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Rocker Geometry 03

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Der wohl beste Artikel den ich jemals über dieses Thema gelesen habe. Teil03

Installation, Measurement and Adjustment Tips

figure01

Here’s the typical stud mounted needle bearing roller tip rocker shown in its CLOSED position. Dimensions are not shown, because they are relative to your installation, which is relative to your NET valve lift (after lash, if used). The important dimension you want to find

and set, to be HALF your net valve lift, is the ROCKER HEIGHT, shown to the upper right, and illustrating the cumulative value between the ROLLER axis and TRUNNION axis. The other references are shown with regard to their heights above and below (respectively) the valve spring RETAINER. All measurements for where the trunnion is sitting, is referenced to the top of the retainer, marked here by TRUNNION REFERENCE. Raising and lowering your rocker’s tail by adjusting the Pushrod as needed, will set this. Although shown in this illustration, the adjuster doesn’t

even need to be here, as it will only get in the way. Let the rocker sit loosely on the study, with its adjustable pushrod and set this trunnion reference as needed to get the trunnion exactly HALF of your NET valve LIFT.

If using a HYDRAULIC TAPPET, be sure that it is fully extended during this check. You can prime the motor to do this. After getting your pushrod length, ADD .020” to allow for hydraulic tappet compression during actual engine operation. Order EXACTLY what you need for pushrod length, rounding off to the nearest ten-hundredth of an inch (two decimals).

figure02

This is the references to pay attention to for setting a SHOE tip rocker arm for your NET valve lift. As mentioned, finding the centerline of the fulcrum’s rotation is required first, since this cutaway only shows where the axis would be. This is not easily seen on the outside of a stamped or cast rocker body so, you need to simulate this rotation and make a mark to reference to.

figure03

As crude as this may look, this is an actual recreation of the drawing illustrated to author by the late Harland Sharp, explaining his original layout of a rocker silhouette on paper before scribing where the roller would be, and using an actual roller on his outline to calculate

this. Setting the roller’s diameter in direct position where the scuffing surface was at, instead of the roller’s centerline is what is wrong here. It was the beginning of a duplicated error that would last over a quarter century.

figure04

As explained in text, this is the real example of what you don’t want to do. But it is very typical, and the problem is propagated more by the head companies luring engine builders into a false sense of “acceptability” to such an installation, by selling their heads with studs in them that

have this stretched over placement, when they’ve had to move the pushrod for wider ports. The better alternative would be to leave the STUDS on the same centerline as the VALVES, and force stud rocker manufacturers to put their offset for the pushrod solely in the rocker arm,

with an offset cup. This is what Ford did on their N-Head, and it is the best way. Otherwise, you need a stand (shaft) system. Here’s the bottom line: You can never have an inline valve array cylinder head, like a SB Chevy, Ford, Mopar, etc., and NOT have the rotating axis of the

rocker’s trunnion be IN LINE with the CAM. Any twist at all, is COMPOUND geometry, and will make it impossible for the roller to lay flat on the valve, or follow the correct straight down path on the Y-axis.

figure05

This represents the STAND (SHAFT) rocker system, with only the most important things to be considered; namely, the STAND, the SHAFT and the ROLLER TIP.

The valve is shown for angle and location, and is shown here in the MID-LIFT point of motion, as noted to the side. THIS IS THE GOAL. The

roller and shaft are to be horizontally in line with each other as measured to a perpendicular right angle with the valve.

figure06

This shows a SQUARE (crosshatched) being used to lie against the valve and atop the shaft of your stand system. The stand is bolted to the head, and a shaft is laid in position to now make this check while the valve is in the CLOSED position. The cool thing about this is it can be done on a work bench. No springs, no anything, just the parts shown. If you are doing this with an assembled head, you can run the square

along the side of the valve springs, providing they’re uniform diameter. Otherwise, you may have to use the valve spring retainer technique from our stud mount example.

figure07

Here’s the stand (shaft) mount system shown in the close position, and our example here has a NET valve lift of .650”, or a MID-LIFT value of .325”. You MUST know this for your engine. It is impossible to set correct mid-lift rocker geometry without knowing your net valve lift. The ROLLERS are shown here in their two critical states, the dashed version representing where the roller will be when it has opened the

valve to FULL lift. But the valve is only shown in the closed position, as is the solid roller atop it.

First, take HALF of your ROLLER diameter; and HALF of your SHAFT diameter, and ADD them together, you will come up with a “standard.”

In this example, that standard would be .521”. This comes from a roller diameter of .480” and a shaft diameter of .562”. Why half? Because this finds our CENTERLINE for each. It is always the centerline that you are setting with rocker geometry.

Second, write down the height of the SHAFT’S TOP above the valve tip.

Third, you need to write down your MID-LIFT value (half net valve lift).

Here’s the TRICK:

With these three things written down:

1. Subtract HALF net valve lift from your STANDARD (.325” from .521” in this case, for a sum of .196”).
2. Subtract the sum of the above (.196” in our example) from your rocker’s height (for our example this would be .350” minus .196” = .154”. This is how much the SHAFT needs to be lowered to bring the centerline of the shaft, half of the net valve lift below the center of the roller). Usually, with shimming, you might have figures that make you ADD shims to get the correct value.

As you might notice, we are using the outside diameters of the shaft and roller to calculate this standard from, because these are easily measured with common tools. But it is their centerlines that are so important.

Jim Miller has been involved in the racing industry for more than 30 years. Jim’s roots stem from amateur racing, starting at 16 but by age 17, he was the youngest Ford trained line mechanic authorized to do warranty work on all of Ford’s factory muscle cars. At the age of 21 Jim was invited to take over Chief Mechanic duties for Dyno Don Nicholson’s Pro Stock Maverick, which Jim respectfully would decline – passing the opportunity on to Jon Kaase. Jim holds a number of patents for valve train design and is the proprietor of MID-LIFT Precision Geometry. Jim can be reached at 1777 Blount Rd., 501 Pompano Beach, FL 33069. Phone [phone].