I believe there is an automatic valve lifter which operates on one valve during starting.
Mine has been ok but I have heard others claiming that these can malfunction.
Not sure if this is a correct diagnosis or not but it might be worth checking closely while you've got the engine apart.
Also worth checking that the short oil feed line from the pump hasn't collapsed. Mine looked dodgy so I replaced it when I had the aforementioned oil feed problem.
And yes, I too haven't done case-hardening for a VERY long time.
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