Hi Kevin,
Some years ago, I refilled the front dampers on my Dragonfly, but like you, I didn't have any idea of volume or grade of oil to use. Just by trial and error, I used 10 grade fork oil, but overfilled the units on the first attempt. The 10 grade oil gives about the same damping rate as was available with new mid 50's units (remember, the damping in those old units was never as good as modern units - and in my opinion, gave a better ride!). The only suggestion I can make as to volume of oil is to part fill the damper body - insert the damper unit and pump it to eject all the air - then top up the oil until it again half fills the body - then screw on the top and pump the damper to make sure there is no more trapped air in the actual damper, but there needs to be some air in the outer reservoir to provide a cushion and prevent a hydraulic lock as the unit is compressed. Don't expect to get super damping - those old Girlings were ideally suited to the Dragonfly and gave a super smooth ride - I once heard it described as being 'like low level flying!'
Regards,
Eddie.
P.S. I used 10 grade fork oil, but any equivalent grade of hydraulic oil should be OK - don't try to get improved damping by using an engine oil as that may 'froth up' in use, giving progressively worsening damping.
