I'd love to help but my pump is buried under 101 other things in the spare room at the moment as I'm redecorating my bedroom..

Why do you want to know? I can tell you that it's a two stage pump (compresses air on both upward and downward strokes) so if you want to work out the internal volume it's not as simple as just working with the ID of the outer cylinder.

If you have had no luck by this time next week (when everything should be normal in my house again) I'll have it to bits and have a quick look for you