valve seat contact with valve problem.
 

I have a set of 3.0L sc heads and have just started a valve grinding job when I found somthing's not correct. the car was running fine when pulled apart.

I cleaned the valves and notised two of the intake valves was black in the contact area.
I thought this was a bad sealing issue,
Here's a picture of the intake valve. All 6 messures aprox 49mm
http://i32.photobucket.com/albums/d5...psb3cb8k6y.jpg

the valve gets a light pass at the valve grinder.
I now put the valve in the seat to locate the sealing surface and i see the contact is way of.
You can see the valve sealing all the way in the end of the 45degree.:
http://i32.photobucket.com/albums/d5...psinkruwbk.jpg

After spending some time troubling and trying to cut the seat to match valve I see that it's not possible. I'm starting to move out of the valve seat with cutter for the top angle and end up with the valve to low in the head.....
This was the best sealing I got....

http://i32.photobucket.com/albums/d5...psd2xbrmq0.jpg

So I checked the rest of my heads and compared them. Turnes out two of the intake seat is bigger than the rest. These are the same as the intake valves was black in the seal surface. So I no see that the there is no sealing here.
I messure the diameter and this is my issue:

http://i32.photobucket.com/albums/d5...pscnxelr6x.jpg

these two valve seat has aprox. 49,8mm and the valve is 49mm
the remaining 4 has aprox 48.8mm.

So is this a bad valve job or has someone mounted to big valve seats? I know the engine had a top rebuild in a german car shop some years ago...

What is the best way to fix this? Change valve seats? maybe insert bigger valves (Is this possible without changing valve seats)?

That's a new one! I would change the valve seats.

Looks like the seat was cut way too deep, you can check it with a valve and measure the stem length, cant be fixed without new seats, they are rather cheap just a bit of work to replace. should have about 1.5mm contact area with the seat.

I would replace the valves with over-sided valves. 50mm
We can supply over-sized valves for most Porsche applications. 1 mm is standard.
2.7 41e and 47i
3.0-3.3 42e and 50i
and so on.
The only caveat it piston to valve clearance. It's always necessary to check clearance. Most valve pockets are machined large enough so clearance is not an issue.

Thanks!

I think maybe me trying to get get it correct now has made the cutting to deep. I think maybe a need to change valve seats anyway....

I had a go on the rest of the valveseats, they are definetly better, but not still not good enough...
So i found a 3.2 head and compared seats. My 3L valve seats has clearly had som machining.
I have the same inside diameter almost in the bottom at the valve seat that the 3.2 has just were 45 degree ends (and the 60/75 begin).
heres a picture that shows that som machining bin done:

http://i32.photobucket.com/albums/d5...ps0pcsjx0z.jpg

Here is another valve / seat. You can se the 45 degree over the valve. and its not possible to get it further down as ther seat is to big.
http://i32.photobucket.com/albums/d5...psnzdjfcw9.jpg

The best one is here: http://i32.photobucket.com/albums/d5...pskqkfi6vh.jpg

1. (the latest image) is this ok or do i want to se space above contact area of the valve to know tit's not over the valve? Do you have a picture that show a good contact location of a porche valve? normaly it would be senterd.... drawing in "how to modify porsche engine" show it all the way at the top and doesn't specify where to locate the contact.

2. the funny thing is that messuring the stem I get 46mm as the book say (46mm +-0,3), but before cutting seats i get aprox 45,3mm. Is the number in the book maximum or am I doing it wrong? I just think it should mach the book before I cut the seats.

3. Do you think a can save the valve seat if I use oversize valves?

I wonder why this cutting has bin done.....??

Try not to over-think this problem.
They (the seats) were probably opened to improve flow (an attempt to be innovative) by the previous machinists and he got carried away.
If possible, over-sized valves is more practical. It will be cheaper for certain, improve flow characteristics and be safer in the long run.
The only Porsche air-cooled seats I've ever seen fall out were replacement seats.
And I've seen it happen from the "best" Porsche machinist in the busyness.

Ok. I will contact you for a set new valves. They should be replaced anyway. but I think I need bigger valves than +0.5mm for the seats I messured +1mm more diameter....?

You might want to think about getting the sealing surfaces machined too:

I am anile about valve work..
A race engine and a street engine can look exactly alike but be exactly different.
Where a seat last the longest is not where it flows the best.
It all becomes a compromise.
The middle is purrfect for the street, move it way north for flow.

I agree. And it seems like 3L and 3.2 seats have inside diameter so big that puts contact face in the upper part of the valvle regardless. It's not possible to get it in the middle unless cutting deep.
So this must me why i had trouble when the inside diameter on my seats was machined bigger.

I Just did a few low mile and untouched 3.2 heads today and that was straight forward and a "easy" job with just a few light cuts to get a clean contact face. Got a good result. So I'm doing it right and my tools are ok. I ended up with 1.5mm face at the top of the valve and after lapping I see that all the 45 degree face has contact with the valve. I just tured the 60 degree light one turn to see that the seat is round so it has the same width all around... so the face doesn't get lower on the valve. The valves still are high in the combusing chamber (valve stem messures 45.3mm).

I end up with a Dremel to smooth all angles to a radi. Usually the seat bottoms are larger than the hole they are in. I make it all blend.
Chuck up the valves . I remove all sharp edges too.
I leave no place for hot spots in the head.

"I remove all sharp edges too."

Yes, including the plug pockets.