Correct Roller Rocker Geometry – Push Rod Length

Do yourself a favour and spend time getting the geometry of your roller rockers right the first time. Incorrect positioning of the rocker arm will place extra stress in the valve train and cause premature ware. At worst it may contribute to a bent push rod or on high lift applications cause the rocker arm to bottom out on the pedestal and break. It is not as hard as one might think to do, especially if you have the right tools.
click on any of the photos for larger detail
The key to getting the geometry of the roller rocker right is working out the length of the push rod you need. Get this length right and your engine will love you, and you will love your engine’s power.
First up get yourself some light tension testing springs, these ones are from competition cams and they are a universal fit spring for all engines. All they are intended to do is just hold the valve up so you can test the valve opening when turning the engine over by hand during assembly. The springs are light enough that you can push the valve open with your finger and allow the lifter to follow the camshaft profile without compressing the spring inside the lifter.
Second tool to get is this push rod length check set made by Crow Cams, because it makes the job so much easier. You install one of these push rod length checking tools in place of the push rod with the roller rocker bolted down. Choose the push rod length checker closest to the size you think you need. The push rod length check tool is adjustable by holding one end and twisting the other and it screws out, thereby lengthening the rod.
NOTE: These push rod length checkers are not designed to work with the actual valve springs installed, they are made to use with testing springs only and to state the obvious you do not run the engine with them either, other wise you will stuff ‘em.
You will need to have the camshaft, a lifter and head installed on the short block to perform this test properly. Assemble the rocker arm and pushrod length checking tool with the lifter on the base circle of the camshaft and nip up the bolts so the rocker arm is not loose. Set the adjuster as in this case about where you have reasonable adjustment. If you have fully adjustable roller rockers with posi-locks and push rod guide plates it may take you a couple of goes of the following to get the roller rocker doing want you want it to.
At no lift we are looking to get the roller rocker sitting just back of the centre of the top of the valve stem.
As the lifter moves up the pushrod pushes the tail of the rocker arm up and the nose of the rocker arm moves down, pushing the valve open.
Now rotate the camshaft until the follower is at full lift, the valve is now fully open and the roller tip of the roller rocker should be in the middle of the top of the valve stem. If it is not, you can now screw the push rod length checking tool in and out and position the roller tip as close as you can to the centre.
You may note that the ideal position of the roller tip is positioned at full lift and not at no lift. Now rotate the camshaft slowly and watch the motion of the roller tip across the top of the valve stem. Essentially what you are looking for is the roller tip moving about the centre of the valve stem and not right out on the edge.
Now this example is on a fairly mild street Ford Cleveland engine with a lift at the valve of around 0.488. On higher lift camshafts you may have to set the push rod length so that the movement of the roller tip moves forward and back over centre as it presses the valve down. The important thing to watch on those applications is that the roller tip is not rolling off the edge of the top of the valve stem when both at no lift and at full lift. You are looking for some symmetry about the centre line of the valve in it’s operation. The beauty of all these cool test tools is you can watch the rocker arm behaviour in slow motion and take the guess work out of it.
You can read directly off the tool the length you need and if you are a bit anal like me you can double check this with your vernier. Now have a look at the push rod lengths available closet to the length you want. Now set the push rod length checker to that length and double check how it will perform. Remember if you are doing a mock up assembly without a head gasket, then when the head gasket is installed it will add ~ raise the head, approx 0.040
Now that you have your geometry right, check that the rocker arm has clearance in the slot that moves about the stud ~ pedestal area. If it is close or it is touching, make no mistake you are going to break something when you start this engine. Do something about it now. First double check you have the right parts, then rework pushrod length and if you still have poor clearances then check if you can get different rocker arms with a larger or longer slot, or talk to the manufacturers about relieving them further for clearance. You may need to use valve lash caps or for fixed pedestal and shaft rockers may require shims or spacers to help get the geometry right, the important thing is you found out now and not after something breaks.
Do the same test for both the inlet valve and the exhaust valve to be sure.
Make sure you read all the factory information when setting up your rollers. For factory information you can download the Yella Terra technical guide for setting up correct roller rocker geometry here.
If you have any tips of your own I would love to add to this article to help others, just send me an email and any pics are real helpful too ………….. Ron .

Roller Rocker Geometry

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Push Rod Length Checking Tool Set ~ 4 Rods with check range 5.800 to 9.800 inch

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