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Have found all intake valves on my Yamaha supercharged engine to be full of little holes. A friend told me that would be usual on this engine.
What is the cause of it?
Most production valves are formed by hot forging, so fine surface irregularities aren't uncommon - https://www.youtube.com/watch?v=dMt1tqDI1nM
Most aftermarket and race valves use a similar process, but may be machined on the face and rear of the head as well, so would be expected to be a smoother finish.
Both may have a harder material welded to them to form a harder wearing seat when machined.
I was told New valves have a totally smooth surface it happens when running to become like that.
See post above - some are, some aren't. To be fair to whoever told you that, to the eye a new valve may seem to be totally smooth, as the irregularities may only be made clear when they fill with darker carbon/deposits and the higher surface around the pitting is polished when cleaning, so there's a high contrast. That you have taken a high resolution picture, and blown it up to many times the original size, just makes it more evident.
Your valves were 99.99% probability stamp forged. I say that because the code on the face is raised and that's impossible when machined but normal if stamp forged- if it were machined I would expect it to be stamped into the face or, more likely, engraved somewhere on the stem.
Thank you. That will help me for further decision.
Something else I should have mentioned is that there are also one and two piece valve designs with the two parts friction welded, this was done because, I understand, because it was to use the best seat material with the best stem material and/or it was cheaper overall costing out the materials.
Some performance valves are made in a similar way, all sodium filled are and some titanium and stainless have hardened tips friction welded to them - these are above the collet grooves, so don't compromise the overall strength of the valve.