Setting Lash Cold or Hot

[bob58o]

Apparently Dyno cams says to set lash at 0.003" when the engine is COLD.

AFAIK, the rest of the world says to set lash to when HOT.

For instance, NR says to set to 0.002" and 0.003" when HOT.
I was under the impression that many just set to zero when COLD, and I was right. Many people do that.

So why does Dynocams say to set the lash to 0,003" when cold? Do they really want it around 0.006" when HOT? With an Aluminum Engine, the lash will open up when HOT.

I feel I've already won this FaceBook argument, but I have been wrong once before. Well I thought I was wrong, but I was actually right, so I was wrong about thinking I was wrong. LOL

I'm sure many (if not most) set the lash when cold and then forget about it until something goes wrong.
BUT what is correct?
And ...
If tighter lash is better for performance, what is to be gained from looser lash?
Longevity?

 

[Whitetrashrocker]

Things swell when they get hot. The lash should get tighter with heat no?

Edit: as I sit and think about it it would get looser. As the head expands the axis of the rocker moves away from the cam. Essentially making the push rod a little shorter than it was with a cold block.

But this is with an OHV engine. Don't know if it hold true with a flat head.

Loose lash is kinda like retarding the cam just a bit. While a tight lash will be more like an advanced cam.

It's a small difference but changing the lash is a part of tuning.

 

[bob58o]

Things swell when they get hot. The lash should get tighter with heat no?


Loose lash is kinda like retarding the cam just a bit. While a tight lash will be more like an advanced cam.

It's a small difference but changing the lash is a part of tuning.

On these aluminum engines with steel valves, for whatever reason (thermal expansion coefficients), the lash opens up when heated and ran. So if you set to zero when cold, it will open up to like 0.003"ish when it is hot.

With loose lash, you are losing lift and duration.
But centerlines will remain the same.
Intake closes slightly earlier. As if you advanced the cam (for lower RPM)
But Intake opens slightly later. As if you retarded it.
This might build higher cranking pressure as the intake closes slightly earlier.

With Zero lash, you are getting the maximum lift and duration.
Intake closes slightly later. As if you retarded the cam (for Higher RPM)
But Intake opens slightly earlier. As if you advanced it.
This might have less cranking pressure as the intake closes slightly later.

Since the centerlines stay the same, I would hesitate to use "advance or retard". I think the change is in the duration, which of course will change ALL timing events Except for the centerlines.

Slightly higher cranking pressure is the only real advantage I can see of loose lash. Not really sure if it would be significant when measured. I'm sure there is a reason for lash, and that too tight of lash might bend rockers or valves or pushrods or something in some cases.

But what I find strange about saying to set lash cold, is that YOU specified that COLD. Meaning you are aware that it will change from cold to hot. So instead of setting on a hot engine, when the change will have already happened, you set on a cold engine and guess how much it is going to change when running and hot??????

---------- Post added at 01:53 AM ---------- Previous post was at 01:16 AM ----------

I didn't think about a more aggressive ramp, which might increase low lift flow.
http://www.hotrod.com/articles/ctrp-1012-setting-valve-lash/

But Dorton also gave us one more trick that you might want to consider if you are qualifying poorly and in danger of missing the field. Although he doesn’t recommend doing this regularly, he says opening up the lash an extra 0.006 will often pick up qualifying times on a typical half-mile track by around 0.2-second. This is because the first few degrees of the opening and closing ramps on a cam lobe are designed to gently lift the valve off the seat and drop it back into place again without damaging the valvetrain. Opening up the lash by six thousandths effectively removes this part of the cam lobe from the process and makes the valve action much more violent. That delayed opening helps create more compression and power, but it’s also very hard on the valvetrain.

 

[KartFab]

Jody with ARC racing said that last grows when the engine gets hot. He says he sets his at zero and many racers set theirs at zero when cold.

I still set my lash to what the manufacturer specifies though.

 

[Whitetrashrocker]

I did say "kinda like" Bob.

 

[Randy H]

One possible issue with zero clearance is valve seat wear can be greatly accelerated. Especially if valves are lapped, as opposed to cut. When things settle in what was zero clearance is now preventing the valve from seating properly.

In racing, clearances are checked frequently and this probably will not be an issue. That and it's a quick easy job.

If these things were motorcycle style bucket and shim lash adjustment I bet no one would be using zero lash.

 

[Kentucky Boy]

I think most people set it while the motor is cold. Just like piston ring gap, crank bearing oil clearance and every other tolerance you can think of. I think maybe Space X and NASA would worry about tolerances that tight but not so much with a go kart motor.

 

[bob58o]

An interesting test....
Warm up the engine quickly set the lash on one of the valves to like 0.006".
Or just measure whatever the lash is.
Let the engine cool.
Measure lash again.

If it measures 0.004" after it cools, you might expect lash will grow 0.002" when the engine heats up. Set 0.001" cold to get 0.003" hot.

If it measures 0.003" after it cools, expect it to grow 0.003".
Set to zero when cold to get 0.003" Hot.

Or set the lash cold, then warm it up and measure while hot, then let it cool and measure again.

 

[KartFab]

here have this wrench

The thermal expansion of steel, stainless steel, chrome moly, and aluminum are all different. All of these metals are found in different types of push rods.