[William Miller]

If you find marks on your pistons, another area to suspect is the guides and valve springs. The cam could relieve the pressure on the valve, but if a guide is bad, valve sticking or spring weak, the piston will hit the valve as the valve is trying to close. (At that point in the cycle the piston is chasing the valve. If the valve is too slow or sticks they will make contact.)

I rebuilt my engine last summer. I found marks on 2 pistons, one minor and one notable. Even though I herd no noise I believe there was intermittant contact with the intake valve. This resulted in the top ring on #4 wearing excessivly and ultimately breaking. I went from a great leak down results to having an engine miss. Matt Demaria identified the weak cylinder with his ocilliscope. I confirmed it with the leak down tester.
50% on that cylinder while others were in the upper 90%.

This engine was supposedly rebuilt a few years earlier. I know the PO took care of it. When I tore it apart I found the valve guides were toast.
I ended up buying new valves, guides, springs and rockers. A machine shop in Kensington did a great job on installing the guides and cutting the seats. Very reasonable price and Quality work.

A few observations:
Upon disassembly, I noticed that the valve seats had been replaced prior and that they seemed to be very high in comparison to the dome on the head. The valves protruded from the head more than I would guess would be normal. When the heads returned from the machine shop they looked normal as the seats were machined for the new valves.

Upon dry assembly and doing the valve clearance checks I noticed the valve clearance was a little tight. (Everything still stock including cams.)
This was based on the procedure in Wayne Dempsy's book. I was able to resolve this questionable clearance by timing the cams to the Euro specifications which is slightly retarded from the US SC specifications. (I understand the cam timing on the 84+ carerra is midway between these two.)

This process taught me how close the tollerances are.


A sticking valve and or weak spring at higher rpms could result in the noise you are hearing. Just hasn't caused any noticable dammage yet.
It may be more likely on the intake because of the way the cycle goes and there is less clearance than on the exhaust valves.

I wonder if you could isolate the noise with a stethescope. Certainly the borescope would tell the tale.

If you find a mark in one cylinder. You can hold the valve closed with compressed air and remove that spring. Then that valve guide could be checked by rocking the end of the valve stem. I guess somewhere there is a tool to check the spring tension.

I hope you find something simple. You may be very lucky that you caught it in time. I did sort of. I just had a broken ring. It could have been much worse.

I also have wayne's book if you want to barrow it.