charliemccraney (5/28/2009)

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Ted,  From your article, it sounds like the first thing I need to do is observe how the roller is traveling across the valve tip. That will tell me if I have to shim or trim the pedestals. When you are setting the geometry, how much do you change the height at one time?

Charlie.  I’m taking this opportunity to start a new post with your question.

 

Although I have a dial indicator setup that tells me exactly how much the geometry is off by measuring the rocker tip location both at zero and full lift, changing the rocker stand height in 0.025-0.030” increments will allow you to sneak up on the ideal position.  When doing the initial setup, using washers of equal thicknesses or valve spring shims work well for raising the height.  By working with only two of the stands while doing this also simplies the process.  If you’re needing to lower the shaft, then having a pair of stands that are cut 0.150” shorter than stock and then raising these in set increments will allow you to determine what the final height of the stands needs to be.

 

charliemccraney (5/28/2009)

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Now that I'm focused on the geometry, I took a closer look at the pattern on the stems. It is slightly off center - the pattern is shifted towards the exhaust side of the heads. Does this tell me anything regarding the geometry?

Due to manufacturing variances and/or any guide work that’s been performed since the heads were new, the rocker tip can be potentially sitting anywhere on top of the valve stem.  But if the rocker tip is sitting outboard of the valve stem center at zero lift, then initial indications would be that the shaft is sitting too high.  In an ideal world, the rocker tip should be sitting slightly inboard of the valve stem center (towards the intake) when the rocker is at zero lift and where ideal valve train geometry is present.  In a less than perfect world, aim for the smallest contact patch on the valve tip between zero and full lift regardless of where the valve tip resides (within reason of course).

Another tip is to run a straight edge across the valve stem tips with the rocker assembly removed to insure that all the valve stem tips are indeed at the same height.  If you find that these are at varying heights, then it’s going to be difficult to accurately set up the valve train geometry.

Lorena, Texas (South of Waco)

I finally got to checking the stand height this weekend. It looks like I need to shim them .125" The pattern was about .039" wide at that height. I had a caliper and my eye ball to use for measuring the pattern. Any point above or below that height and the pattern widened. With no shims, the pattern was about .078" wide.

Do these numbers seem to be in any ballpark, Ted?


Lawrenceville, GA

Charlie.  Sounds like you’ve found the ideal rocker stand height at this point as you’ve found the narrowest sweep pattern on the valve stem tip.  While ideal rocker arm geometry will be with the least travel occuring on the valve stem tip, the maximum lift that's possible at the valve will also be achieved.  Any rocker stand height adjustments above or below that ideal point will make net lift at the valve also less and this is assuming the pushrod length is being readjusted to correspond with the change in rocker stand heights.

 

The rocker arm sweep mesurement itself will vary depending upon the lobe lift of the camshaft as well as the rocker arm ratio being used so I don’t have a specific travel or sweep value that I’m looking for.  Just go for the narrowest sweep and if you have to err to one direction or the other, always err to the rocker shaft sitting  at a taller height than on the shorter side (speed secret #1448).  Be sure to readjust the pushrod length to keep the rocker arm ratio the same or you’ll lose some of the valve lift gain you get in raising the rocker stand heights in that the rocker ratio will reduce as the adjuster is lowered to reach the same or original length pushrods.

 

While you have your aluminum rocker arms off the vehicle, be sure to check the rocker arm to shaft clearance and make sure you have at least 0.004”.  Have seen some recent issues with the aluminum rockers where the clearance has been on the tight side.  Be sure to especially check the rockers with the 'centered' adjusters as those are the ones I've had particular issues with.

Lorena, Texas (South of Waco)

Thanks, Ted.



I was just thinking, I was running out of rocker adjustment at .150" of shim. I probably could not have gone much higher without changing to a longer pushrod. Will the change in the rocker ratio affect my findings significanty? Should I check it again using my adjustable pushrod so that I can maintain the same ratio?


Lawrenceville, GA

Charlie.  The pushrod being too short will affect the rocker arm geometry to some degree.  You’ll likely find that once you increase the pushrod length so that the adjuster is at the original location before raising the shaft assy that the rocker shaft is now sitting slightly on the high side.  This is due to an increase in rocker ratio that occurs when raising the rocker adjuster nut to back where it was.  In this instance and by increasing the pushrod length, you are restoring the rocker ratio back to where it was before you started relocating the shaft assy for a narrower sweep across the tip of the valve.  But if the rocker shaft is no more than ~0.020-0.030” higher than that perfect spot where the sweep is at a minimum value, then I’d suggest leaving it alone as low lift flow at the valve is actually improved by the rocker shaft being biased to the high side.  I hope that's not as confusing as it sounds.

Lorena, Texas (South of Waco)

One more thing came to mind. When shimming or trimming the stands, does pushrod clearance usually become a concern?


Lawrenceville, GA

Pushrod clearance at the rocker typically isn’t a problem but especially check it when moving the adjuster up as that’s where any clearance issues will come to the forefront.

Lorena, Texas (South of Waco)

Oh, sorry, I meant pushrod to head clearance.


Lawrenceville, GA

I checked again with the adjustable pushrod and it seemed to like .150" while maintaining the ratio. That seems like a lot of shim.


Lawrenceville, GA

Charlie.  Non-stock valves?  Anytime longer than stock valves are substituted for the originals, the rocker shafts do need to be adjusted so that they are appropriately higher.  The stands on my roadster engine place the shafts ~¼” higher than stock simply due to longer valves which in turn were required by the valve springs which in turn were required by the 0.600” lift at the valve.  The dominoe effect was is in full force here in that one thing led to another and another and……  The point being that each change required another and ended up being a cascading effect.

 

As far as pushrod clearance issues in the heads go, they actually diminish as the rocker shafts are raised.  Not raising the shafts while going with longer than stock valves will tend to aggravate any existing pushrod clearance problems but by the same token, lengthening the pushrod will also tend to improve the situation by lieu of moving the adjuster more outboard or closer to the intake manifold.  I’ve found that these clearance issues when present are typically at the inside bottom of the pushrod hole at the deck surface of the head so I just try to remember to simply grind in these areas while the heads are disassembled and being worked on.

Lorena, Texas (South of Waco)