with the c-clip make sure you get the right thickness and strength
the bov i bought had a normal c-clip fitted and it was useless, leaked very badly.
https://lh5.googleusercontent.com/-qk_7HyUoK_8/UBJwloyogpI/AAAAAAAAACE/_P1dNMVnIds/s512/Clip%20diamater.jpg
0 ring = 4.1cm on the inside (d no2)
c clip = the other is 5.5cm to (d no1) but the ring goes into a groove so would need a c clip that can go in the groove to about 5.8cm,
if you are determined to buy a normal c clip then make sure you do a leak test
i know the oring size because of changing mine but i dont want to take the c clip off to measure it because its a pain to get back on