Valve lash rant

[451Mopar]

Was watching bad videos on youtube about adjusting valve lash. The find zero lash and tighten 1/2 turn stuff without explaining where that comes from, and why it is usually wrong for our Mopars. Most of that 1/4 to 1/2 turn adjustment is from adjusting stud mounted rockers, not shaft mounted rockers. When adjusting a stud mounted rocker arm like that, it is like a 3rd class lever. The lower the rocker ratio, the greater the adjustment has to the pushrod end. It also does not account for thread pitch of the adjuster which might be 3/8x24 tpi or 7/16 x 20 tpi (Stud mount rocker arms.) For example, a 1.5:1 rocker arm on a 20tpi stud adjuster, 1/2 turn is 0.0417" preload. With a 1.6 rocker and 24 tpi adjuster, 1/2 turn is 0.0339" preload. Not a big deal except for when suggesting using 1/4 turn. With thermal expansion the lifter could end up with zero preload.
On our Mopar shaft mounted rockers, the adjuster is at the pushrod, so figuring preload is easier. 24 tpi is 0.0417" per turn of the adjuster, and 20 tpi is 0.050" per turn.
On the Stud mounted adjuster, it only takes about 1/2 turn to get the same preload the shaft mounted rockers would require 3/4 to 1 turn of preload.

 

[Cranky]

Wouldn't the push rod expand some once it was heated up which would increase lifter preload? I usually shoot for .040 preload max cold with the juicers.

 

[IQ52]

Wouldn't the push rod expand some once it was heated up which would increase lifter preload? I usually shoot for .040 preload max cold with the juicers.

Yes the pushrod will expand some. But so does the cylinder head, aluminum or iron (though aluminum expands more), which lifts the rocker shaft away from the camshaft and the valve tip. Thereby decreasing the preload.

 

[HEMI-ITIS]

Iron heads & PRs might grow a thou or 2 that's it. When I had the iron heads the would grow 3 thou,with the aluminum they grow almost 6 thou!

 

[451Mopar]

I really didn't explain how I calculated the adjustment difference. Normally the rocker arm is looked at as a first class lever with the pivot between each end, so a 1.5:1 rocker arm would have some distance from pushrod to pivot, and the distance from pivot to valve would be 1.5 times that distance. For example pushrod to pivot = 1, and pivot to valve = 1.5. Total length is 2.5.
When you adjust preload by adjusting the pivot point, the ratio is the total length / the length from pivot to the adjuster, but now the pivot point is at the valve stem (third class lever like an engine hoist.) Example total length = 2.5 / pivot to adjustment = 1.5, so ratio is 2.5/1.5 = 1.667:1, so if the stud adjuster is turned in (down) 0.020", the pushrod would be pushed down 0.020" x 1.667 = 0.03334".
As the rocker ratio gets higher, the ratio decreses as the adjustment gets closer to the pushrod. Example of a 1.7:1 rocker, 1.7+1 = 2.7/1.7= 1.588:1. So the same 0.020" at the adjuster is now just 0.020x1.588= 0.0317" at the pushrod.

 

[PurpleBeeper]

Wow, I'm a "math guy" and my head hurts. Could you dummy this up for me so I know how many turns of pre-load I should put onto my hydraulic lifters with the old Isky ductile iron rockers (I think 1.5 ratio, but not sure) that I bought in the 80's? I'm about to adjust them again. I went with 3/4 turn in last time.

I get the "general idea" of what you're talking about with lever arm length and thread pitch, but not the finer details. Is there a relatively easy way to adjust the rockers that is better/more precise vs. the desired thousandths of an inch pre-load? I've always just heard "1/2 turn in"

 

[451Mopar]

Trying to state the differences between adjusting the stud mount rockers vs shaft mount rockers and how the "turn the adjuster" a specific fraction of a turn can result in different preloads depending on the setup.
The amount of actual preload to use has always been debated. Some racers turning high RPM like near zero lash with the engine hot so the hydraulic lifter will not pump up and hold the valve open. Many OEMs like > 0.080+ so the lifters are really quiet and have plenty of adjustment tolerance. Just about every performance cam manufacturer recommends somewhere between 0.020" minimum and 0.060" maximum. This is the preload with a warmed up engine, not setting the preload cold.
With iron block and aluminum heads, I use 0.010" of expansion from cold to hot. Figuring about 0.008 to 0.012 depending on actual head expansion for aluminum, and a but less for iron heads.
Anyhow, for the shaft mounted rockers with adjusters over the pushrod, I just use 1 full turn cold for most setups.
If the adjuster is 24 tpi, this is 0.0417" preload cold, and hot it should end up around 0.030" to 0.036" preload
If the adjuster is 20 tpi, then 0.050" cold, and 0.038" to 0.044" hot.

 

[451Mopar]

Don't get me started on what many think is valve train "Geometry"

For a good, but long read on geometry by Jim Miller:

http://www.sbintl.com/tech_library/articles/rocker_arm_geometry.pdf

 

[MoparLeo]

And yet they still run. Just goes to show that you don't need a degree to make your car run. You make life too complicated. How many out there have broken any parts whether you went 1/4 turn or 1/2 turn. What does the FSM say? There are enough variables in an engine to give anybody a head ache.

 

[dvw]

Preload doesn't mean squat on a hydraulic cam as long as you have the folowing
A: the valve spring controls the valve train in the engines usable range
B: the plunger isn't bottomed
C: there is enough preload to have a oil cushion under the plunger at all times
My 2 cents
Doug