I just tried it on a different PC, and it works as I described. I even tried to rotate the piston, but it doesn't matter if it's horizontal or vertical. I am, however, running linux; you're probably running Windows?
About that shredder.
It turns out the problem was not a glitch of such, more an update that prevents PSTN pushing objects into any WALL apart from ABSORB WALL. The save is fixed now, but keep in mind the cumulative effect of changing elements.
About 4 of my saves have being broken in updates, and one of those saves was distributed to several others and a giveaway homepage, which are subsequently broken now and the users have left. (unable to fix them)
Sorry I didn't mean to take it out on you.
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Also it wasn't the PSTN fix that broke most of them. FIRW to EMBR was a bigger culprit.
edit: Although after three years of playing and only 4 saves broken out of 70 (and those only in need of minor fixing) I'd say thats a pretty good ratio.
When the default firework explosion was changed to an EMBR explosion all existing saves that had fireworks already in movement lost all velocity.
At the time this broke my first fp "The golden skull", and my fireworks display which I have since fixed. The original fireworks display was actually given to someone to create a front page for them, but when the original is loaded, the fireworks already in the air fall catastrophically and melt the electronics in the save.
That same fireworks display was given to several people actualy, as I alternated between it and a vector art homepage when people asked for one.
The third save that was "damaged" with the update was a firework bomb (that was one of my first saves) that now can't even melt qrtz as it's not hot enough. My other bomb "Awesome explosive" had to be scaped and rebuilt to keep the same amount of heat, and no longer looks as pretty :(