Thanks for the informed responses skypig and Nathan S. Does the same logic apply to enduro bikes as well as motocrossers? For example WR400F and WR426F, and CRF250X etc etc? Judging by how cheaply some of them can be had, I can only assume it does...
Tex
The recently superseded Yamaha 250F motor is an interesting example on this one... Same basic motor in the WRF & YZF (although the WRF kept using it for longer).
When you cut through the guff, the YZF doesn't last well. Cranks are short lived, clutches are hopeless, pistons and valves aren't great (but nor are they the worst).
Meanwhile, the WRF has a deserved reputation for lasting much longer than most 250Fs - the difference is in the usage: the YZF is way more likely to have the shit revved out of it, and that brings the lifespan way down. And an anal-rententive maintenance schedule for air filter & oil/oil filter doesn't do much to improve that.
So it really comes down to how they're used - and the same goes for cranks in CRF250X vs R.
I've also heard a knowledgable bloke aim that he's seen Honda heads with the valve seat cut 0.6mm off centre to the valve guide (not a typo!). I don't even know how you could do that if you tried, but if you wonder why the valves last so little time on some Hondas, despite the owners giving them all the care in the world....
Final thought while I wait for lunch time:
The other thing a that a modern 4T will generally run perfectly well until the moment it sprays it's innards all down the track... Because a lot of the blow ups are caused by fatigue failures, rather no than simple old "wearing out".
A typical 2T (or TT/XR) that wears out, gives you plenty of warning: low compression on the kickstarter; hard to start; rattly; vibey; blows more smoke; etc. and you can usually get away with ignoring those symptoms for ages before they bite you.
