Tom Lear wrote, "No doubt one of you guys will know where I might
obtain springs of a similar size but with a higher compressive
resistance than standard jack springs."

Consult the Organ Supply Industries catalog.  They have conical pouch
springs that come in at least three strengths.  These are the ones that
are found under the pouches in an electro-pneumatic pipe chest.  The
variety of strengths are for the variety of wind pressures.  They return
the pouch to the upright and valve-closed position.

One end is large enough to press against the outside of the solenoid
case or frame and the other end has a small hole in it for the push rod
to go through, depending on whether you are using the solenoids in an
upright or upside down position.

D.L. Bullock
St. Louis
www.thepianoworld.com