The rear bearings should be checked from time to time by feeling how smooth the prop turns and listen(if mounted in plane) for roughness in a quiet house. If it feels and sounds like there's sand in it, replace the bearings. To continue operate the engine with bad bearings can damage it beyond repair.

A good place to find affordable bearings and great info on how to do it. http://www.mcintoshcentral.com/rc-bearings/

The most common bearing for most 46 sized engines are: Front bearing size= R6. Rear bearing size= 6902 BUT MAKE SURE.
Remove one seal/sheild on the front bearing leaving one to the front or both if your engine came that way. Remove seals/sheilds on the rear bearing if any. All races I use are bearing steel, not stainless. The rear bearing can have ceramic balls for long life and speed. I try to get bearings that use plastic retainers, its mostly what these folks sell anyway.

Most important thing is to clean the engine's outside real good first without pushing dirt into intake, exhaust and front bearing before disassembly. Keep your hands and work space clean, lay out paper towels or news paper. I like to wipe all the parts down super clean with paper towel and put a little oil (ATF or after run oil) on them just before reassembly. Did I say keep it clean?

When mounting the cylinder head, thread all the screws in about one turn from touching, now while holding the head down by pushing the tip of the glow plug run the screws down till they just touch in a cross tightening pattern. Look into the exhaust port to see if the sleeve turned, if not keep tightening in a cross pattern working up the torque gradually several times until its tight.

It's easy, take your time, follow the above site's instructions and think it through and don't force or bang/hammer on anything.