Valve Clearance as Engine Warms?

[Motorhead-45]

This may sound like a dumb question but does the valve to rocker clearance increase or decrease as the engine warms?

[unclebilly]

So the head is aluminum, the valve is ferritic steel. Aluminum has a thermal expansion coefficient roughly 2 - 2.5X that of the steel. So as the engine heats up, the aluminum will 'grow' more than the steel so the valve clearance will increase.

[Motorhead-45]

So an increase in valve lash and associated noise as the engine warms is the norm right? I'm asking because I just went through this motor (my first Porsche) and I'm pretty confident that I had the cold gap set right on the mark. I set them all while the engine was on the stand and checked and rechecked. After about 5 minutes of warm up I now hear several of them talking to me.

My concern is because when I had the engine apart replacing a broken cylinder stud I also installed the cam oil line restrictors and was afraid I am possibly starving the valve train of oil. I've only put about 150 miles on the motor.

Opinions wanted ....should I swap back the cam oil line fittings and see if the clatter goes away or just wait for a couple of 100 miles and then do a complete valve adjust? I guess the swap-out would be the initial no-brainer answer as they can be easily changed in about 30 minutes or less....valve adjust is a bit more involved.

[unclebilly]

yup, valves get noisier as they warm up - the oil also thins out too. You might want to recheck your clearances tho.

[Motorhead-45]

Thanks for the explanation....I'll probably put another 100 miles or so on it and then do a full valve adjust and check head torque at the same time....

[earlyPguy]

..

[afterburn 549]

1st time people will always (almost) adjust too loose, the feeler needs to be a tight drag out of the gap..if you do this with a dial ind. and check one for the other you will see what I mean.

[movin]

Mentioned before, on the Carrera series inside the engine deck, the sticker says 0.002" to 0.004" for valve lash. Set them to a tight 0.004".

[afterburn 549]

0.002? really ? ? this not a snooty remark just never herd of two b4....

[movin]

Check the sticker posted inside of the engine deck lid of the Carrera series.

[Wil Ferch]

Which Carrera series?.... I'll double-check but I think it says 0.1 mm ( 0.004") all around for my 85 Carrera....

...been that way for almost forever.....AFAIK

- Wil

[mb911]

logic tells you that as the engine warms up the steel valve and the steel (cast iron) rocker expand and become closer when warmed up. I have verified this several times to prove a point with a friend of mine once

[Motorhead-45]

Originally posted by mb911
logic tells you that as the engine warms up the steel valve and the steel (cast iron) rocker expand and become closer when warmed up. I have verified this several times to prove a point with a friend of mine once

So it's your belief that the clearance decrease rather than increases?

Hum.....

[ChrisBennet]

You could always test it yourself like this guy did:

Originally posted by Scott Crawford on Rennlist
Tue, 9 Feb 1999 15:13:7

Author: "Scott Crawford"
Subject: Valve adjustment particulars Part 2

Body: Hello gang,

I sent out a plea for help on the finer points of valve adjustment about
two weeks ago. I got a terrific response, I guess confusion, frustration
and misery love company. It's really not all that bad, I guess.

Anyway, I was stuck on finding out a couple of things. For instance, how
tight is too tight? At what point is a gap of less than .004 dangerous to
the engine? How can you tell it's too tight BEFORE you munch your engine?
Toward understanding the ramifications of this setting, I also was trying
to find out what happens to the gap as the engine gets hot? If the gap
gets bigger with temp, it seems to me the .004 is gravy, margin for error.
If the gap gets smaller with temp, then it becomes insanely critical. So I
set about finding out conclusively.

I measured #4 exhaust overnight cold. The .004" guage went in tight, the
.003" went easy, I could not get the .005" gauge in. The .003 and .005 are
"conventional" feeler gauges, about 2" long, 3/8" wide, difficult to get in
there. Of course, I made sure that piston was TDC.

I put it back together, drove around for about an hour to get the temp up
over the first hash mark, pulled the passenger side exhaust valve cover off
to measure the same valve at TDC.

The results: .005" went easy, .006" went easy, didn't have .007", .008"
went tight. All these sizes were a "conventional" gauge, therefore a bit
difficult to work with, but they went in. Thus, the valve gap gets bigger
with heat, and the cold gap is something of a safety factor.

All that said, there is no substitue for a correct setting. It's
comforting to know that there is a margin for error there. Hope this is
helpful for someone.

The best news is, the burns aren't permanently disfiguring

-Chris

[Motorhead-45]

That pretty much confirms what I thought, as the engine warms the gap increases. I think the path ahead that I'll take is first, get another 100 miles or so on the reassembled motor to allow things to settle, do a complete valve adjust, probably set them at .003 since "newbies" tend to set them loose, remove the cam oil restrictors and give up the increased numbers on the oil pressure guage (probably a placebo anyhow) to eliminate any possibility of under oiling the valvetrain. It was idling at a solid 1 on the oil pressure guage before the restrictors and about midway between 2 and 3 with the restrictors.

Thanks guys...appreciate all the feedback!

P.S. Anyone know where I can buy .003" for the Porsche feeler guage tool sold by Pelican or do I get to make my own?

[mb911]

very interesting!

[Wil Ferch]

I'd vote for 2.7 Racer's method ( Doug) here...brilliant....measure from the easy-to-access backside against the cam lobe directly....

--> valve adjustment

- Wil

[RoninLB]

BA has also mentioned that clearance "increases slightly" when warmed up.

on my rebuild I checked the head stud nut torque and valve adjustment after the 1st day's ride. Then all again after 500mi.

[afterburn 549]

hope fully a real engineer will check in here....too my knowledge and exp. a couple of sec at diff heat ranges is going to equate into tons of diff readings.....one can hardly (imposable 1! ) tell what the clearance is in real life running- as the temp is a lot higher or maybe lower then 30 sec after running as that will only measure static and residual heat, not real conditions. As a i.c. engine warms up (or cools down ) it goes threw growing pains, at some pts the clearance will be greater and at others less . I do not think you will find it Lenear as the metals are diff and the heat wave is diff from instant to instant .

[unclebilly]

A real engineer? I was the first to reply - what is unreal about me.

The clearance should increase as the engine warms up. As I tried to explain before, the steel parts do expand as the engine warms up but the aluminum parts expand more (the CTE of aluminum is roughly double that of steel) so the valve train moves further away from the valve seat.

Think of it this way - the head studs failed on early cars because the aluminum cyclinder expanded more than the steel head studs did and as a result eith the studs broke (SC cars) or the head studs pulled out (mid year cars).

If you're at all interested in how thermal expansion of dissimilar materials can be used to your advantage, check out US patent # 6835486 - It was my first one. www.uspto.gov

Afterburn - you are correct - thermal expansion is nonlinear and there are infection points in the groth rate. Under certain circumstances, there can be points where fits between components of different materials will tighten and loosen slightly during a thermal cycle.