I was hoping someone would check my math; I feel like I've got this right, however most of the threads I have read suggest cam's with a .600+ lifter usually receive 7.425 or larger push rods. Here is what I did and what I got.

I have a comp cams adjustable push rod 7702-1 that goes up from 6.800 to 7.800 in 0.050 increments. There is not a lot of space to spin this thing around once its in the push rod bore so I had to keep taking it in and out in and out, finally settling on 9 turns which was just slightly loose. To give you some background the specs of the cam are 223/235@.050, with an advertised duration of 274/285, and a .625/.625 lift and a 114LSA. I have Morel 5206 Lifters I just installed as well. They call for a .060 preload to be happy.

So at 9 turns on the adjustable push rod I was able to use a feeler gauge on top of the valve in between the rocker to find actual zero lash. I thought it was better to use a feeler gauge then to fuss with the push rod because of the space and the fact that I couldn't feel for the lifter bucket moving down.

Most of my measurements were pretty close... out of my 16 valves, the smallest gap was .018 and the largest was .033, a .015 difference. Morel calls for a preload of .050-.060.

6.800 push rod + 9 turns X .050 per turn = 7.25 + .016 average of feeler gauge lengths = 7.266 + .060 lifter preload = 7.326 Total Push rod Length


I feel like this figure puts my preloads all between .050-.060

Now here is where the confusion comes in. I turned out my push rod 10.5 turns which comes to... 6.800 + 10.5 X .050 = 7.325... when I tighten my rocker arm to 22 ft/lbs I am STILL only getting 3/4 of a turn. I think the suggested amount of turns from zero lash to preload where 1 1/2.

Am on on the right track or what?

DG

 

@ .060 Preload

Cylinder 1

Intake 7.325
Exhaust 7.340

Cylinder 2
Intake 7.330
Exhaust 7.330

Cylinder 3
Intake 7.335
Exhaust 7.340

Cylinder 4
Intake 7.335
Exhaust 7.342

Cylinder 5
Intake 7.335
Exhaust 7.330

Cylinder 6
Intake 7.335
Exhaust 7.343

Cylinder 7
Intake 7.335
Exhaust 7.342

Cylinder 8
Intake 7.335
Exhaust 7.330

I forgot to mention the cylinder heads were milled .004 and had a valve job done. There values are calculated at .060 preload. To get a value between .050-.060 what would be the best pushrod size ASSUMING THIS IS ALL CALCULATED CORRECTLY --- 7.325, OR 7.335

DG

 

"finally settling on 9 turns which was just slightly loose"
That doesn't sound like 0-lash, .050-.060 preload should be between 1.06 and 1.27 turns when tightening the rocker arm to 22 ft/lbs from 0-lash.

 

True, but zero lash should be 9 turns + a feeler gauge in between the valve tip;

For example cylinder one exhaust side was 9 turns (0.45) + I slid in a feeler gauge in at the valve tip that was .030 thick putting me at zero lash (no rocking movement at rocker)... then I added .060 for the amount of preload I need.

DG

 

Okay, I read you first post again and what you need is a 7.350" which should give you .060" preload, or right about 1.27 turns.

Try setting your PR length checker to 11 turns, it should give you right about 1.27 turns of preload.

 

Ill try and see what happens however I am surprised its not working already as I already added + .060 preload to the equation. That size push rod puts most of my values at around .065-.075 Preload, but puts the first cylinder at 0.085 preload. Would that be too much?

DG

 

What's the preload on a set of LS2/LS7 lifters... Maybe the fact that I have Morel's is changing how many turns I am getting?

DG

 

I had 7.290 with no preload. I put in 7.350 rods and ended up with about 3/4 turn required to set the rocker. I believe you are fine, the old small block Chevy usually only had about 1/2 a turn preload.

 

Does anyone know if using the feeler gauge on the valve tip side changes the mathematics of the pushrod length?

I just tired 11 turns on pushrod length checker and came up with 1/2 turn now from zero lash to preload....

DG

 

Technically 1 full turn from 0-lash to 22ft.lb. should be .0492 with the 8mm 1.25 rocker bolts, but Thunder Racing seems to stick by .047 preload with 1 full turn from their experience.
Yeah, something is definitely not correct because increasing the PR length from 10.5 turns to 11 should result in a increase in preload ; not a decrease.
I think you're right about the feeler gauge, your .030" feeler gauge is see the increased gap from the 1.7 rocker ratio, so the actually gap at the PR is more like .0176"; not .030".

 

Any lash measured at the valve tip would have to be divided by 1.7 to get lash at pushrod.

 

You have a length checker, why are you using that bolt-turn method and feeler gauges? Find the length rod to zero lash and add preload to that.

 

Rocker arm math.

The rocker ratio is 1.7 to 1.0
This means the distance from the trunion pivot to the push rod can be expressed as "1"
The distance from the trunion pivot to the valve stem can be expressed as "1.7"

The bolts securing the the rockers do indeed have a thread pitch of about one thread per .048 inch..
HOWEVER!!! We are not measuring preload at the trunion where the bolt is. As we apply preload the rocker pivots on the valve stem while the trunion and pushrod are both forced downward. The pushrod end gets pushed farther than the trunion. How far??

Glad you asked. Here is the math.
(1.7+1.0) divided by 1.7 equals 1.58

The pushrod will travel 1.58 times the distance of the trunion. Since we know that one full turn of the rocker bolt moves the trunion .048 the pushrod will move .078. 3/4 turn on the bolt will move it .057.

You should be good. I verified all this on my motor 10 minutes ago.

 

I thought something wasn't right. Time to recalculate



Every full turn of the push rod checker is .050, I would imagine every half turn is 0.25. The rocker has to be removed every time I set the push rod length. If I had to do that for all 16 it would take hours. I am trying to get the most accurate measure using math.

As for the bolt turn method, I am just using it to double check my work. If the pushrod length and preload is set correctly, I should in theory wind up with somewhere around 1.5 turns,

DG

 

EXCELLENT POST, THANK YOU!

Just to pick your brain for a second more. The rocker arm is tightened down to 22ft/lbs so we do not have to concern ourselves with thread pitch. Lets use the example from post # . The pushrod is just slightly loose allowing the rocker to rock at 9 turns.. or .450...... Add that to the 6.8 pushrod and we have 7.25. I take 7.25 which is the pushrod length thus far on the pushrod side. I need to add the valve side. The valve side has .017 feeler gauge in between the valve tip and the rocker. So would I divide 0.017/1.7 = 0.01 + 7.2768 = + the lash .060 = ... So we would be looking for a 7.34 pushrod @ 0.60 preload?

DG

 

Yeh, that looks right. You can get 7.35 rods which will give you about .090 preload. Or a little over one full turn on the rocker bolt. No problems.

 

I am actually going to sit down a recalculate all the valves. Then I will pick the ones the reoccur most often. I want to get all the pushrods in the ball park of .050-.070 preload.

Engine is all aluminum should I calculate in an Extra .010 for expansion or just let it alone?

DG

 

MALLDER THAT IS A BEAUTIFUL CHEVELLE YOU'VE GOT THERE!

I have a 1971, that's in need of a restoration. How do you like having all that new LS3 technology in a 40 year old car?

DG

 

I love the new technology stuff, but you give me to much credit. It currently has a gen 1 406 with a Holley stealth ram FI system. I am currently putting together a 408 LS3 with the new Holley ECM. I should have it swapped in this winter. Ls3 408 + T56 equals fun.

As for preload, I believe anywhere from .050 to .100 preload will be good. The lifter plunger has over .200 of travel available. It's not super critical that it be dead on .060. This setup was designed to be forgiving.

 

Mallder,

Thanks again for helping me with my equation. After recalculating all my valves, the range was from 7.33-7.36.... In short as mentioned prior, I need a 7.350 Push Rod!

I will order those tomorrow from Lunati,

Thanks,
DG