OEM Half Rings - $5.00 or $6.00 each
Above left - 4v and newer 2V half rings
Above right - older 2v half rings
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These OEM half rings are used to secure the closing shim to the valve stem. These sometimes break, necessitating replacement. Valvetrains with excessive valve lash contribute to broken half-rings. I've performed valve adjusts where I had to replace EVERY half-ring. While that event was a rare one, it isn't uncommon to replace a half-ring or two on any given valve adjustment. I do not stock nor use aftermarket half-rings. Aftermarket half-rings are a pain in the ass to install and remove and often require custom opener shims. I have found that once a shimstack beds in, shim replacements are rare.
To determine which style you use, I offer the following guide:
All 2 valve motors EXCEPT the 695, 696, 796, 1000 and 1100 - use the older 2V half-ring.
All 3v and 4v motors (except 851*) + the 695, 696, 796, 1000 and 1100 2-valvers - use the 4v half ring.
* The 851 used a third varient of half-ring that I do not stock. Source them from any Ducati dealer.