I first read it when I was somewhere between first and third grade, in the early ‘70’s, and, unfortunately, don’t know what its cover or illustrations looked like, since I read it in braille. I have only the vaguest description, but hope this helps me find its title so I can now read it through adult eyes. It involved several kids who built, I think, a mechanical horse that time-traveled (maybe). Their workshop was someone’s basement—I remember they had to move some “lead weights” periodically, and I remember this only because, at the time, having never encountered the word as pronounced “led”, it took me a while to catch on that they weren’t “leed weights,” but were, indeed, “led weights”. I’m guessing it may have been 150-200 pages in print.