Hi Janey,
I agree with you - and if I tweaked the tension a bit either way I'd get the "unsquare" problem happening on the other side - where I ended up was about as close to perfect as I could get it.
But - I have an update that means Hooligan and Watson were kinda on to something just the same. I got a replacement shuttle from (a proper Singer original vintage one for this model) and the machine instantly ran more quietly. I thought this was a slightly noisy machine - it turns out (it seems) that it was a "noisy shuttle"! Well - blow me down!
So I set the tensions the normal way I do - just like I had on the other shuttle. It seems to have solved the problem. I stitched 6 perfect corners in a row. Three turning clockwise and three anticlockwise. I'm not quite sure what the difference is. I'd checked the shuttle clearances on the other shuttle with a feeler gauge but perhaps there's just a tiny little difference beyond that which is making this shuttle both quieter and able to work with the rest of the machine to turn better corners.
It will take wiser heads than mine to know if that might have anything to do with it but thought I'd report it right away (this news is only a few minutes old as I write it).
I'm very keen to hear anyone's ideas on what might have been (or was in fact) the "secret sauce" from changing to another shuttle.
Ta heaps... Megan