The RC51 comes from a long line of Honda race engines that used spur gears to drive the cams.
They have spring loaded split-gears to eliminate the gear lash, but they still have that wonderful gear whine that says "I was a purpose built race engine detuned for the street".
I love that whine. It's the most accurate and reliable way to time cams.
Could be just the cams actuating the tappets.
How loud is it? I mean I can hear my valves ticking at idle, but they're not what I'd call loud.
Too be sure, have the valve clearances checked.
The engines are known to be very good at keeping clearances in spec.
My buddy raced his CBR600 F3 the whole season and was at 14,000 RPM all the time.
After season ended, his valves weren't out of spec one bit.
My engine only had one exhaust valve slightly out of spec after 24,000 miles.
Be careful and not over tighten those nuts and shear the stud.
That would ruin your day.
It's good practice to use a high-temperature anti-sieze compund on the threads and torque them down.
When I was removing mine I had no idea how rusty they may have been from the previous owner, so I was almost cringing when I started to loosen them.
Fortunately they loosened right up and I breathed a sigh of relief.
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