Much has been said about valve regulation and what piece of measuring
equipment should be used.  To get a fair measurement of the upwards
travel you need to ensure that any resistance imposed by your measuring
tool is kept to a minimum.  The force exerted by the valve onto the
upper seat will be diminished significantly if your measuring tool is
exerting too high a downwards force.  You will therefore record
a smaller travel than would occur in practice.

People have used all sorts of words to describe a Dial Test Indicator,
or DTI for short.  This is a sprung feeler connected to a highly geared
pointer on a circular scale.  It is not a micrometer or caliper or any
such variation thereof.  A DTI is a bit over the top for such a job.
We are not looking for such levels of accuracy.

I made a suitable tool in less than half an hour.  It is simply a
pivoted length of thin wood with a pointer and scale at one end and
a vertical rod pivoted downwards at a point 5% along the length.  This
rod goes through the hole in the valve top seat and sits on the valve
itself.  The pointer then gives me a twenty times measurement of the
valve travel.  Any resistance to valve movement is kept to an absolute
minimum due to the low weight of the components involved and the fact
that no springs are used.  It is easy to use and can be moved from
valve to valve in seconds.

Half an hour and a few pieces of scrap, to do a reliable and repeatable
job.

Yours practically,
Nicholas Simons, GB