hi ive reached the point where i can start to rebuild my 50 cc engine. Im aware of using heat and cold to fit the bearings into the engine housing for the crankshaft but what order is best? Should i fit the bearings to the crankshaft first by chilling the shaft and heating the bearings and then chill the whole unit for fitting into the engine or should i fit chilled bearings into place in the engine halves then fit the crankshaft afterwards? I know both methods work but is one better than the other? So bearings onto shaft then into engine or bearings into engine then fit crank?
I personally fit bearings to cases first then crank, also have a pile of washers/spacers to help pull the crank in if you don't get it in fast enough. The spacers/washers need to be slim enough to sit on the inner bearing face and use the nuts to pull it in just snug, then fit seals!
Usually we would throw the bearings in a sealed up baggy and into the freezer for 12hrs+ get them super cold. then you would heat up the crank case halfs , one half first, grab a cold bearing and seat it into the case, sometimes you will find the bearing just slots right in as the expanded case and the freezing bearings are just right to get in, if not its a tap home with a rubber mallet and a suitable tool ( so you do not damage the bearing itself) Then its the same process as the first half , give it a good heat up whip bearing from freezer and bag and drop it into case, again use the rubber mallet to seat it home if need be. then your add a little bit of gease to the crank rod bearing seats. and slip it home into one side of case, marry up the second case side to the crank and slip it home. if it dont seat fully down some then fit the crank case bolts and tighten them down little by little in a staggered pattern ( dont forget to liquid gasket seal the case half before fitting so its a good solid seal ). Thats one method, im sure the other guys may have other methods they will be willing to pass on
