Crystals and case sizes for fit

[Alastair]

If a 60's/70's case has say, a 30mm aperture for a plain acrylic non tension-band crystal, what size must the crystal be for good fit...?

Or is it a case of, a 30mm crystal is for a 30mm case aperture and the tight interference fit is taken as part of manufacture...?

[clockboy]

Not an exact science I have found. I order (if not in my stock) A few sizes up from the actual size. Sometimes just one size up works great & sometimes although it fits it is not tight so I then move up to the next size. Thats my method but probably wrong but it works for me. Crystals tend to increase by 0.1mm increments. 

[StuartBaker104]

Usually I would go up .2mm, so in this case would get a 302. As Clockboy says, not an exact science and quite difficult to measure accurately. From Cousins, the postage is more than getting an extra crystal or 2 in different sizes.