Push rod length question

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Feb 22, 2002 | 11:20 PM

88-406

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Burning Brakes

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From: Tracy California

St. Jude Donor '05

Push rod length question

If you have to be off center a little, wich way is better? Towards the intake/manifold or the exhaust side? mine are off towards the center of the engine and I'm wondering if I should get new pushrods or not.

Thanks

Kelvin

Feb 23, 2002 | 12:08 AM

CFI-EFI

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From: The Top of Utah

Re: Push rod length question (88-406)

I assume that this is at exactly half the valve lift? What I would do is adjust one rocker arm to the exact center of the valve stem at half lift and measure the valve clearence like you would a solid lifter cam. Then multiply that clearence by the factor of your rocker arm rato, to see just how much longer a push rod would be required. If in order to get the rocker adjusted properly, you have to tighten the adjustment, count the turns you adjusted, down from zero lash. Knowing the thread pitch (20?) and the rocker ratio, you can figure the how much shorter a push rod would be needed. Now you will at least know how far off you are. I don't know the limits of the acceptable tolerances. At this point, I would call the cam grinder and consult with them. By following the above proceedure, you'll have better information for them, than "off center a little".

You MAY get other, simpler answers, but this is how I would approach the problem.

Feb 25, 2002 | 01:05 AM

88-406

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Burning Brakes

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From: Tracy California

St. Jude Donor '05

Re: Push rod length question (CFI-EFI)

CFI-EFI

Correct me if I'm wrong, Zero lash is set on the backside of the cam right? Do you know if the roller will move outwards "towards the exhaust" only? or will it move allover?

KM

Feb 25, 2002 | 07:45 PM

CFI-EFI

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From: The Top of Utah

Re: Push rod length question (88-406)

The lash (or preload) is set when the lifter is on the heel (backside? OK) of the cam. By roller, I assume that you mean the point of the rocker arm that contacts the valve stem. Normally, it would start to the exhaust side (outside of the engine) of the valve stem in the closed position and roll towards the intake (center) of the engine at full lift. It does depend on where the rocker arm is in its arc. Try to draw an imaginary line through the center of the fulcrum (pivot point of the rocker arm) perpendicular to the rocker stud. If the point of contact between the rocker arm and the valve stem is below that imaginary line, then the arc of the pivot point will be below the center line of the circle that the arc of travel will be a part of. Therefore, all of the rocker arm tip motion will be towards the rocker stud. I wish I had a way to draw this. I know what I mean, but I doubt that I am doing a good job of conveying it. If the arc of the pivot point crosses the perpendicular line, the contact point will first move away from the stud, then begin to approach it. I'm going to quit here, in hopes you understand the concept I'm trying to describe. If I'm not being clear, feel free to ask further questions. In the meantime, I'll try to think of a better way to express it.

Feb 26, 2002 | 11:40 PM

EBSDallas

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From: Dallas TX

Re: Push rod length question (88-406)

Kelvin:

I assume you're talking about the line the rocker arm makes on the valve tip.

If the line is offset towards the exhaust (outside) of the valve tip then the pushrod is too long. If it's offset to the intake (inside) of the engine then the pushrod is too short.

Lunati has a very good illustration of this on pp 181 & 182 of their 2002 catalog.

I'm of the opinion that any offset, one way or the other, is not good. It should be centered.

Everett

Feb 27, 2002 | 01:33 AM

88-406

Thread Starter

Burning Brakes

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From: Tracy California

St. Jude Donor '05

Re: Push rod length question (CFI-EFI)

Thanks guys!

I have a checker, but somehow, after putting everything together I'm comming up a little on the short side and was wondering if I could get away with it. I'll measure it again and get new ones. Anyone need a set of unused 7.3".

KM

Mar 7, 2002 | 04:28 PM

JAKE

Le Mans Master

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From: Kempner Texas

Re: Push rod length question (88-406)

Here's what you'll be looking for:

With the lifter on the base circle of the camshaft lobe, the vertical centerline of the roller on the rocker should be slightly inboard (manifold side) of centerline of the valve stem.

At mid valve lift, the vertical centerline of the roller on the rocker should be centered over the centerline of the valve stem.

At full lift, the vertical centerline of the roller on the rocker should be outboard (exhaust side) of the centerline of the valve stem.

Hope this helps.

Jake

[Modified by JAKE, 4:52 PM 3/8/2002]

Mar 9, 2002 | 10:11 AM

mountainmotor

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Re: Push rod length question (88-406)

Kelvin,
It is good to see one that is paying attention to this crucial part of assembly.Expect many small base circle cammed 383's are down on power and longetivety do to not addressing these type issues.

Are you using a STD lenght valve?