Drine is on the right track here, but I guess I would suggest double checking the length of your bolt head and the overall length of your carrier. How much cocking tube gap do you have with the standard 8mm rollers in? Could be that you had zero gap and a ground bolt head, with a shortened carrier to get CT gap on the ground head. Best to get measurements of everything so that you can get the big picture of what is going on. Especially on a Century gun.
See if you can get your hands on a pair of calipers to measure the bolt head, bolt windows and CT gap, You can use a ruler to get the overall length of the carrier. Get the following measurements and report back....
Bolt head length
CT gap on the standard 8mm rollers
Bolt head window width