Put on your blindfolds, into the inner and outer world of valve springs we go!
You could squeeze your fist with more pressure.lmao.
A lot more accurate statement than many realize.
And when I say 4K RPM, I mean for an instant, varying. That's just how I've always done it. Our auto shop instructor showed us that and it's always worked for me. I have YET to wipe a lobe. Knock on wood.
But back to the valve sprAng subject....I've also broken in a good many with dual sprAngs and never taken the inner sprAng out. Never worried about it. The key is, I believe, not letting it drop below a certain RPM. "WHATEVER" that may be.
I guess my attitude has always been, if something goes wrong at break in, I want it to happen RIGHT NOW, rather than on the road.