G39B Valve Guides
The head is round to allow you to fit them properly. Screw into place in the valve, mark where they are to be cut, and remove. Install them in the G39 Valve Guide Fitting Tool and hand file or power grind to slightly oversize. Install on the valve and hand file to the exact size. Thread is 3-48 which fits most valves. For smaller valve guide screw holes, use a D220W #47 and a D200, 3-48 tap to enlarge the hole.
Available in dozens or hundreds.