.004MM valve clearance?

[cjmurph]

This is probably a simple question but I just completed my first valve adjustment and I am curious.
I found myself using the feeler gauge to check and simply running the adjustment in until I felt the slightest resistance. Backed off, then tightened making sure that the slotted end looked about right. I would then reuse the feeler gauge to again check. Did this several times but noted to my unexperienced self that it would be much easier to just set minimal gap with feel rather than trying attain the exactly 004. My motor is out and on the ground so the work is pretty easy but I thought that the highest lift possible both intake and exhaust would be the most efficient.
Obviously, .004 is not very much but if you could set to a minimal standard with feel and using rocker movement, should you? Why .004mm?

[yelcab1]

you mean 0.004 inch, or 0.1mm.
What you are doing seems to be right, almost an overkill but no harm.

[cjmurph]

My bad. Your're correct I meant .004".

Is there a mechanical reason for the gap?

[don gilbert]

the gap helps your valves transfer heat.

[hcoles]

don't know how they came up with 0.1mm - lots of variables, temperature, valve train wear, valves need to seal, valves need to transfer heat/etc.
In all the times I've adjusted my valves I haven't got the idea that things wore much, maybe 0.0005" max. and that is a bit hard to feel. A solid drag adjustment on a 0.004" feeler seems to work fine. It is nice to do the adjustment with the engine out.

[cjmurph]

Thanks fellows. I appreciate the responses. I really am a novice when it comes to mechanics.

- I've heard of the "heat transfer" necessary. What exactly is that and how would a gap promote it?
- Does the .004" ensure that the rotating lobe allows the valves to completely close?

Thanks,
Curt

[Shoepop]

On your "by feel" question sure you can. I have been doing it that way for 15 years. Since your a newb set a couple with feeler gauge wiggle then set a few by wiggle and check. You will catch on in no time.

[cjmurph]

Shoepop On your "by feel" question sure you can. I have been doing it that way for 15 years. Since your a newb set a couple with feeler gauge wiggle then set a few by wiggle and check. You will catch on in no time.

Thanks Mark, thats kinda what I thought.
I'm quite sure my current "gap" is variable. I'm also sure I could make some of them closer and still retain a small (less than .004") gap. My difficulty was retaining .004".
My logic says that the closer I get the higher the valves and the more efficient the engine will run. I have a feeling my logic is missing something.
My covers are off and I'm debating on tossing the feeler gauge and simply letting the flathead slot with the rocker movement tell me when I contact the valve end. I'm sure I could be more consistent but I wanted to understand why the .004" spec.

[BigD9146gt]

Alex says; the correct answer "What is thermal expansion"

[cjmurph]

Thermal expansion of the valve? The valve stem?
I think I might see your point. You set the gap cold. Metal expands when the motor is hot. Cam, cam lobe, bearing, valve face, and even stem. Rocker arm is simply under cover but likely does not see the heat the aformentioned items do, hence .004".
Do I got it?

[snbush67]

Exactly, thermal expansion, that is why the valves are adjusted cold. When your engine is at operating temperature there is no gap. This cold measurement allows your valves to fully open and to fully seat while warm or cold.

If you adjust your gap too tight your valves will not seat and your valves will not transfer heat, you risk burning a valve.

If you adjust your gap too loose you will loose a lot of power from your valves not opening enough and your engine wont breath.

The .004" is a standard conversion from the factory metric setting.

Shane

[cjmurph]

Thank you very much fellow pelicans.

[Flieger]

Heat transfers from the valve to the cylinder head via the seat when the valve is closed. If your valve is open, not enough heat can transfer out and valves burn. Having a large valve clearance means that less of the camshaft duration has sufficient lift to lift the valve off the seat. This means the valve is closed more of the time and more valve cooling heat transfer occurs into the rest of the cylinder head with cooling fins.

[efhughes3]

I'd say doing it by feel is not only lazy, it is dumb. The potential damage on one end and loss of valuable power on the other would justify the effort in my mind.

I honestly do not understand what is so tough about valve adjustments that causes so many to look for some easy way out. It really is a fun way to spend part of a Saturday morning, and it isn't like it needs to be done every month.

I think I know my way around my engine reasonably well. I also know that the valve adjusts I've "rushed", I've not been happy with. Ironically, that has happened twice-both times with the engine out.

IMO, if I couldn't do this the proper way, I'd pay a pro or buy a 993.

[brentrussell]

Did my first valve adjustment on Sunday morning, .... a little tricky to start with but once done a few, all good.
Suggestion, when trying to insert the feeler guage give the Rocker a small lift to initiate the gap. I would definitley use a feeler guage !! to much at risk by feel.

[psalt]

Thanks fellows. I appreciate the responses. I really am a novice when it comes to mechanics.

- I've heard of the "heat transfer" necessary. What exactly is that and how would a gap promote it?
- Does the .004" ensure that the rotating lobe allows the valves to completely close?


The valve clearance on a hot 911 engine is actually larger than the cold clearance. The purpose of the gap is not for thermal expansion. The exhaust valve is the hottest part of the engine and it gives off most of its heat when it sits down on the seat. Another misconception is that noisy valves just need adjustment. Many first timers are disappointed when the valves are noisier after adjustment. Valve noise is more often wear in the guides and rockers than too large a gap. The normal wear pattern is for the gap to close over time as the seat erodes through a process of welding and tearing. Each MFG choses a gap based on the wear pattern and the service interval, so that the gap is never allowed to go to zero and burn the valve seal. Recommended clearances range from 0.004" all the way up to +0.020" for different engines and 0.004" should be considered the minimum. Some MFG's recommend wider clearances for racing, so the valve has a longer period to sit down and give off heat.

[cjmurph]

Again, this was my first valve adjustment, as well as first porsche clutch, as well as my first engine drop. I've never done one before and I agree that it was an enjoyable experience.
Previous to this venture, I didn't know there was a built in gap at the rocker ends of vehicles with specific regards to heat transfer or metal expansion. It makes sense to me that the exhaust valve would be the hottest.
I had convinced myself that increased lift would be more efficient. Intakes would draw in more fuel, exhausts would evacuate cylinders better. In my mind a higher exhaust would be a cooler running engine. That is why I asked the questions.

I would be very curious of the results if you set the .004" and had the willpower to remove a hot valve cover (especially on the exhaust side) and measured the gap. I'm sure some racers have probably done it and have documented changes. Whether it would be case expansion or rotating assembly changes with heat, I believe the design was intended to be set cold as .004" but that when the engine is running at operating temps the cam lobes and rocker arm relationship has minimal if any designed gap. I not convinced that this is entirely correct. I think that maybe spring deflection rate with rpm may also have a play in this. I could be wrong and I want to know more about "psalt's" theory specifically with valve seat wear prevention.
I'm confident that I currently have a fairly consistent .004" +/-.001". I found the car to have 3 seperate exhaust valves that were considerably tight (I think I had to back the slotted end down 3/4 of a turn to even come off the valve stem end). A couple of the intakes may have been a bit over the .004".
The car hasn't moved in 17 years and the clutch gave out within our first 100 miles of drive so it's been a massive learning curve.
I appreciate the help, the insight and the details.
Curt

[Shoepop]

murph, you sound as if your gonna be a great P car owner with a head on your shoulders. A lot of good knowledgeable folks here and as with all can sometimes seems like the Mayberry sewing circle. You will quickly learn to distinguish between those who like to run up their post count while offering little and who really has something to offer.

[psalt]

Curt,

If you are interested in this subject, I suggest looking at an intro to IC engines text book. Most of them will show a diagram of temps in the exhaust valve, the flow of heat and the resulting burnt valve when there is not enough heat transfer through the seat. You can also find a lot of discussion about the mechanism of valve seat wear in the studies done on the elimination of lead in fuel. The lead in fuel slowly built up a coating on the seats and valves that interfered with the wear process and harder material were adopted after the lead was removed. Some of us have 20 and 30 years of experience with engines and anyone who has actually measured hot clearances knows they are greater. I have measured them on SOHC, DOHC and pushrod engines and the biggest variable is the material the engine is made from, not the design. Aluminum engines have the largest increase in hot clearances and usually have tight cold specs. All mechanical valve trains require periodic adjustment to maintain a clearance to maintain compression and keep the exhaust valves from burning. The hydraulic lifter was invented to eliminate this requirement and reduce noise, but there is some lost motion. It is difficult to compare clearance requirements in different engines. Simple camshaft designs with steep ramps usually have tighter clearances because of acceleration rates and noise. In the 60's, more complex parabolic cam designs with softer ramps allowed wider clearances with less noise. Noise and emissions standards, 100,000 mile tune ups, and variable lifts have pretty much killed off the mechanical valve train and the periodic adjustment. The 911 is a piece of cake, the DOHC Jaguar XK with shim and bucket tappets takes about 8 hours to adjust the valves and the early Aston Martin requires removal of the head and grinding the stems to fit.

[cjmurph]

Thanks Mark. I appreciate that. Paul that is good stuff. I like engines and I like learning about them. I have no fear of admitting my ignorance and sometimes it is difficult not sounding obstinate via the typed word.

I think I've got some conclusion. If at the least, I received some very needed education. I think most everybody that responded was correct in their reasoning. Please excuse the un-technical terminology.

The valve seat time is important due to;
A) They need to cool via heat absorption from the valve face to seat contact.
B) Valve seat wear is prevented by ensuring that combustion takes place with valve completely closed.

Gap parameters at stem end at cold setting are .004" due to;
A) Thermal expansion of motor and various components.
B) Ensures cooling as noted previously
C) Ensure proper valve lift during running conditions. Vac, gas mixture and exhaust evacuation.

My process has changed with respect to the responses. I think I would error on the high side (+.004") instead of concentrating so intently on being close to the minimum.