I used 403's because I had them and am using Robart #662 struts - they are 7/16" diameter. If I were buying a set I would go with Spring Air 700's they are bored for 7/16" struts I believe. The 403's are larger than needed and require the use of the 1/2" adapter that comes with the Robart's. OOPS - I didn't see you were referring to the 50 size - then I would go with the 100 series retracts and struts that adapt to 5/32 or 3/16" wire gear - you can get the 100's in either size.