Pat,

The information Bill, Mark, and Dan provided you is correct.  I just replaced the ball valve in my forward head with the Groco part number IBV-1250BSPP.  Because it is a straight thread you will need to use a thread sealant to both seal the threads and to fix the ball valve in a position where you can access and turn the handle.  I used Loctite 592 Thread Sealant which has worked out well so far.
 
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Mark McGovern
SM #440 Cara
Deale, MD USA