Originally Posted by DaveC
It's not that they are doing anything, it's just that the Chinese don't often tap the holes to the bottom. You screw in the studs too shallow and they pull out easily. For $12 you can get a 40 piece Metric tap set from Harbour Freight. If the engine is assembeled then plastic baggy the piston and use tape to seal off the insides. Use a heavy grease to lube the tap and carry off the chips. Tap to the bottom of the hole being careful not to try and run it in too far, you can tell when you hit bottom. Replace the junk bolts as soon as you can helps also. That was one of the first up-grades I did was to get the SBP head stud and bolt replacement sets. I HAD to tap the head studs on the Z80 BGF motor I got. The head studs would not thread into the stock M8 holes, they were just *slightly* undersized and the SBP stud would not go in.
Another thing about the badly tapped holes, you can screw the head stud in all the way but because it doesn't bottom the acorn nut will bottom it's threads and then you get false torque readings and head gasket leaks or pulled out head studs.
you can tap the threads even deeper if you grind off most of the taper on the top of the tap.