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push rod length

 
foxcamaro81
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Posted: 04/24/12 02:08 PM

what push rod length should i use with vortec heads and a comp 268 xe cam on a goodwrench 350 or will i have to get a push rod lenght checker?  

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pepsi1
I mow my lawn and find Chevys
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Posted: 04/24/12 04:07 PM

foxcamaro81

Try this first:

I would check the valve train geometry with the stock push rod...

Bring #1 cylinder to TDC on the compression stroke. Clean the tops of the #1 cylinder in/ex valve tips.. Now using a magic marker or blueing..Something that will stay on the valve tips...Now carefully adjust your valves on that cylinder and with that being said rotate the engine BY HAND atleast 2 to 3 full revolutions. Make sure you bring it back to TDC....

Now remove the rocker arms and check where the rocker arms left a mark on the valve stem tip..Please Note for correct geometry the marks should be in the center of the valve stem....If not you will need to adjust it accordingly  

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Pontiacman2
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Posted: 04/24/12 06:23 PM

1 The first step is to install a solid lifter and an       adjustable pushrod. Mark the tip of the valve with a marker.
Verifyvtgeometry1

2 Install your rocker arm and set it up with zero lash. Rotate the crankshaft clockwise several times. Remove the rocker arm. The contact pattern of the rocker tip will be where the marker has been wiped away from the valve tip. The pattern should be centered on the valve tip, and as narrow as possible. If it is not, experiment with varying the pushrod length to yield the best pattern.

Verifyvtgeometry2

3 Rotate the crankshaft clockwise several times. Remove the rocker arm. The contact pattern of the rocker tip will be where the marker has been wiped away from the valve tip. The pattern should be centered on the valve tip, and as narrow as possible. If it is not, experiment with varying the pushrod length to yield the best pattern.
Verifyvtgeometry3

4aPushrod Too Long: Notice how the pattern is wide, and shifted to the exhaust side of the valve tip
Verifyvtgeometry4a

4bPushrod Too Short: Notice how the pattern is wide, and shifted to the intake side of the valve tip.
Verifyvtgeometry4b

4cPushrod Length Correct: Notice how the pattern is narrow and is centered on the valve tip.
Verifyvtgeometry4c  
Pontiacman2
Pontiacman2

Professional Hi-performance engine builder

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pepsi1
I mow my lawn and find Chevys
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Posted: 04/25/12 12:43 PM

foxcamaro81

Now how about that Pontiacman if that doesn't tell the story I don't know what will as always a picture is worth a thousand words....

Thanks Pontiacman can't get any better then that!!!

Bob  

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Pontiacman2
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Posted: 04/25/12 04:06 PM

Icon Quotepepsi1:
foxcamaro81

Now how about that Pontiacman if that doesn't tell the story I don't know what will as always a picture is worth a thousand words....

Thanks Pontiacman can't get any better then that!!!

Bob


You bet bud.  
Pontiacman2
Pontiacman2

Professional Hi-performance engine builder

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foxcamaro81
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Posted: 04/26/12 01:14 PM

thanks for the info guys  

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Pontiacman2
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Posted: 04/27/12 05:39 AM

Your welcome.  
Pontiacman2
Pontiacman2

Professional Hi-performance engine builder

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johndavid24
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Posted: 01/09/13 04:07 PM

I know this is an old board but, this info above sent me in the wrong direction.
Shorter or longer pr's will move the sweep, but first make sure the sweep is moving out towards the exhaust then back to the intake, my sbc sweep from intake side to close to the exhaust side,according to many post online ,i was told to shorten the pr length, what i actually needed was to lengthen the pr by .300 .The sweep was still towards the exhaust side but it was very narrow.

The quickest way I found to get close is to use a square , set the rocker on the stud without the pr, set its pivot heigth 90 degrees to the valve stem approx 1/4" down on the stem to simulate the valve being open halfway .Measure the heigth at the pr end of the rocker, then reinstall the pr and rotate the engine to get close to midlift, it should be very close to the measure at 90 degrees.I know this a very rough measure but it will tell you if your way off like I was.
ie.... roller tip moving from intake towards exh only,way too short. moving from exh side to int only,way too long.

Simply to say shorter or longer pr's according to sweep pattern left with a black marker may not be wrong but it's surely not complete .

ps... to get the sweep in the post above, i think the pr length is way off! prob .2 to.3 idk... just a quick look?  

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pepsi1
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Posted: 01/09/13 04:46 PM

You did mention the type of cam your using.

Stock length for a flat tappet cam for SBC is 7.765"

Stock lemgth for a factory hydraulic roller is 7.178"

Stock length for an LS1 5.7L is 7.400"

Stock length for a BBC is 8.250"int and 9.250"ext.

Bob  

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johndavid24
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Posted: 01/09/13 10:09 PM

The geometry works the same with either, just different specs.
Image 4a shows the sweep 3/4 of the way across the stem, that 3/4 point should be where the rocker tip starts it's return sweep,not the end point. Knowing the roller tip can roll out to the 3/4 mark tells me that it can't be centered on the stem, as image 4c shows.

There are some web sites that show this in detail,near the center of page ,the first diagram i believe. circletrack.com, Rocker Arm Geometry And Valvetrain Alignment

http://www.circletrack.com/techarticles/ctrp_0611_rocker_arm_valvetrain_geometry/viewall.html

I am only trying to help, but the info above doesn't make sence  

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pepsi1
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Posted: 01/09/13 10:58 PM

Please bare with me okay. The sweep you refer to is that when the rocker arm goes across the head of the valve then stops on the valve?

Roller rocker arms will do that. Especially on a high lift Mechanical Roller Cams. It has to sweep it has a lot of built in ramp clearance. It will actually slap the valve open, and pop it closed, the spring controls that...

I haven't used a .100 longer push rod until the lift of the cam gets near/or over .600 thousands lift. Unless the heads are angle milled then the geometry will be off. If you get the same results go with it. As long as the Rocker Arm is in the center of the of the valve the geometry is correct.
  The valve can be actuated at either the outer tip or the inner tip of the valve head. But it stress's the valve train. We want to get the geometry correct to avoid valve guild wear and get the complete valve train working in harmony. There is a lot more to it.

Bob  

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johndavid24
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Posted: 01/10/13 02:10 PM

having the roller tip centered on the valve is ideal, and if you have a sweep around .080 or less ,it could very well be correct.imo
Having the valve stem and push rod close to 90* to the fulcrum @ half lift will give the best results.Here is another site that shows what i'm saying..



according to eveything i know and have read and used .... don't worry about centering on the stem,try to get 90* to the fulcrum at half lift, that will give the narrow sweep , I think we are talking about the same thing , described differently  

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skyeking
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Posted: 01/11/13 03:54 PM

And Interestingly precice...Hi Guys!!! All the best.  
skyeking

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johndavid24
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Posted: 01/15/13 02:19 PM

I'm not trying to be precise, fyi.
Since you obviously believe that if your pushrod moves towards the exh means you need a shorter pushrod , you just keep on shortening it, let me know how it turns out.

For somebody that has never done this before, they may just shorten the pr and it may 'pull' the pr in, but it's wrong, and the geometry wil be worse then before. Am I the only one that see's this?  

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pepsi1
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Posted: 01/15/13 03:59 PM

John
Sorry I didn't answer you sooner I'm having a heck of a time getting on any of the 18 Forums I'm on. I think it's just a termanoligy thing. Not to worry. You follow those pics and PMan will get you there. Big mechanical roller cams slap the valve open and they pop the valve shut. Thats why the correct valve spring NO MATTER what cam or Cam Maker you use you need the correct valve springs for that cam. Thats really the controlling factor. I always use a rev kit. Just in case a lifter breaks (BEEN THERE) you don't loose the engine, when the oil pressure drops to Zero. But I'm talking about big mechanical sticks, over lifts over .600+.

Bob  

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