I’ve found out through bitter experience that a lot of people on here don’t realise that all 104BCD Shimano middle chainrings are not the same! Although the bolt holes are in the same place, Shimano have been playing about with the length of the arms over the last couple of years.
Starting at the beginning – the M542 chainset is the same as the Deore M590 but with different surface finish. So, the M590 rings will fit perfectly, as will any of the other recent ones like M532, M572. The M510 will also fit, but it has shorter arms, as you rightly noticed, so there will be a small gap between the arms of the ring and the lugs on the crank arms they are supposed to sit on. The few quid saving by buying the M510 versus say the M532 is not worth it not properly fitting in my view.
But, the ones to really watch out for are the new ‘composite’ middle rings (like come on the 10 speed deore’s or the 9 & 10 speed SLX and XT chainsets), which have longer arms on the middle ring, so won’t fit your M542 without either filing the ring arms shorter, or filing the lugs off the crank arms.
However, I happen to really like the composite middle rings, as I find they give silky smoooooth shifting, with the durability of steel. So, I chose to file off the crank arm lugs on my M542 so I could fit an SLX M660-9 composite 32T ring. Works fine for bike path & commuting, although I probably wouldn’t do it on a true off roader.
Which is all an exceedingly long way round of saying – you’ve done the right thing by ordering the M532!
