I'm puzzled by something that doesn't add up to me. I believe my double hung windows to be relatively standard. The two sashes are 1 3/8" thick with a 1/2" parting stop which adds up to 3 1/4 inches total. This means the distance from the blind stop to the back edge of the window stool is also 3 1/4". Replacement pocket windows seem to be 3 1/4" wide and will fit in this gap nicely. However all of the replacement window channel kits I've seen, and even the replacment sash kits are 3 1/2" wide. They won't fit unless the blind stop is removed or the stool is narrowed, neither of which should be necessary according to the manufacturers.
When I asked the guys at the local big box home supply store about this they said they've sold hundreds and have never heard this question before.
What am I missing?
Thanks, Tom