I have only had two GM engines do this. One LS that I could not see why it did it, so I scraped the bearing. The other was a BBC and #3 bore was too small. The boring bit must have broke (saw evidence of that), they fixed it, but didn't finish the hole, just kept on moving forward. I had to turn the OD of the cam bearing down a few thou and install it (used green loctite on that one). Fords; maybe a few snug ones that I had to scrape. Mopar; SWEET Jesus, damn near every one needs to be scraped. The reason for this is that after the cam bearing bores are cut, the block is drilled from the mains, and all of the boogers from the hole and any gouges from installing the bearings remain because the bearings are semi-finished and final bored for the cam after installation.
Now, when a block is rebuilt down the road, "finished" cam bearings are installed in the block with the crappy cam bearing bores, and you wonder why the damn cam is too tight. All of the imperfections create high spots, and interference fit at those points. One of the last steps I do, mostly on Mopars is to use a ball hone, and clean up the cam bores to knock off the hole drilling boogers, and possibly any gouges. If there are any bores that still will cause a problem, I will clean it up with a file and/or emery cloth.
Yes, I have seen shops just slam in cam bearings and maybe the cam, but that doesn't mean we ALL do. I require the customer to bring in their cam for me to test fit after cam bearing install when they are assembling the motor. Hell, I had a friend who picked up a motor that another shop did, and he was asking me to do the rear main rope seal as he had no clue how to. First, the crank was tight as hell, and I told him that he needed to align hone the mains, then I tried to turn the cam, and it was totally immovable. I told him that needed to be fixed and his comment was 'I will just run it with the impact until it loosens up'. I rolled my eyes and walked out...
