Worn Out Valves

[1982911SCTarga]

I got my "new" heads back today and I'm looking over the worn-out valves I got back, too. All six exhaust valves were worn out, along with four of the intake valves. Two intake valves could have been re-used, but I decided to replace them all. Here's the pair from No. 5. Now follow the pencil point. A little worn?

Brian

[cgarr]

They are cheap, Its hard for me to justify grinding valves at 8 bucks each when you can get new ones so cheap. Check out the Intervalves (IV) They are great quality, the others QC seems to really be dropping off.

[1982911SCTarga]

No question. It was an easy decision to replace all of the valves. It's just interesting to see how these things wear.

My rebuilt heads look superb -- like brand-spanking new!

Brian

[hcoles]

sorry if this is a slight highjack - I read on another thread that someone was using Viton stem seals instead of the "white" seals because the "white" seals were causing wear where they seal. What is the current/latest thinking re. type of seal and who makes them?

[1982911SCTarga]

Not a hijack at all. The white Teflon(R) valve seals are considered too tight and hard, sort of unfriendly to the components they live with in their harsh-environment home. The Viton(R) seals were recommended and installed in my rebuilt heads.

Other things I also learned from my head rebuilder (I didn't do any disassembly -- I just packaged them and sent them off):

-- My heads had a mix of Viton and Teflon seals, and 11 of the 12 seals were crumbling apart.

-- Six of the 12 valve springs were installed upside-down. Porsche valve springs are progressively wound, so there is a top and bottom orientation to them when they're installed (this is not that uncommon). The upside-down springs is a sign of an inattentive previous engine rebuilder.

Interesting stuff.

Brian

[cgarr]

It is fun to see what you get in the shop, I like to sample a few of the installed height too, I had one set come in with a few broken springs, checked the height and they were way tight, I think I had 20 or 25 extra shims when I got done..

[hcoles]

another valve job question - my machine shop bought new a Serdi type seat cutting machine. He showed me how it worked and the way the cutter is aligned to the centerline of the guide. Was very impressive to me. Is this tool now common at the better 911 valve job service shops? If it was me.. I'd like to see that the shop had one of these.

[1982911SCTarga]

I don't know how common a Serdi machine is, but that's what Anchor Atlantic uses.

After you have your valve seats cut on a Serdi, go have lunch at Sardi's.

Brian

[1982911SCTarga]

I think I had 20 or 25 extra shims when I got done.

Craig, is this the old "more has to better" approach to engine building?

Brian

[911pcars]

"-- Six of the 12 valve springs were installed upside-down. Porsche valve springs are progressively wound, so there is a top and bottom orientation to them when they're installed (this is not that uncommon). The upside-down springs is a sign of an inattentive previous engine rebuilder.
"

I'm interested in the spring dynamics at work for a progressively wound spring. Why does the orientation of the spring matter? As it compresses, its length decreases; the section with the lower spring rate compresses first, then the higher rate spring section next, yes? What difference does it make?

Perhaps a portion of the spring contributes to the inertial mass of the valve train. The lower tension section has fewer coils per inch, thus less mass to contribute, thus extending the engine speed when valve float occurs. Is that in the ballpark or is there a more accurate answer?

Does it matter how a progressively wound coil spring is installed in a suspension application? Taking the valve spring example, a suspension coil spring contains a portion of which is considered sprung weight and the other end unsprung weight. One would think the unsprung weight would be less if the lower rate spring section (lower mass) were attached at the control arm instead of the chassis, yet I observe most prog. coil springs are installed with the lower rate coils on top.

Hmmmm,
Sherwood

[1982911SCTarga]

You raise an interesting point that I'm not really qualified to answer.

I found this definition here, which might be helpful:

Progressively wound valve spring:
A valve spring with variable spacing between its coils which helps to improve responsiveness and thus to reduce the load on the valve train.

The orientation may also have to do with the windings of the inner valve spring, but that's a guess on my part.

Brian

[1982911SCTarga]

Here's what Car and Driver says:

Progressive-Rate Spring
A spring with an increasing spring constant. For example, if the first inch of spring motion requires 100 pounds of force, the second inch would require more than an additional 100 pounds, and the third inch would require still more. Progressive-rate springs become stiffer as they are compressed, unlike single-rate springs, which have a fixed spring rate.

So, in the Porsche world, the progressively wound valve springs are better able to handle potential valve float troubles at higher RPMs?

Brian

[sabeo.m]

1982911SCTarga, did you get your heads done at Anchor Atlantic? I need to get mine done...

[1982911SCTarga]

Yes, Anchor Atlantic did my head work, and I'm extremely pleased with the quality of the job. Bob does a lot of work for us East Coasties.

Brian

[hcoles]

re. progressive wound springs - I think I also read somewhere that it can help reduce unwanted harmonic vibrations

[sabeo.m]

Originally Posted by 1982911SCTarga

Yes, Anchor Atlantic did my head work, and I'm extremely pleased with the quality of the job. Bob does a lot of work for us East Coasties.

Brian

Awesome, look's like I will be sending them there..