I just went through this, in fact my motor is still at the machinist. I got into my motor and because of the cam going so quickly (about 150 miles) it ate the heck out of all the bearings. It was only one lobe even. I couldn't imagine if more than one lobe went bad what it would have looked like. I even had to have my crank turned. I was using a comp XE268H. I went to the XE284H this time and the machinist told me to run it for a whole hour at 2000-2500 rpm. This is the first time I have heard a whole hour for the break in. Thats gonna be fun. I will be using old weak springs to break it in. He told me to use compressed air through the spark plug hole at 120psi to hold the valves in while I change back to the good springs. Good luck with the change and I hope your's comes out with less damage than mine did.
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Semper Fidelis
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