roller bearings are used in a 2 stroke as the oil is mixed with the fuel and the ful is the lubricant. useing roller bearings instead of sleaves like a 4 cylinder is composery as they will be longer lasting and wont lock up as a 4 cylinder sleve bearing. roller bearings are caged so that grease or lubricant can be kept inside the bearing and keep it rolling smoth and efficant for the engin. on a two stroke the caged roller bearings are located on the crank .SPLIT TYPE BEARINGS
split type bearings are used on a 2 stroke crank shaft so that the crank and engin can be serviced. they are split from factory and are just cracked and sloted on . not having a split type bearing will make sericing the engin extremly hard as you would have to pull apart the crank peice by peice to slide the bearing out. PISTON RINGS
the sslots in the side of a piston are for piston rings. the piston rings stop fuel and leaks down the boar past the con rod or piston head. the slots in side the piston ring slots are called locaters and are used to locate the piston rings. the piston rings must be sloted in right with the ends on the out side of the locater. no doing this can cause leaks ,damage to the boar and make it hard to install the piston.the piston works by sealing the boar, it seals the boar when the engin heats up the metal stretches and so dos the piston rings. one of the piston rings are cut on a diaginal axis so that fuel can go in behime the piston ring and push the piston ring out seal the boar. parts of the piston rings you would messure is grove deepth,side clearince and n and end gap. the grove depth is the depth of the slot that the piston ring sits in. side clearince is the play that the piston ring has in its slot and the end gap is the distance between the end of the piston ring tips. 
REED VALVEa reed is locate on the intake manifold and is a one way valve that works of atmospheric pressure. the reed valve can be tested by either boling through it or looking through it to see if you can see and day light .
OVALITY AND TAPPER
ovality and tapper in the boar is a term used when servicing a engin. to check for this you would use a boar gage and micromiter. recording data when messering is a help when resolving the problem. when checking the boar you should mesure top midel and bottem, and 3 places in each section to be acurate. causes of ovality and taper can be due to and offset piston wearig against the boar. to fix this you would get the boar re boared as part of the service.
2 STROKE 4 STROKE
the difrence between two stroke and four stroke is lubrication, bearings and cams. in a 4 stroke the engin has a sump and the oil is seperated from the fuel. in a 2 stroke the oil and fuel is mixed also causeing the diffrences between bearings. in a two stroke the valves are on the side and dont usauly have over head cams. in a 4 stroke there are allways over head cams and some times DOCH wich are double over head cams. also 2 stroke piston heads are curved and shaped so that the fuel can circulate around the top of the head and not straight out the exaust port. on a 4 stroke the pistons are flat although some are ajusted to suit valve clearices.