valve clearence anomoly

[Enrique]

I bought my 914 w/ Weber 40s and aftermarket cam. Among the paperwork was a card with the cam specs. It listed "checking clearence" as .009 in. and .010 ex. I adjusted the valves to those specs some monthes ago, but the valves sound loud (I think it's the valves), and I hear a slight pounding sound.

So I checked the valves hot (twice), but strangely clearence still measured .009 and .010. True, it took me 4-5 minutes after shutting off the engine to place the feeler gauge, but I still thought it strange. Anyone else have experience with valve clearence when hot?

Here's my real question. Should I tighten the clearence, and if so, by how much? Any cautions?

Thanks as always.

[Steve M]

I thought the valve's get adjusted when the engine is cold?. Am I wrong?

[bpick84]

The noise you hear is probably the rockers slapping themselves silly. Valves should be adjusted cold. As in let car park overnight cold. The clearance sounds about twice what it should be. The valve are not going to open enough and reduce your power. I would use the stock valve clearances.

[Germain]

Enrique,

First Steve is correct, valves should be adjusted with the engine cold (sitting at least 4 hours). If hot, the metal is going to expand, causing the measurements to be smaller (i.e. a .10 gap cold becomes a .08 gap warm).

Now if your trying to say that the valves were hot, you need to recheck. Since the gaps are bigger, the valve would not open fully. This would cause less air to get in, less exhaust out, but complete cooling of the valves. If the gaps are too small, thats when you get valves that are stuck open and burn up.

I can't tell you where to adjust them at, but I don't see why the cam manufactuer would change the stock position.

Measure them cold, anyone telling you to do it with the engine hot is wrong. I just hate to think of burning yourself on the exhaust.

[John Rogers]

I would recommend that you call the maker of the cam and get the info from them. Most air cooled engines adjust the valves cold and many cam makers use the stock clearances when designing the cam, but not in all cases. For example my race car has roller rockers and the cam is designed with ZERO clearance. So again, I would call and check. Good luck.

[paulhagedorn]

Don't know if I am right guys, but I think Enrique is trying to say that he adjusted the valves while cold, then checked them when hot. In theory, the valve clearances should be zero then because of thermal expansion.

Am I correct, Enrique?

[Enrique]

Sorry for the confusion. Paul is correct, I set the clearences cold, then checked them hot thinking that, if properly adjusted, there would be little or no clearence right after the engine had been running. I'm confused because clearence was the same hot as cold, but then maybe the metal just contracted in the few minutes I took to get under there with a feeler gauge?

In short, I want to adjust the valves closer or to stock specs, but I'm wondering how and if any harm could come of it.

[Germain]

Well, park the car over night. Adjust them in the morning at normal temps (not 10 below ). Then before you finish up, spin the wheels and recheck all the gaps.

*NOTE: not trying to be mean or hurtful*

I'm guessing that either the engine was not cool enough when you did the first adjustment and therefor were adjusting them warm. Or the screws and lock nuts are slipping causing the readings to be off. Or your not doing the readings correcty, a go/no feeler guage with the correct measurements helps alot of beginers do valve adjustments.

Within 4-5 minutes of cool time the gaps should be very very small. Heck within an hour the gap should be very very small.

[Steve M]

Here is a mistake I've made myself and I'm sure some others have as well.

Make absolutely sure your getting the right cylinder at the right valve pair.
I've done it where I tripled checked it and was still wrong. Lately it seems I've been getting the hang of it.
On the other hand the engine would not have started if I had it wrong. It may be worth the effort.

You also may want to check with the original maker of the cam as suggested above.

[Conrad W Peden]

As far as the gap staying at .010 for the hot engine....

The most likely cause is that the adjusting screws are old and have lost their hardness. If you adjust them and give them a little twist the gap is gone as soon as you fire the engine up.

My suggestion is to try new screws they're cheap. Or should I say 'inexpensive' depends on where they are made. Brazil/China/Germany etc..

Also.... Are the rockers stock? The cam may have come with different rockers requiring a different gap.

Conrad W Peden
72 L20E in rust-o
73 L20E in resto. soon to be raced-o http://members.rennlist.com/a914lover


[This message has been edited by Conrad W Peden (edited 09-21-2000).]

[Enrique]

I had a Porsche mechanic listen to the car and he said the valves did not sound loud considering how wide I had the clearences set. He became interested in the mystery and through a shoestring trail of evidence, he managed to have a phone conversation with the man who ground my cam in 1981! (turns out this guy had reground quite a few 911 camshafts for this mechanic in the more than 10 years ago)

The mechanic also mentioned that valve clearences are there to counteract wear to the valve seats (mostly the exhaust). I had previously heard that valve clearence was there because valves expand as they heat up. Now if they expanded .005", how could they still move back and forth in the valve guides? What am I missing here?

Question: has anyone else checked their valve clearences HOT just to see how much they really expand? As I wrote before, I did it twice, and both times found less than .001" difference between hot and cold readings.

BTW, I do have new valve adjust screws in there. (from Pelican!)

Thanks all.

[Enrique]

Damn, I forgot to mention what the cam guy said about my cam: the characteristics of the ramps and lift of this particular grind require greater valve clearence, thus .009" and .010".

[bpick84]

To answer your question on valve expansion, all metals expand and contract with temperature change. Expansion is viewed in a linear change in length. Therefore, a valve would expand in length and in diameter. Since the diameter is proportionally smaller than the length, the expansion is not as noticable. Expansion is figured by using the coefficient of linear expansion. For mild steel it is 0.00065 inch per inch per 100F temperature change. Stainless steel is 0.00099. I am not sure exactly what the metal is in a Porsche valve, however the exhaust is sodium filled for heat transfer. To figure exact length change would be difficult due to heat transfer. More than you wanted to know and still no answer.