I know that the earlier 1600 xflows had 681 on the block and rounded main bearing caps and The 711 is the best one to tune due to the thickness of the block. However, just to be awkward mine is a 691. It's a 1600 and has square main bearing caps. Does anyone know how far I can go with a rebore. It's blowing smoke out of the rocker box filler and I've gone through every piece of advice given (including the breather system) and it all points to worn rings (even though the compression test on all four cylinders is good and within 10% of each other)!! No point in just changing the pistons without a rebore. Any suggestions?

Could be stuck rings or knackered valve stem oil seals.
They'll all bore +0.060 Thou (1.5 mm) ok but the 681f doesn't like extreme states of tune. If you're 691m hasn't blown up in its current state then going to the next size won't be a problem, though anything above +0.060 should only be considered on 711m as this block was designed as an updated block for competition used that ended up becoming the production standard

Is that correct?
I would have said left to right was 1.6, 1.3 and 1.1 pistons.