> Way back in high school, I got a great deal on an XK 120
> Jaguar..Straight six with chain driven dual overhead cams..The rich kid
> who's mechanic told him the engine was shot didn't check the cam chain,
> which had a broken tensioner..about $50 bucks as I recall..that allowed
> the exhaust cam to skip one tooth over and made the motor not run real
> well.

Ever seen what a broken camchain does to an engine? Ask a Kawasaki Z1300
owner...

>   Ducatti had a desmodromic valve train..One tower shaft driven gear
> between the cylinders with some kinda linkage to both open and close
> each valve...no springs..

Yep. Imagine what happens ig the open & close cam settings interfere with
one another... no wonder Ducatis are normally buggered before the warranty
period runs out (a local used-bike importer refiuses to touch 'em for this
reason). I wonder why Ducati dropped desmos...

>   And the two stroke motorcycles...no valves at all, just ports that
> are opened and closed as the pistons go up and down

The old 60s-70s Honda CB450 Twin used torsionbar valvesprings for its DOHC
head. Almost zero camfollower clearance and no valvebounce at 14000+rpm.