3.0 head valve extension too short

[OhioRick]

First post for me:

I bent some exhaust valves last year and I'm rebuilding my 3.0 heads for a 3.4 upgrade with new exhaust valves. Overall valve length is in middle of the correct range, but the stems don't extend as far as the specs say for my 1983 SC engine. I finally noticed a footnote in Wayne's book that valve heights may need to be modified on SC and later engines, but I can't find anything saying what they should be.

I then checked my existing intakes that were professionally rebuilt only 14k miles ago and they are also "too short". Valve seats are all still the originals. Logic tells me that my heads didn't get thicker, so I have to conclude they were built this way without any problems, other than my apparent overreving.

Should I sweat this since the adjusting screw will make up the shortfall? Any help will be appreciated.

[cgarr]

That stem length is used to measure if your seats are still in spec with new valves. I know there is a max length which would indicate your seats have been cut to their limit. The only way to increase the stem length measurement would be to cut the seat deeper which in my opinion is not something you want to do. You would most likely just use less shims when setting the springs.

What do they measure? I shoot for around 1.800

You have the correct valves?

[OhioRick]

all my exhaust valve overall lengths are 4.265" to 4.268" so they are right in the middle of the tolerance range (4.258" to 4.278") so I think they are correct.

However, most of my valve heights are 1.781" to 1.789" which is under the low end of 1.799" or 1.800". With no shims, of course.

I think something must be wrong with that height spec, since a typical new valve won't meet it and I have not changed the seats. I don't think I should have to grind the valves to get there, do you? Wayne must have been aware of this when he put the footnote in the book, don't you think?

[cgarr]

I would go with it, any cutting on the seat will only make them longer. I would not cut the seats just to bring them into that number. Its interesting all these specs Porsche puts out there and they never followed them in their own factory. I get a lot of first rebuild heads in that never had the 3 angle seat cut. It would be interesting to measure the depth of the spring pocket and see it they were not consistent, that's were you measure the seat depth from..

[OhioRick]

that's my thinking, too. I am not going to worry about that height since it will just take the adjusting screw another quarter turn to compensate. Thanks for increasing my confidence.