I was scared to go any tighter... last thing I wanted was to bend a pushrod when the engine got hot!
Just for grins, try this- - - -with the piston at top dead center on the "overlap" stroke (exhaust closing and intake opening) use a lever bar or a big screwdriver to push the intake valve down until it touches the top of the piston. I think you'll be surprised at how much safety margin is built into the design. It's virtually impossible for an exhaust valve to touch a piston, unless a valve spring disentigrares or the split keepers come off and let the valve fall into the cylinder. If a timing gear strips a few teeth, it might be possible for an intake valve and a piston to get together. I don't think adjusting a valve clearance too tight could do any damage without creating a dead miss of the cylinder by holding a valve slightly open all the time.
Jerry
