Does anyone know the name of the part that fits on top of the spring and keeps the rocker arm from falling off? It's shaped liked a mini dime with two flanges on the outsides that act as guides.

I think that is called a valve adjust shim. They come in different thicknesses to compensate when your adjusters are bottomed out in the head and you can't loosen the valves anymore. Happens as the seats get worn. They are on fastlane but don't have a picture so I don't know.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6

My Series 114-115 service manual simply calls it a "pressure piece". It further states that a thinner one can be substituted in the event that the correct valve clearance can't be obtained with the normal adjustment.

Happy Motoring, Mark

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DrDKW