Mmmkaaay............ sales@greenhouse-vent-opener.com

I have 2 univent style openers - they are at least 8 years old and I have had no problems. I use a counter-weight to balance the load on teh windows.
I did order an extra 'power tube' to try and make a specialized opener - they run about $20. I don't see how they could get cross-threaded unless the nuts where the wrong size. If you have access to a die kit you could cut the threads back onto the rod.
If you post a picture it would be helpful.

http://www.charleysgreenhouse.com/3513-Replacement-Cylinder-for-3515-3527-3533--C3513-.htm

Thanks chadinig for chiming in! My friend turned the stuck housing cylinder off the cylinder on his metal lathe then chased the threads on the cylinder. I know nothing about machine work, but my friend said it was cross threaded. I 've finally located the Denmark manufacturer's website (see above link) & ordered a couple extra cylinder housings......... As I posted earlier, they seem to work fine initially, then suddenly they're stuck.........