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I didn’t say removing scratches from a tappet in situ would be easy, Kaspar…
But a picture of the delinquent may clarify if any work is necessary? As for using an engine cleaner; it may be a bit excessive? A change of oil and filter - then observing flow with the cam covers removed as suggested by David?
Frank
xj6 85 Sov Europe (UK/NZ)
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You didn’t: Ill say it’s functionally impossible.
Yes, sounds impossible to do that in situ.
On my Yamaha XS project I had a shim that was 0,3mm too thick and decided to sand it down, couldn’t wait to order a new one, it took forever… they are bloody hard.
If a tappet is scratched you could maybe put it upside down?
The bottom is just pushing on the valve so it doesn’t matter if it’s scratched.
It is not the shim, it’s the tappet (bucket) that is scratched. A few little scratches don’t hurt, polishing I don’t know, they are hardened, but I don’t know about getting new because they are a very close fit.
You can certainly polish them…just use the proper buffing wheel on your grinder/polisher with the appropriate rouge…I used the “white” rouge which is recommended for hard metals like SS. But first, I surface-plate sanded mine to expose any wear divots using 400, then I moved on to finer papers. You may expose a circular divot where the cam contacts the bucket…much easier to evaluate any particular bucket once you have a shiny surface.
That works, I think I was still thinking of the tappet in the head.
I‘ve sanded down valve shims and no ill effects so far, they are hardened too, and took long enough for any effect. Doesn’t have to be perfect.
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The important bit, when adjusting valve clearances, David, is to measure the shims with a micrometer. To absolutely verify their thickness - with or without shim thickness adjustment…
It’s difficult enough to make one-shot adjustment even with measured shims…
Frank
xj6 85 Sov Europe (UK/NZ)
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Don‘t tell me. I‘m very lucky to have access to a good selection of shims, but if you need five of the same or an odd one isn’t available, tough luck. On used shims the letter eventually disappears so the micrometer is always in use.
Still no guarantee of course. 
As you say, David - even with the most careful attention to details; rechecking valve clearances after adjustments is a moment of apprehension…
Frank
xj6 85 Sov Europe (UK/NZ)