Generally water cooled motors will go way longer than air cooled. Now ALWAYS though. I swear the best motor in the world is them Honda horizontal 50 cc engines! They's air cooled, but last longer than you want them too! Or Goldwing engines! Those things go for ever and ever! I's read about them with some over 500,000 miles, no trouble, just routine maintainence, still running strong! Most of those have transmissions or other things go before the motors. I believe Elite 150 cc scootes have no trouble going 100K before a rebuild, probably more if you take good care of them. Helix and Elite 250 as well. Water coolers generally last longer, because heat isn't an issue most of the time. H-Ds will only last about 100K miles before they need a top end rebuild. Not that good compared to a Water Cooled Honda Shadow, those will easily exceed 300K. Again this is all with proper maintainence. Although I've herd Yamaha Roadstars [also air cooled] doing 260K+ without any work other than oil and regular maintainence. H-Ds probably can last longer, if you don't ride hard don't know. Just generally not always, a water cooled engine, kept stock [no more than maybe pipes, NOT straight pipes though], that doesn't run tooo high of RPMs/WFO all the time, with good maintainence and oil changes with quality synthetic oil, will last a long time.