Got to thinking about how to do this right. How do readers here decide what height to set, and how does one measure it? My thought process is that spring pads have to be a uniform height relative to the rocker shaft center-line to use them as a consistent reference point for setting stem height. If they aren't, stem height from valve to valve will change relative to the rocker fulcrum. Then, according to one philosophy, the stem height should be set so that the rocker tip is perfectly centered on the valve stem at exactly mid-lift. Or, if one believes in faster low-lift motion, maybe the rocker tip is set to center of the valve stem at zero lash? That should make things a little quicker assuming the rocker tip doesn't leave the stem tip at max lift. Probably not a problem unless you have a big sold roller (then lash caps if needed), but also in theory harder on the valve guides, as if it would matter in this application. Does anyone pay much attention to this, and how/where do you decide to set the rocker tip placement on the valve?
Your mixing terms here kinda . there is “ Overall valve installed height “ which WILL effect rocker arm geometry and max lift and then there’s valve spring installed height to achieve a needed spring pressure. These three things out of the many that effect overall valve installed height but yet do not effect spring rate are 1) milling the head. 2) decking the block. 3) placement of keeper groove on valve stem. This list of factors effect both spring pressure and installed height/ geometry. 1) overall valve length. 2) freshening up the valve job will sink the valve deeper into the chamber . 3) adding a back cut can sink the valve deeper. 4) a multi angle valve job will sink the valve deeper in conjunction with or without number 3 above. 5) on all stock factory heads and on some out of the box aftermarket heads the OD of the valve job is less then the OD of the valve . Maximizing this OD to better use the valve size and in turn air flow numbers will put the valve deeper into the chamber and make for a taller installed height. 6) in opposition to number 5, installing bigger valves will make for a shorter / lesser installed height. 7) keepers can be had to give a .050” taller spring installed height. 8) custom valves and modifications to aftermarket replacement valves can provide a whole range of changes in terms of keeper groove heights , and keeper groove to valve top heights. 9) valves with hardened tips can be safely shortened by .025” to help set the geometry.
Steve, those are all good explanations on how a person can either deliberately or accidentally change the final resting place of the valve tip and get springs installed at the right height afterward. Although I do have a hard time visualizing how a back-cut or multi-angle valve job can sink a valve as long as the seat has already been established and is not [re]cut along with either modification. Setting spring height has lots of helpful options once the stem tip location has been set. What I am feebly attempting to ask is how do our more careful builders here decide WHERE the final location of the valve tip shall be set while using all of those methods you describe in order to do so. Primarily to get the desired rocker geometry/contact point with the rocker tip. Is there a useful reference point one can measure from (like spring pads), without scrutinizing the tip-to-tip rocker contact on every single valve for a given setup, considering some of the variables Steve has mentioned? Are the cam, block deck, head deck, and rocker shaft pads all adequately true with each other in most instances that it doesn't interfere with critical valvetrain geometry on a max-effort engine? Are rocker dimensions different on the valve end of any of the aftermarket rocker brands or ratios? Does anyone manipulate the valve tip locations based on the lift of a given cam to be installed? e.g. tall valve stems, lash caps, or ?? All things I can think of that might influence where one desires the closed position of the valve tip. Does anyone do anything different than a simple "center of the valve tip at half-lift" rocker tip travel pattern? I realize this is an esoteric question for most builders, but hey, there haven't been any new tech questions posted in the race section for a long time, so I figured, "why not ask?" Surely, some folks have investigated and decided at the very least, that a given valve tip location is appropriate for all but the most radical of setup, and if so, I'd like to know how they determine that measurement has been achieved. I got to thinking about this why thankfully being able to duplicate an already-established stem height while replacing some intake valves.
The factory provides the spec for proper tip height. There is no reason to deviate from that unless there is some radical changes made.
So the "Racing 400/430/455" section is not the place to inquire with those who are likely to have made "radical changes?" Please define what is radical so I know whether I qualify or not.
Adam, Valve tip height is an assembly spec, and not something one would use to alter valvetrain geometry. Head deck side cutting, block deck cutting, lifter cup depth and design are all issues that are handled on the other side of the rocker with pushrod length. Buick heads, like all heads that use single shaft rockers, run into issues with the geometry due to the fixed fulcrum point of the rocker. Once you get that valve tip too high, you create an over-center situation with the rocker arm. This causes all kinds of havoc, and must be avoided at all costs. This is really a problem with iron heads that have been thru multiple valve jobs. I actually have had to use 409 Chev valves in the past, to get the valve tip height back into spec, they are shorter than the Buick valves, I don't recall exactly how much.. The Buick Spec on the BB for valve tip height is 2.050. Measured from the valve spring pad to the tip of the valve. On an iron head, you can go out to about 2.110 or so, before you run into issues. Farther on an aluminum head, but we like to keep them down near the minimum. Not really an issue on alumium heads, because you can just replace the seats if they get sunk too deep. The TA alumium heads have rocker stands that are .100 taller than the Iron head.. this was done to allow more "room" before you get in trouble with tip height. As a general rule, cutting the valve spring pads, to increase depth, is a no-no. Not only have you lost the blueprint dimension for the pad, often on ported heads, there is not a whole lot of material there to begin with. Lash caps are used only on Titainum valves, all our Buick valves have hardened tips. A custom valve would be required to run a lash cap, the oal would have to be shortened to accommodate the .045 or .060 thickness of the lash cap. Keep the valve tip heights correct and consistent, get the right pushrod in it, and get your valve spring pressures right and you will not have any issues. It is really rather simple on these engines, due to the limitation of the single shaft type rocker system. JW
Adam I'll try to take a picture of my rocker/spring/valve tip setup on the dragster this week. That would be radical Beyond radical is the new Super Wildcat heads from KBR. Even with those heads the roller is staying in the center segment of the valve tip . JW can definitely put out a clear description and reasoning for why the math is so important. Beyond radical.
Thank you, Jim for some thoughtful explanation. Obviously, it is a compromise, and if a person can't adjust the fulcrum height, you're not left with much. My estimate is that relatively low-lift cams that complement available port designs are the reason this works acceptably. My point on the spring pads, is if they are the reference point for valve tip height, they had damn well better be in-line with the camshaft center line, or else pushrods will need to be a different length from one end of the head to another. The spring pad modification limitations must be a challenge for people with some roller cam spring combinations, but maybe not for the reason up top. I watched a video on Jesel shaft rocker conversion installation; both they and T&D have specific jigs to set the shaft/valve tip geometry for a fairly narrow range of maximum lifts. It does not add up that we have some sort of special luxury that these other shaft systems don't. Apparently that "luxury" is that we're stuck within the limitations of what we have. Although I seem to remember reading of some folks shimming shafts on iron heads way back when. TA's modification to the rocker stand height is evidence of that we don't have a luxury, other than their effort to correct geometry for cams with an anticipated higher lift. I'm using a straightedge to maintain consistent stem heights on my 8 new intake valves. A little tedious (not as tedious as cutting down oversize valves), but so-far, within a few thousandths. But as I also mentioned, this wasn't in an effort to re-engineer my engine, but to get a better understanding of what the limitations and compromises of what we have are.
John, that would be really cool! I like to understand why things are done the way the are, and not just ignorantly place faith in engineering designed to do something that has since been changed. Seeing what is done for the outer limits is even better. Do the KBR heads have any repurposed rocker hardware or is it all bespoke?
Jesel did the rocker design for KBR based on the valves in the heads here for the 560 NA going in the dragster. I'll get a picture when I get back to the shop.
