This is a handy tool that most any enthusiast can make. It allows the valve train geometry to be set so the correct length pushrods can be ordered for a specific engine combination.
Start with a pushrod that is the correct type and nominal length for the engine and valve train that is being used. A used pushrod is fine as long as it's straight and the ends are smooth. Cut the pushrod in two and remove about 1/4" of material from one of the cut pieces. The missing length will be made up by the threaded adjuster segment that will be added later.
Thread the ID of both cut ends of the pushrod to receive a length of threaded stock that will give an overall length of plus 0.200" to minus 0.200" of the stock length (the exact range of adjustment is up to the builder, the figures given are arbitrary). Thread the pieces and size the threaded stock so at least 1.5 times the OD of the threaded stock is engaged into the cut ends of the pushrod. This will give the strength needed to resist bending under valve spring pressure when the engine is turned over during the checking process.
Using a fine pitch thread is a good idea to give a finer adjustment, but not an absolute necessity. The main things are to keep the threads parallel to the pushrod and to make them on the tight side so there's no wobble. One end can have loctite or JB Weld applied to reduce any chance of excess movement.
The adjustable pushrod is now ready to be used for setting the valve train geometry.