Piston to valve clearance

[1987 Porsche]

Here is a little background before my question: The car is a 78 911 with a 3.0. I recently changed out the pistons and cylinders to a later SC. So now I have the 9:3:1 Mahle pistons. No case machining. A .25 base shim was used between case and cylinders. John Doughtery did my camshafts to 964 grind. Cylinder seating areas machined .012.

Today I did my cam timing. I had two dial indicators and two z blocks. I was able to time the cams at 1.26. I then moved on to the valve to piston clearance. Here are my readings: Intake for cylinder one was good at 1.5mm but did make contact at 2.0mm. Exhaust valve for cylinder one made contact with piston when set at 2.0mm, but was good at 1.5mm. Intake for cylinder four was good at 1.5mm. Exhaust valve for cylinder four also made contact with the piston when set at 2.0mm but was good at 1.5mm.

Is the exhaust valve clearance a concern since it is not within the spec (2.0) wayne recommends in his book?

Sorry for the long explanation.

Lou

[Flat6pac]

If you’re not comfortable with the numbers, back the cam to 1.20. That will give you more space and still be in spec.
Bruce

[1987 Porsche]

Bruce,

This is my first build so I haven't been comfortable at all since the beginning. Just joking but kind of serious. Would you be ok with those numbers if you had them on a build? I'm sure I can get a little bit more out of the exhaust valve if I went out and retested but at 2.0mm it was contacting.

Thanks,

Lou

[Neil Harvey]

I have never read this "book". It seems this book has become the bible of engine building. Not to be critical of anything that can help, but it seems that this book does not explain there are tolerances that can be had and why. Its not all absolute.

I think from what I read here often, this book should be called, "old lady with a new table".

If this helps, we often run engines with a lot less that 1.0mm valve clearance and just enough piston to head clearance to wipe off the carbon.

It depends on what you are trying to achieve. Street engines that are driven within a RPM limit of 7500 and the gear changes done carefully, you can run close. Even race engines can be run closer, just learn to drive. Yes. mistakes can happen, so this is why you may want to error on the safe side and give yourself more clearance.

One of my issues is the over springing of the valve springs. It does help in an over rev situation, but more often than not, the springs are installed with seat pressures very high without the notion of an over rev. Every compromise you make for safety sake you are taking performance away from your engine, so understand why you are doing this.

Its a shame that this book doesn't explain this stuff. Maybe it wasn't considered or understood either.

[1987 Porsche]

Neil and Bruce thanks for the replies. I went out in the garage today to see if I can get anymore than 1.5mm on the exhaust and was able to get just a hair under 1.75.

[911pcars]

There have been updates and revisions to Dempsey's book that provide more accurate numbers. I would use factory specs to back up the numbers in any aftermarket publication. After all, it is your engine.

Sherwood