Originally Posted by
Coxsteve
Everything needs to be accurate as crank alignment when assembled is adjusted by the rather rough sounding methods of squeezing it it a Vice hitting it with a soft copper mallet to twist the halves in relation to one another or spreading the halves with a large radius cold chisel, The aim being to make sure the main bearing journal sections of the crank halves are as near as perfect aligned in all planes.
All this whilst still making sure the side clearance for the con rod thrust washers stays in spec not quite as easy as it sounds.
A jig to aid alignment when initial pressing together helps but experience a slow careful approach aided by some straight edges such as a machinist square on at least two points on both crank halves at the same time will keep things within a margin of error where the above standard practice with the copper mallet etc will suffice in final alignment.
Some engines such as Kawasaki two strokes use a satin finish on the crank pins where they fit into the crank halves, With these get it as right as possible before final pressing as they are a ***** to fix afterwards if it is out too far as the crank halves do not like to rotate on this satin finish.
If a multi cylinder like Yamaha TZ350 twins race model cranks it really is worth the effort to make a good jig to aid assembly, As for triples definitely make a jig it really does make it not just easy but actually possible to do the job in a reasonable timeframe