Engine design - is it an 'interference' or 'non-interference'

[Sprocket]

As above really, I can see there are cutouts on the top of the pistons which initially led me to believe the pistons could never touch the valves if you put it together wrong (asking for a friend!). However, the shape of the cutouts isn't valve shaped, it's heart shaped and that makes me think it could be there to help combustion and gas flow.

Anyone know the truth? Can the valves hit the pistons?

[K2-K6]

Yes,  definitely "interference" type engines.
 
You've probably got 2 teeth out either way on camshaft sprocket and they'll be touching the pistons.  Not much room for error and most bikes are the same.

[ka-ja]

You don't need to be two teeth out with a "sports" cam fitted, one tooth out will make the exhaust valves kiss the pistons.

[Sprocket]

I see, thanks all! 

[Trigger]

Even if the valves just kiss the piston this, can result in bent valves. Even if the tappet clearance is too tight they can touch.

[Sprocket]

I'm going to go ahead and guess that as soon as you have a touch, the valve is bent and you'd notice the engine pretty much stop 'engining' fairly rapidly?

I have a tapping noise, once per revolution that sounds metallic.

I had trouble getting the exhaust gaskets in, and I can't be sure they're actually sealed properly (have ordered another set and will clean the area with a dremel to make sure new ones fit properly)

The bike starts and runs and revs absolutely fine. I was a little paranoid so last night checked the clearances again and they're all good. Also dipped a boroscope camera thing in there and couldn't see any evidence of any damage at all - so I can't imagine I'm in a situation where any of the valves has touched and I've not noticed.

[Nurse Julie]

It could be an exhaust gasket leaking, someone else had an exact noise on a 550/4 and all was resolved when the header clamps were tightened. Another thing to check is the rocker arm spring positions. If the springs have been put in the incorrect sides of the rockers, the tappet clips the side of the valve stem top and not full face on and this also gives the noise you are experiencing.

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[Rob62]

Too tight a tappet cleararnce can lead to touching the piston with the valve!..... doesn't leave much range for head and base gasket thickness variation then.. considering that the valve clearance is only 0.002"........

[Nurse Julie]

Also, i wouldn't rule out a bent valve(s). If it were just a minor dink on to the piston, the engine would run with no noticeable problem at this stage but a few seconds of hard acceleration will burn one of the damaged valves and result in a leak.
Edit.....purely out of interest, what pistons have you fitted with the heart shape valve recesses?

[Sprocket]

purely out of interest, what pistons have you fitted with the heart shape valve recesses?

Cruzinimage, exactly the same shape as the originals though - except the word 'Honda' wasn't cast into them!