ANOTHER valve geometry question....

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Veepster
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ANOTHER valve geometry question....

Post by Veepster » Wed Nov 05, 2003 6:40 pm

so all of the threads and manuals and such state that the valve and the adjusting screw need to line up at 50% lift....

so what do you do with a set of rockers where the adjusting screw is on the pushrod end? :?


Peace........BartG

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amskeptic
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Post by amskeptic » Wed Nov 05, 2003 10:59 pm

No matter where the adjuster screw is located, the principle remains the same, you want the valve "striking surface" exactly midway through its arc with valve half open. All you need to do is visualize where best the geometry reduces side loads on the valve.
Colin

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Post by Black77 » Thu Nov 06, 2003 1:23 pm

Just a suggestion, but have you considered lash caps to prolong your valve stems? It's appearent that you have some seriuos ratio rocker arms to have them adjust from the push rod end of the arm. This way you can run a hot cam with a good lift and still have the roller tip in a strategic position on the tip of the valve stem to reduce side load and wear on the stem. Another advantage to these is, the oil that collects in the area betwee the tip and the cap when the valve is not compressed, is shot out in all 360 degrees onto the valve spring, therefore cooling it. :idea:

Guest

Post by Guest » Thu Nov 06, 2003 1:28 pm

Thanks guys..

Yes I have lash caps...and am currently waiting for a fax from Autocraft with geometry install instructions...I will post when I receive it..


Peace......BartG

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general_lee_jr
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Post by general_lee_jr » Thu Nov 06, 2003 3:29 pm

Those lash caps are awesome. Make sure you have the Porsche ones and not the Ford ones. The ones from Ford with crack and break apart. Just my personal expierence. :D

Eric Allred
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Post by Eric Allred » Thu Nov 06, 2003 8:52 pm

I align the adjuster to either the valve or the pushrod depending on the rockers.

I had to add .160" of rocker shim to get mine lined up. That's what I get for running Chevy valves and lash caps. :wink:

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A_67vdub
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Post by A_67vdub » Fri Nov 07, 2003 8:17 pm

OK, I've never set valvetrain geometry before, but I've read about it and I think I understand it.

So, when the valve is at half lift, a line going from the center of the rocker shaft (the point that the rocker pivots on) to where the adjuster/rocker tip touches the valve should be perpendicular to the valve. Is this correct?

So, how far do you screw in the adjuster? Half way? It seems to me that changing the adjuster will change the geometry, which brings me to my next question.

How precise do these measurements need to be? You somehow use a micrometer and protractor? Or just eye-ball it?

Thanks,
Steve

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Post by amskeptic » Fri Nov 07, 2003 11:52 pm

A_67vdub wrote:So, when the valve is at half lift, a line going from the center of the rocker shaft (the point that the rocker pivots on) to where the adjuster/rocker tip touches the valve should be perpendicular to the valve. Is this correct?
Yes
A_67vdub wrote: So, how far do you screw in the adjuster? Half way? It seems to me that changing the adjuster will change the geometry.
Halfway is a good start to give you flexibility. You understand that the adjuster only answers to the lash requirements. You have to trial & error a bit to get your shim thickness correct AT the correct lash with valve closed AND perpendicular at 1/2 lift. And yes, each change affects the others. This is the most enjoyable part of engine rebuilding . . . .
Colin

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ANT
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Post by ANT » Sat Nov 08, 2003 3:36 am

When building a radical engine another thing you need to look out for in the valve train is clearence between the rocker arm itself and the valve retainer.

Should they ever hit you may cause the keepers (locks) to pop out resulting in a dropped valve and a modern art sculpture where the engine used to be.

The devil is in the details, and the Deville is at the Caddy dealer.
-ANT

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Post by britegreenVWSB » Sun Nov 09, 2003 6:06 pm

A_67vdub wrote:OK, I've never set valvetrain geometry before, but I've read about it and I think I understand it.
I haven't either, and I still don't understand it.
So, when the valve is at half lift, a line going from the center of the rocker shaft (the point that the rocker pivots on) to where the adjuster/rocker tip touches the valve should be perpendicular to the valve. Is this correct?
I still don't get it. Perpendicular in what sense? Doesn't this mean that your engine has to be perfectly square? If it is tilted a little, how could you tell if the measurement is really "perpendicular?" This is something I don't think I can understand just reading it without someone showing me.

TG

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Post by amskeptic » Sun Nov 09, 2003 7:55 pm

The Hill Billy's Guide To Valve Geometry

Your rocker arm is a hammer. Your valve is a nail.
The hammer has to hit the nail square.
Your arm is a rocker. If you are too far from your nail you will bend it downwards. If you are too close to your nail you will bend it upwards.
You can easily see the arc that the rocker arm describes. You just want the rocker arm to be positioned "square" at half-lift on the valve.
That's all we are doing, trying to get the stupid arc to be exactly at the middle when the valve is half open. Then we get to divide the sideways forces in half so there is a little "up" against the guide and a little "down" against the guide during a valve opening event. If it is not correct, you'll quickly get too much "up" force against the guide or too much "down" force against the guide, and the valve will turn the guide oval, and then the valve will wobble and then the valve will burn and then it will drop and everything...
Colin

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ANT
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Post by ANT » Sun Nov 09, 2003 8:02 pm

That's too funny! And a beautiful description to boot. Kudos!

When we speak of "square" it's in relation to the parts in question and not the angle of the dangle of how the engine is sitting.

Think of it this way.....if you hold your arm straight out by your side while standing perfectly upright and then lean to one side, your arm is still square to YOU. See?
-ANT
amskeptic wrote:The Hill Billy's Guide To Valve Geometry

Your rocker arm is a hammer. Your valve is a nail.
The hammer has to hit the nail square.
Your arm is a rocker. If you are too far from your nail you will bend it downwards. If you are too close to your nail you will bend it upwards.
You can easily see the arc that the rocker arm describes. You just want the rocker arm to be positioned "square" at half-lift on the valve.
That's all we are doing, trying to get the stupid arc to be exactly at the middle when the valve is half open. Then we get to divide the sideways forces in half so there is a little "up" against the guide and a little "down" against the guide during a valve opening event. If it is not correct, you'll quickly get too much "up" force against the guide or too much "down" force against the guide, and the valve will turn the guide oval, and then the valve will wobble and then the valve will burn and then it will drop and everything...
Colin

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Post by britegreenVWSB » Mon Nov 10, 2003 1:14 am

amskeptic wrote:The Hill Billy's Guide To Valve Geometry

Your rocker arm is a hammer. Your valve is a nail.
The hammer has to hit the nail square.
Your arm is a rocker. If you are too far from your nail you will bend it downwards. If you are too close to your nail you will bend it upwards.
You can easily see the arc that the rocker arm describes. You just want the rocker arm to be positioned "square" at half-lift on the valve.
That's all we are doing, trying to get the stupid arc to be exactly at the middle when the valve is half open. Then we get to divide the sideways forces in half so there is a little "up" against the guide and a little "down" against the guide during a valve opening event. If it is not correct, you'll quickly get too much "up" force against the guide or too much "down" force against the guide, and the valve will turn the guide oval, and then the valve will wobble and then the valve will burn and then it will drop and everything...
Colin
LOL! That IS funny! Okay, I figured out the concept by drawing a picture of the arc of a rocker arm. However, I'm still fuzzy on how to actually measure/figure out if the rocker arm is perfectly perpendicular at half lift without the benefit of a few expensive tools. Would this work?

1.) Take a 6" ruler; hold it against the head with the valve fully closed and mark on the ruler the height of the valve stem fully closed.

2.) Install rocker arms with push rods. Turn engine over until full lift is acheived. Mark on ruler this location.

3.) The exact center between these two points is half lift. Turn engine over until this point is reached.

4.) Now I should be able to see if the valve stem and the push rod end of the rocker arm are perpendicular, BUT...

How do you determine if this line from the valve stem, through the rocker shaft, to the push rod end of the rocker are is TRULY PERPENDICULAR? What on the engine do you use as a reference to DETERMINE whether or not this theoretical line is actually perpendicular? Do you see what I'm asking? The reason this has me hung up is because no matter where you place the geometry at half lift, you will always be able to "draw" a straight line from the valve stem, through the rocker shaft, down to the push rod end of the rocker arm. The way I'm visualizing this you need to have a fixed reference point.

Also, isn't it enough to do this procedure on one valve per side?

TG

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ANT
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Post by ANT » Mon Nov 10, 2003 2:06 am

Frankly this is getting tiresome. Did the original poster do any research before asking this question? I mean it's one thing to ask a question (that's been asked 4398232 Katrillion times before) and then it's another to actually try to FIND the info. Everyone wants the easy way out.

Sorry for the rant, but come on people, that's why there are search engines. Yeah yeah, I know, we're all here to help each other cuz we're all feeling warm fuzzies of the fraternal order of VWs, but there's just no excuse for laziness.

You can start with these:
http://www.shoptalkforums.com/search.php

http://www.google.com

Hmmmm, could this possibilty be ALL the info that you seek that's been discussed HERE BEFORE?

http://www.shoptalkforums.com/viewtopic ... n+geometry

http://www.vdubn.com/sandrailing/engine/valvetrain.htm

-ANT (on a rant)

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amskeptic
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Post by amskeptic » Mon Nov 10, 2003 9:38 pm

britegreenVWSB wrote: How do you determine if this line from the valve stem, through the rocker shaft, to the push rod end of the rocker are is TRULY PERPENDICULAR? What on the engine do you use as a reference to DETERMINE whether or not this theoretical line is actually perpendicular? TG
Just to obfuscate a little for you, I don't do the perpendicularity thing.
Once I have determined that the valve is half open, I sight the adjuster screw along the valve stem. I'm looking for a straight line. I have not had a difficult time doing this by sight.
Colin

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