No. 6 Cylinder Leak Down Problem
 

1976 930 newly rebuilt (less than 100 mi).
While chasing a terrible noise problem....
[see this thread: http://forums.pelicanparts.com/911-engine-rebuilding-forum/1043749-whats-noise-newly-rebuilt-930-engine-2.html (spoiler alert: flywheel bolts loose)]

....I performed a leak-down test of the engine. All cylinders were between 0 and 3% except No. 6 which was 16%. It was isolated to the exhaust valve. Removed the rocker, tapped the valve/spring, retested-same result.

Pulled the heads here's a couple of photos:
http://forums.pelicanparts.com/uploa...1572711473.JPG
http://forums.pelicanparts.com/uploa...1572711473.JPG
Note the bright area on the valve and the small scratches on the piston at about the 4 o'clock position. When looking at these I can imagine them being at the same location (Does the valve rotate while operating?)
Maybe some piston to valve contact? Bent the valve slightly? I didn't think these low compression 930 engines (flat top pistons) were an interference engine. Anyone know for sure?
Not sure what to do now other than remove the head from the cam housing and replace the exhaust valve.
Let me know if anyone has any ideas.
r/
Bob

Removed the #6 exhaust valve. valve looks good to me, springs look OK.
http://forums.pelicanparts.com/uploa...1572715237.JPG
http://forums.pelicanparts.com/uploa...1572715237.JPG
http://forums.pelicanparts.com/uploa...1572715237.JPG
http://forums.pelicanparts.com/uploa...1572715237.JPG

Only thing noticeable is that defect near the seat. Note it doesn't touch the valve, but maybe it affected the seat and its ability to seal.

Couple amateur thoughts...

Did you get a 1 angle valve job, or 3 angle? Looks like 1?

Your valve seat is sitting low, at least to my untrained eye...

Your valve seat doesn’t look “shiny” like the valve was making contact...

You sure your valve just wasn’t set tight?

Glad it wasn’t anything horrible...

A lot of oil deposits for 100 miles. Is the stem seal still attached to the guide? Maybe just crusty oil buildup on the seat, which as noted, doesn't appear clean, and certainly looks too wide. There are rebuilds and there are "rebuilds".

I didn’t have a valve job done. I thought it was done years ago before I bought the car (it had been sitting for over 20 years).
I lapped the valve and put the heads back on. Leak down now is at 8 percent on this cylinder. Calling it good! In fact when I was researching this, I read over and over that up to 20 was OK. Probably wouldn’t have went to all of this trouble if I would have known that. Getting to these valves ain’t as easy as a small block Chevy!!

20% leakdown is bad...

20% leak down is bad.
8% leak down is not something I would tear the engine down for.
Revisit in 1000 miles.