cyclocrossworld.com heres the link http://www.cyclocrossworld.com/thorn...hainring-guard

If it says 42 on those then its larger than the tips of the teeth..

But yea, as your Brompton spider crank is machined to just have the chainring on the inside face ..

Its different from a double crankset that you also could install .. there the shoulder for the outer chainring
seats a chainguard disc perfectly ..