Ive just been to check. Im very reluctant to stick my digits between the cocked barrel and the cylinder single handed but a rough guide is that a 3.5mm Allen key will fit equally into the .177 and .22 Vixen t p so I would guess approx 4mm each.
As for the leather piston sealed '35 (.22). Again 'digit preservation' foremost a 2.5mm allen key would fit but not a 3mm so I would say 3mm.

Sorry its so vague but the next time I have them apart I will measure them accurately.

HTH
Ian