Ask anybody who has professionally raced Sportster motors which i haven't especially on dirt and they will tell you the bottom end is bullet proof. The Sportster is the most proven , most durable motor Harley Davidson makes be it in 883 or 1200 versions. I sometimes look for a 6th but i don't need it. Like some have said the Sportster motor is designed to be ridden hard. Unfortunately most Sportster riders lug there motors in town and i'm guilty. 35mph in 3rd is borderline lugging your bike. I don't have a tach but i know i've shifted out of 3rd into 4th at around 60 not on the rev limiter. 70 in 5th is purring not even pushing the motor. Potato, potato is cool and all at idle but if your puttering around at low rpms your doing your motor a disservice . Wind that sucker out!