Quoting John Rodgers <inua@CHARTER.NET>:

> Just reading the post about "New Engine Conversion" - and I wonder -
> besides cost, is there any reason why engines have not had meshed
> gears
> to drive the cams. Piston type aircraft engines all have gear driven
> cams. No body wants to try to pull over and park on a cloud when a
> timing chain fails.

The only engines I know of with gear-driven DOHC are Honda's VFR and NR
bikes. reason? Expense, nothing else... excepting perhaps mass.