Mis alignment of the cases is possible..Chinese engines have no corner on bad QC[]....I have an Air Hobbies 4.2 here with the front bearing bore .015 higher in the case than the rear...It couldn't be assembled that way, so someone ground the rear crank by hand so it was smaller than the bearing bore...The person I talked to at AH said it wasn't possible...I'm a machinist, had a granite surface plate and indicator to check it with...It's still here...It could be fixed by putting a sleeve in the rear bore, then boring it to size in the right location...Then the rear crank extension would be too small and have to be ground smaller and a new sleeve pressed on it...Hardly worth the trouble for an overweight 4.2....