Several years ago, a pilot who had relocated from Florida to Moriarty with an ASW-20C had me install an O2 system. By enlarging the fuselage bulkhead hole down by the pilot's right elbow, I was able to fit a Mountain High AL-682 cylinder. This is the largest capacity cylinder available for use in a -20. It is also the one I use in my Pegasus, which has a nearly identical cockpit. The diameter of the AL-682 (and the shorter AL-415) is 4.4 inches. It fits in the existing clamp, but the padding must be removed. If you do not have a clamp, one must be fabricated or possibly ordered from Schleicher.. Call or email me if you need more info. You have my number and email address.