When planning for new spring caps why not do the perfect job with split collets with three grooves ?? So the valve can rotate whenever it wants to as the collets don´t lock the stem. There must be hundreds of these stupid wire clips orbiting the moon from trying to fit them. Can´t see why they came up with these, standard split collets were easily avaliable even then and no different or more expensive to do - but a hassle to do or undo the wire clips. I have three groove collets in all engines I do since decades .
Certainly you cannot turn the grooves in the lathe safely with carbide tipped tools, too hard , so I grind them either with air grinder on the tool post, or with hf spindle - easy and slow with cutting oil and diamond file profiled stone, 2.70 mm pitch.
Three weeks ago had to do 36 grooves for the old Ford V 6 , so for a Vincent twin this would be only 12 to do. Type is Mk-8H (TRW) , for 8 mm stems - 5/16 " for those on remote islands - standard since many decades and perfect. They are 1 € each in my country,, so look around at VW, BMW Ford etc. for 8 mm stems, used are allright as well.
Spares Co. valve collets : "Split the pair and remove tang one pair per valve" - - - in 2021 they must be joking . . . . .
Vic
valve collets MK8-H