Dont forget the gearing. The WR is setup for trails. The yzf is setup for mx. Not only do 4 strokes have more moving parts, they HAVE more parts then 2 strokes.
The WR has a lower 1st gear so that the bike will climb steep hills at low speeds and putter along trails , and a taller 5th to allow it to cruise roads and highways without screaming its' guts out and going too slow . The ratios in between are also spread further apart .
The YZF has a taller 1st gear and all the gears are closer together to keep the engine in the best power range all the time for the quickest possible acceleration during racing conditions .
4 strokes have more moving parts - but they are lubricated by thick oil so they don't wear very quickly whereas the 2 stroke is lubricated by a thin petrol/oil mix and they wear pretty quickly in comparison .
Drain the oil from your pit bike and fill it with 2 stroke mix then take it for a thrash and we'll see how long it lasts - then you'll get the drift .
The main reason for the maintenance on four strokes is purely because they took the lead out of fuel which is what valves , upper bores and top rings rely on for lubrication . The lead virtually eliminates valve wear and valve seat recession . IF you ran your engine on leaded fuel - you'd rarely have to adjust valve clearances . Formula 1 cars run ELF high octane fuel - ELF stands for Extra Leaded Fuel in case anyone didn't know . They run the leaded fuel because they wouldn't finish a single race running on unleaded .
Anyone who isn't running a lead replacement such as tin , valve saver or castor oil in the fuel on their four stroke is a fool . Lead also reduces detonation - the main cause of four stroke blow ups and engine damage .
Car and road bike engines are four stroke and they turn over billions of revs for years over thousands of miles without needing constant rebuilds . The YZF cylinder is simply a cylinder off a YZF 4 cylinder road bike engine so WHY would they wear out quicker in a dirt bike when they can be revved for miles and miles , hour upon hour , month in , month out for years in road use ?????
What dork brains have to realise is that you can't run a water cooled engine revving its ass off at low speeds around a track - radiators require high AIRFLOW thru them to cool the engine .
Most dirt engines that have had problems will be getting damaged by detonation from over heating from being ridden at too low a speed . The only other thing that could cause damage and wear is lack of regular oil changes and dirty air filters .
Excess heat from continual slow riding , dirt and mud blocking the radiators would cause the oil to go bad faster . Water cooled street engines have thermo fans which kick in when the vehicle is going too slow for the air speed to provide adequate cooling . So WATER cooled dirt bikes without thermo fans should purely and simply NOT be ridden slow - doing donuts etc . Water cooled 4 stroke MX bikes are designed to be ridden fast and they need to be kept moving at a decent speed to stay cool . Flog one around your back yard or small local track for any decent length of time and you'll blow it up in next to no time at all . Air cooled bikes on the other hand can be ridden at slow speeds in filthy heat ALL day long - your local posty bike is concrete proof of that - and pit bike engines which are ridden harder have increased cylinder fin sizes and can be fitted with oil coolers to get rid of the extra heat . I'd just LOVE to see someone try to deliver letters all day in the middle of summer on ANY water cooled bike ~ without having electric fans blasting away at the radiators .
2 strokes get away with it because they pull their intake air thru the crank case and that's similar to being fan cooled ! IF you ride slow - stick to air cooled 4 strokes like XR's - they last for donkeys years with the minimum of maintenance .