Probably a stupid question...
I know how important is the correct push rod lenght.
I 99% understand the use of a adjustable pushrod to find the correct lenght.

What I don't understand is how simply errors can be made...
I explaine:

Roller block stock lenght 7.200 (or 7.190) this measurement is not important now to focus on my question.

Say we WILL know that at the end the correct lenght WILL be 7.300
(now we don't know that this 7.300 will be correct lenght to use...)

Say I start with the adjustable pushrod set to a lenght of 7.600

When I put the rocker on the stud , PROBABLY I will be tempted (or forced) to put the rocker on the very first threads of the stud, placing the rocker on a very high position (related to the full lenght of the stud).

Now I search for the 0 lash (fine tuning the adjuster of the rocker).

Now I check for the roller tip position on the valve stem, and I probably see that the roller tip is correctwith the lifter on base circle, mid lift and on max lift.

This can lead me to think that I found the correct pushrod lenght.... (even if we know that the correct lenght is way less...!)

We "think" that the correct leng is 7.600 JUST ONLY BECAUSE WE placed the rocker on a very high position (related to the full lenght of the stud).

How to avoid this kind of error?

 

Probably because you want to see the sweep of the rocker's roller across the valve stem to verify the correct length of the pushrod.

When the lifter is on the base circle of the camshaft lobe, the rocker's roller should NOT be centered on the valve stem tip. Ideally, it's only centered on the valve stem tip at mid valve lift.

Jake

 

You won't get that kind of error. The only way this kind of 'error' would be possible is if you raised the valve stem by the same amount. When you raise the point that the rocker seats on the pushrod, the side that rests on the valvestem moves toward the rocker stud. You cannot raise or lower one side without affecting the position that it will rest on the other side.

 

The height is determined from the pushrod and the valve stem, as the pushrod height changes based on the camshaft, the bearing/stud only preventing the pivot height of the rocker arm from changing thus making the roller tip move on the valve stem/depress the valve stem as the pushrod height increases.


Simple powerpoint image.
Note: I didn't necessarily try to make the alignment on the valve stem tip accurate, though it does follow the same general pattern of movement it should still be mostly centered on the valve stem.

The key here, is that with or without the rocker stud, the pushrod on the base circle and the valve stem closed have the same starting height (left) with and without the rocker stud. Get it?

 

Put some whiteout on the valve stem tip and cycle the engine through on complete 4 stroke event. The rocker should leave a mark on the valve stem tip that is centered around the valve stem tip when you have the correct length pushrod. If it is not centered you need to either lengthen or shorten the pushrod until you have a centered sweep pattern.

If you have the rocker too high the pattern will not sweep across the center of the valve stem, it will sweep across the inside edge towards the valley of the engine.

 

You should always start out with the adj pushrod set to stock length.

 

Thanks to all!
Thanks, I realized just some seconds after i posted this question that the valve stem it's always in tha same location.
Mr.Dave you explained it very good !

A question :
How this can be done using a rocker different that the one used on the build?
I say... how the blue plastic one (used on soem kit) can be used with accuracy?
I think that the best way is to use the rocker you are using in the build.
-Beppe-

 

http://www.compcams.com/information/Products/Pushrods/

 

The following link has some great information I got from Lloyd a while back when doing my heads and cam, and has some nice photos that illustrate things a little better.

http://www.biggrizzly.com/Lloyd_words.html

 

 

good info,im in this process now
i need a set of new pushrods but i donno which lenght i need !
I have a 355L98,ZZ4 Cam,and now 1.5 roller tip rockers on exhaust and 1.6 roller tip rockers on the intake ...will i have to buy different pushrods for intake and exhaust?
at this time im running the stock ones

 

they are usually close to the amount of correction done to the motor. plus, they only come in .050 lengths.

on a stock roller block you can almost bet on 7.150" rods if you shave the heads, deck the block and run thinner head gaskets. if you only do minor things like a head resurface with stock gaskets and a stock block, chances are the stock rod will fit better. this all assumes that you have a standard head casting with stock length valves.

using an adjustable pushrod and looking for the correct sweep is a good idea, i just have yet to see a stock valve length L98 or LT1 use anything besides 7.150 or 7.200

you would have to shave some serious numbers to approach a 7.100 and 7.250 would only be needed if you added length to the valve or somehow raised the head higher from the lifter.

 

Shouldnt matter what brand rocker arm you use as long as its SBC/ratio.
With the typical pushrod length checkers they recommend you not turn the motor over, youll break the tool.
If you arent running much valvespring you can probably get away with it though.

Solid roller spring, youll hear a satisfying "snap" if you try to compress the spring with it. Just something I heard
If the tool they give you is too wide for the guidepates if you use them just pull the stud and guidepate out, put a headwasher (same thickness as the plate) under the stud and try it.

 

thanks,i think i'll stay with stock length and see what's going on
thanks

 

Usually aftermarket heads requires .100" longer pushrods.

On my engine (made with pride by Pete K) the pushrods are 7.400 due to the fact that the cam is a small base circle cam.

 

Calderone
I was just wondering why you put the 1.5 rockers on the exhaust and the 1.6 on the intake? I know cars with LTR style intakes usually favor higher lift on the exhaust side.

 

Hi uv!
Because of this :

ZZ4 cam :
Duration @.050 208/221
Lift: .474/.510
112 LSA

 

Makes sense now didn't know the lift on a ZZ4 cam, should of checked befofe asking. Thanks for the info. If I ever get around to my my intake,cam and heads swap I'm going with an LT4 hot cam with 1.5 rockers on the intake 1.6 on the exhaust which should give me similar results.

 

i was about to get that Hotcam
But it stoles a lot of low-mid range torque and put it up on the
range,with my stock TPI intake that would be useless..so i'll stay with the ZZ4
for now.-

 

RPM range for the hot cam is 1500-5500rpm and I'm hoping with the heavily modded runners, plenum and base I should be able to pull close to 5500.








Thought I had pics of the base too,but apparently not. It's an original style Accel base opened to a Fel-pro 1205 on the head side and almost 1.9" on the runner side.