I've been struggling for ages with a cheap (and quite nasty) Fox hollow chisel morticer.
The problem is that there is significant play between the main casting and its guides which translates to about 1.5 or even 2mm at the tip of the tool. Naturally, the weight of the head tips it forward, resulting in a backward movement of the chisel, then when you start to cut, the reverse happens, the cutting pressure pushes the casting back the other way - the chisel tries to move forward - result sloppy mortices off line and not truly parallel to the sides of the work piece. The plan was to replace it, but a visiting friend suggested drilling and tapping the casting, then fitting four brass bolts to take up the slack.
Today I did just that and it works beautifully. Of course the brass bolts will wear quite fast, but I'm a hobby woodworker, so no problem to tweak it once a month or so... Thanks Geoff !
I could not find hex head brass screws locally, so made do with csk ones, but I think I'll cut them off short and cut slots like grub screws just for tidyness.