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Valves do not deal tight after cleaning and lapping

Tiny
LYNCH890
  • MEMBER
  • 1989 AUDI 80
  • 2.0L
  • 4 CYL
  • 2WD
  • MANUAL
  • 150,000 MILES
I took the cylinder head out for valve seal replacement and discovered that the cables had excessive carbon deposit, so I decided to clean and lap them by hand with a professional lapping compound.
After everything was done and assembled it was time to do a leak test by pouring gasoline on the valves with the cylinder head upside down. To my bewilderment the gasoline went right down past the valve edges in no time. I leaned over to take make a visual inspection and sure enough there was about 1mm gap between the valve head and the seat. I could literally see light coming through the gap from the intake/exhaust ports.
Now I know that it couldn't have been my mistake, for I lapped the valves with a lapping stick by hand.
This sure seems a mystery to me. What could have happened here?
Is it possible the previous owner to have done a lapping or resurfacing which could account for this?
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Monday, February 11th, 2019 AT 1:14 AM

5 Replies

Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello,

Did you make sure to put the valves back to the cylinders they came from? This is important to do because they do not wear evenly. The way to remedy this is to lap the valves to cylinder head with valve lapping compound. I have managed to get a lapping tool into a drill and lap the valves this way. It does the job a lot faster and easier. I hope that this helps.

Thanks,
Alex
2CarPros
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Monday, February 11th, 2019 AT 4:03 PM
Tiny
LYNCH890
  • MEMBER
  • 13 POSTS
Yes, I made sure each valve went into the place it came out of.
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Tuesday, February 12th, 2019 AT 1:11 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

That's great. My suggestion to you would be to get the valve lapping tool, or a suction cup device into a drill and go about lapping them that way. I have included the valve seat and valve measurements in the diagrams below. I hope that this helps.

Thanks,
Alex
2CarPros
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Tuesday, February 12th, 2019 AT 1:54 PM
Tiny
LYNCH890
  • MEMBER
  • 13 POSTS
Well, today I decided to take the head apart and try to find the cause of this oddity. When I removed the camshaft the valves sealed tight against the seats and I even did the leak test to confirm that there was no leak, and sure enough there wasn't!
When I put the cam back in and tightened it properly, the valves once again developed this gap.
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Thursday, February 21st, 2019 AT 3:31 AM
Tiny
SCGRANTURISMO
  • MECHANIC
  • 4,897 POSTS
Hello again,

Do you have any worn camshaft bearings? I am including in the diagrams below troubleshooting, procedures, and specs for the cylinder head, camshaft, and lifters for your vehicle's cylinder head. Please go through them and let us know what you find out.

Thanks,
Alex
2CarPros
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Thursday, February 21st, 2019 AT 12:52 PM

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