Cold valve lash settings - tighter or looser?

[Tom68]

So the consensus is:

Steel : lash gets tighter when hot
Aluminum: lash gets wider when hot

Right?

Frank

Cam suppliers publish the information.

When Honda Dirt Bikes were having all the intake valve recession issues they'd be running fine for the day. The next day when the engine was cold, they wouldn't start due to the valves now leaking.

Side valve Briggs and Stratton Lawn Mowers went the other way. When the exhaust valve got hot it would increase in length and not shut. Hot engine stall, would restart when valve was cold enough. Increase lifter clearance,m no more hot stalls.

No rocket science involved, just differing thermal expansion with differing thermal concentration. It's all calculable.

[CamKing]

So the consensus is:

Steel : lash gets tighter when hot

No.
With Iron block & heads, the lash may grow a small amount, stay the same, or get tighter a small amount.
It depend on all the components in the valve-train.
With most of the SB Chevy engines we see, it grows about .002"-.003".

[dynoflo]

I must be crazy because on probably 100 race car steel heads steel block solid lifter engines it didny change from hot to cold. Maybe .001 and thats not worth adjusting valves again. Aluminum heads yes it will make a differance. I never had a aluminum block and aluminum heads on my dyno. I did some cheating on hydraulic lifter rules by having cam ground for .004 solid lifter clearance. Made big differance and car won almost every race. Got teched after winning 3 in a row and tech wiggled rockers and said must be fast leakdown lifters.

[FishFry]

With Iron block & heads, the lash may grow a small amount, stay the same, or get tighter a small amount.
It depend on all the components in the valve-train.
With most of the SB Chevy engines we see, it grows about .002"-.003".

Thanks, got it.

Follow up question: I run a hopped up Chevy 261 - so all iron, solid lifters.
If it all gets looser when hot, what if I would set my cold lash to zero - and than maybe back of a quarter turn for good measure?

From what I understand, that would advance my cam a hair (but nothing to write home about).
Would there be any adverse effects?

Thanks, Frank

[Alaskaracer]

When I was running an iron block and aluminum heads, the difference in my last from hot to cold was right at .006". So I set them .006 tight cold and they were just about perfect when hot. When I went from an iron block to an aluminum one, the difference became .012-.016 depending on engine temp....i'll make an initial adjustment cold to get it started. Warm it completely and lash all the valves while hot....let it cool and recheck. Any differences in the valve adjustments from valve to valve I'll correct when cold, then check again when hot. Might sound like a lot to go through, but once I have them set, they rarely change lash unless there is an issue. I do run shaft rockers though, and that helps.

[FishFry]

Let me rephrase my question.

Given that on a 261 with solid lifters the lash gets wider when hot, I was questioning my self - why would you want anything more than zero lash (when cold)?

From what I know, hydraulic lifters always try to keep it zero lash, when the engine is running. So zero lash (or as close as possible) is a good thing - right?


With solid lifters we can't have that - the next best thing would be to set the lash to zero (cold) and live with whatever gap they end up when hot.

Did I miss something? Would there be any adverse effects?

Why is there a spec for anything more than zero (when cold) anyway, when the gap widens at operation temperature?
Why would I want a gap in the first place - what is it for?

Frank

[CamKing]

Let me rephrase my question.

Given that on a 261 with solid lifters the lash gets wider when hot, I was questioning my self - why would you want anything more than zero lash (when cold)?

From what I know, hydraulic lifters always try to keep it zero lash, when the engine is running. So zero lash (or as close as possible) is a good thing - right?


With solid lifters we can't have that - the next best thing would be to set the lash to zero (cold) and live with whatever gap they end up when hot.

Did I miss something? Would there be any adverse effects?

Why is there a spec for anything more than zero (when cold) anyway, when the gap widens at operation temperature?
Why would I want a gap in the first place - what is it for?

Frank

Every mechanical cam profile(Flat Tappet, or Roller is designed for a give Hot lash. Above the point, is the lift curve the cam designer wants, to open and close the valve. Below that point, is the lash ramp that the cam designer designed to take the lash out on the opening side, and to gently seat the valve on the closing side.
The lash ramp is much slower then the lift curve above it.
If you set your hot lash tighter then what the cam designer designed it for, you will be opening and closing the valve in the middle of the much slower lash ramp. This will increase duration, without adding almost any area. This will turn a good cam design into a lazy cam design. This is why you need to set the Hot valve lash to where it's designed to be set.

[1980RS]

Of all the things I have run I have found a stud girdle or shaft rockers to be and keep the lash the most stable on any of my engines.

[FishFry]

Thanks for your input guys.

I find it fascinating how much different (and often opposite) opinions (like in "I did this for 35 years, and it always worked perfect") one can get on a question, that should have one simple answer based on physics and engineering 101.

After doing some research and asking a lot of people with much more experience than I have - here and elsewhere - my bottom line is:
It depends. Engine and valve train materials/combination seem to play the main role. But there are probably a ton of other variables in play.

After all I was just looking for best practice and maybe a better/smarter way to set lash.

There is still a lot of stuff I'm trying to wrap my head around (like the question of zero lash setting at cold).
I was told it would wipe my cam in no time - thought that lash only exists, when there is no pressure on the lifter in the first place (base circle) - as soon as that gap is taken up you have zero lash anyway etc.

I mean solid lifters are around forever and I'm pretty sure somebody had the same thought before - so there is probably a good reason for having a lash when cold. I'm just trying to really understand all this in depth, instead of just blindly following the rule book.


Frank

[Stan Weiss]

Frank,
Let say that you could set cold lash at zero and everything would work fine. Is cold lash set at 20C or 0C or -15C ...?

Stan

[FishFry]

You tell me Stan - I was asking myself the same question.

Cold is cold (as many say) but how cold is too cold?
Same goes to hot - GM recommends 30 minutes of driving, before setting the lash in the manual, others say let her idle till it reaches operation temperature. I guess at least the oil has to be hot enough - but I don't know.

Some say set the lash with the motor running and go by ear (till the ticking goes away), and that this is the only right way, because dynamics and all.
Others say that all race drivers set the lash cold andwhatnot.

To many opposite opinions - it's complicated.

But when an actually cam designer like Mike here tells me that I need a lash, I tend to trust his advice.
I mean, it's probably not getting more competent than this.

Frank

[Tom68]

so there is probably a good reason for having a lash when cold. I'm just trying to really understand all this in depth, instead of just blindly following the rule book.


Frank

Because you can't accurately set 16 valves hot with engine turned off, the temperature is changing as you go.

Manufacturer does the hard work for you and supplies cold figures.

Having said that I always done them hot once, take the cold reading, then just do a cold check after every race meeting.

[Geoff2]

Most lash figures from American cam grinders are hot lash, not cold.

[Tom68]

Most lash figures from American cam grinders are hot lash, not cold.

I was thinking OEM, aftermarket give guidance on hot cold clearance change for different material combo's, hence the hot set cold follow up measure for reference when cold checking.

[n2omike]

In the simplest basic terms:

Aluminum expands twice as much as steel with heat. Therefore, the aluminum head/block grows more than the steel pushrod as the engine heats up. This will increase lash.

Iron and steel have very similar rates of expansion. Therefore, engines with iron heads will see very little change in hot/cold lash.