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Do your tailstock up tighter, that will stop it slipping make sure there is no dirt under the tailstock or a bit of paint. Also the "plate" that runs below the bed may have a burr. Also the nut under the plate may want tightening as you could be running out of movement on the levers camBack to the cross head issue , when making an engine I often turn a small point onto the piston rod and this can be used to mark the correct location for the hole in the crosshead
First side is looking good. Not that I’m really qualified, but, put on the optic visor and get eyeballs to eyeball with the other side and watch it with the patience of a world class chess player as you slowly turn it over by hand. With enough patience, I’d bet an RC Cola and a Moon Pie that you will see where it’s actually binding. Remember Caddy Shack? Be the engine Cletus
I find that over time, the nut on the bottom of the clamping plate will loosen and I have to tighten it up to get a good lock with the cam