steam radiator: knock apart

Twenty years ago (wow!) I shortened a steam radiator by taking out a section. I have to do it again for other radiators. I have taken out the connecting rods but I can't get anywhere. Could it be a solid radiator (4 feet wide, three feet high and 6 inches deep)? Is there more for me to take off before the sections separate? Or am I just 20 years weaker? What's the problem? What do I do?
Thanks, Frank
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Maybe the sections are joined with screw nipples instead of push nipples?

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MARK,
Thanks for help,
Could you describe how I would know the difference? What is a screw nipple or a push nipple.
Thanks, Frank

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Frank,
I think they're just being stubborn. Use more muscle.
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