This little tool will let you change out valve springs and valve seals without removing the head. It's a great way to deal with a set of leaky seals, for example. Oil smoke on idle and overrun? Easy fix.
How about a new set of valve springs? Now you can swap those out without pulling the head off as well.
It's designed specifically for the NA/NB Miata and bolts on to the head. The valve is held open by a screw so you have both hands to work.
You will need to prevent the valve from falling out of the head. You can use a leakdown tester to provide air pressure in the cylinder or you can feed rope down the spark plug hole so it piles up on the piston and supports the valve.
Fits all NA and NB Miatas as well as many other twin-cam engines.