I have a sabertooth z77 mobo and a cooler master silencio 650 case however the i/o shield does not fit in properly only 2 corners are fully secure. When this happens the bottom bit pops out a little bit and does not let me align my mobo to the motherboard standoffs
The physical size of the shields and openings in the case are standard - so the most likely issue would be that the motherboard isn't aligned properly in the case. When you install a mobo, take these steps:
1) Make sure all standoffs/mounting holes are present (if extra standoffs and/or missing standoffs, fix by adding or removing standoffs).
2) Lay the case where gravity assists you (case on bottom, mobo lies on top - usually with case on it's side.)
3) Insert screws through the mobo into the standoffs - do not tighten all the way, just insert ALL screws.
4) Make sure that the i/o shield is aligned properly.
5) Insert a couple of PCI/PCIe cards (if used) to make sure they are aligned properly.
6) Tighten screws one at a time, using the tire mounting strategy (assuming a clock face over the mobo, start at 12:00, then 6:00, then 3:00, then 9:00, etc...Do not over tighten screws, just a snug fit. You may have to remove PCI/PCIe cards to accomplish this - make sure they align properly after tightening screws.
All mobo mounting holes have a slight "play" in them allowing you to align the mobo to the case. In some instances, you will find that your case may not be perfectly "plumb" (meaning all angles of the case aren't exactly 90 degrees). Sometimes you can correct this problem - but if it is too far out, you might not.