Valves touching pistons

[Seabeowner]

Took my 500k1 apart to replace the rattly primary chain and all was going well. Didn't even look at the head when I took it apart. It's been running ok but could probably do with new rings (original bore looks good) But horrified when I turned over the head and spotted the marks on the inlet valves. This bike had the top end rebuilt about 14 years ago, mainly to smarten up the appearance and since then has done about 4000 miles but ran fine and never revved near the red line. I have another old cam and the lift is the same.
Inspiration needed: Sticking valves : thin head gasket  :  Skimmed in the past?  :
Picture show 3 & 4. 1 & 2 are better but there is a witness in the carbon.

[Trigger]

Tappets toooo tight, most probably burnt the valve face and seats .

[Ashdowner]

Have you disturbed the cam chain?

[Bryanj]

cam timing 1 tooth out?

[AshimotoK0]

Check that pistons are marked  323 on the side of the piston, not 330 (ie xxxxx -323-xxx  not  xxxxx-330-xxx parts). 330 pistons are sold as fitting the CB500K1 but the cutouts in the top of the pistons are slightly less as the 500F pistons and may require slight modification. Otherwise, dimensionally, they are identical

http://www.ebay.co.uk/itm/GENUINE-HONDA-CB125S-CL125-SL125-CB500-FOUR-PISTON-ONLY-NOS-13101-330-000-/122381232934?hash=item1c7e7d6b26:g:bl4AAOSwgY9XfSOv

[Seabeowner]

Thanks for the replies. The pistons are all 323s. Have measured the block from bottom surface to top and the head from bottom surface to where the cam cover sits and compared them to another old engine I have and they measure the about same.
A compression check before taking apart gave 150-160psi. Not brilliant but thought that I would mostly leave the top alone for this year.
Engine was very quiet, so tappet clearances could be tight, but checked them by the book 6 months ago. Can see that a tight tappet will also change the timing slightly as it follows the cam.
I'd like to think that it was cam timing, but would an engine run reasonably well with the cam chain a tooth out?

[Ashdowner]

But it's an easy job to take the rocker cover off and check the camshaft timing again so why don't you do that to make sure and eliminate it as a problem?

[Ashdowner]

I have a mark on 1 of my pistons from the inlet valve having hit the piston, and the head of that cylinder shows past major trauma. I don't know what it was because someone rebuilt it and all is now well.