Well, i overhauled my 638 engine and it has now 38.000 trouble free km's on it. I've been told the 638 were made in very poor quality for the 'home'market and better for the export market. Though also over here the 634 engien is said to be manufacured with more care, but has iron cylinders which would make me afraid for piston jams in hot wheather and f.i. mountains or traffic jam.
also a big difference is that the 638 engine has 2 bearings in the middle of the crankshaft, which are prone to go first, but the crankhaft hbs are of VERY soft material.. maybe hence the choice for a double bearing..
As for my 190cm, the 638 is a little higher and better seat..