454 when they machined it. I've known the owner since high-school. He's a good guy. But I have learned to check EVERYTHING he does, especially if what his shop does was actually performed by his part-time help.
Passage between main saddle, and cam bearings after machining and cam bearing installation, once I stroked a brush through the oil passage. I bought my own cam-bearing installation tool, I don't let the machine shop do that any more. I don't let them install any oil gallery or cooling system plugs, either. I want all those passages open so that "I" can clean them to my standards.
