Depends on the bike and what it's life is. I'm an old fart, ran Bultaco's in the early 1970's and raced all the way to 2001, also had stints on Suzuki ( early TM's , then late 80's RM's) CZ, Ossa, Maico, KTM's, then Kawasaki's & Honda's. Had some goofy stuff like a year on Cagiva 200's in the 250 class which eventually turned into Husqvarna. ( Long Story ). My Air cooled 125's needed rings and clutches every couple of races. Sometimes would go half a season when I was a novice/armature class racer. I remember the first KTM 420 (1978) went a season without Anything required except tires, chains, sprockets and SPOKES. Motor was rugged. 1977 Maico waisted ignitions and rear wheel hubs. By the mid 1980's the water cooled Honda's would last. 250's most of a season , 125's just cleaning exhaust and rings. Point? Can't make a "blanket" statement. Motocross ... the "practice" and training was where the time was compiled. I ran a 1982 Husqvarna xc430 in AHMRA races in the 99-2000 time frame and it didn't need a thing other than wear items like tires/chains etc..and brake shoes. But the big difference is the design, dirt bikes had evolved with power valves and pipes for wider power bands. They were longer ( square vs. over square ) stroke than chain saw designs and ran at a lower RPM although the piston speed relative to the cylinder wall is roughly the same at peak RPM's. Most dirt bikes aren't run flat out under load like a saw....although I could argue 125 expert class in the late 1970's was flat out continuously for the race time, also were air cooled therefore more like a saw in application. But point is different design parameters, bikes had more weight and space latitude. And I've said for years the 800lbs gorilla in ALL these little motors is the ability to rid the excess heat. THAT more than any other parameter is why saw motors are relatively low horse power designs. They make enough power for the job at a light enough weight, which is ANOTHER thing. The "mass" and surface area of the cases are effectively a part of the cooling system on saws. Start adding horsepower without a balance of cooling systems and u have a impractical for work design. And cooling systems add weight, complexity therefore cost , and size to the package. Take a design where the cooling system is meant to be "stable" at 5hp and turn it into a 20hp motor...it will run. Even at full load, just not long enough to do a typical tree job before heat "limits" the fun.
