Help with pushrod length and geometry
 

Hello everyone,

I am a new poster on these forums, but have used them extensively before for information. I have a 1975 Trans am that I am knee deep in building a 465ci stroker motor for and I have a question about pushrod length. My usual method to determine pushrod length is the contact area on the valve stem, and trying to get it exactly centered. I looked up the correct way to do it with a roller cam, and I did the valve geometry method, where you find half lift and get the valve and rocker at a 90 degree angle. However, both of these methods have resulted badly. The contact area method landed me with a 9.5 in pushrod and horrible geometry, and I had no thread left on my poly locks. The half lift method resulted in a 9.1 in pushrod and a contact area (see pic attached) on the intake side and wider than I want. I have 1.5 ratio Comp rockers, and comp evo hyd roller lifters. Am I doing something wrong or can I just run it with the 9.1 in pushrods? I have ample thread with those (around 6 turns to tighten on poly locks. Also my poly locks are too long and hitting my OEM chrome valve covers, can I shave them down to get rid of 3/8 of thread so I can use my stock valve covers? Sorry I know this is a long post but I know somebody had probably had my same problem before. Thanks.

Let’s forget about the poly lock issue just for now.

With any of the push rod lengths you have tried how close have you come to the amount of lift you should have in regards to contact patches one way or the other?

Contact traces that are towards the intake port side of the head mean your push rod is too short , if it’s the other way and on the exh side of the stem then there too long.

Make sure that if your using roller rockers that the rocker body is not touching the retainer.

As the ratio of the rocker goes up above 1.5 the push rods static position gets drawn closer to the rocker stud.

Do not minus your cams recommend lash out of your checks your making .

In general you want the narrowest patch you can achieve while being within .040” or so of the valve stem center while getting within .012” to .015” of the lift you should have.

If I get the contact patch in the middle of the valve stem, the pushrod is 9.5in and i run out of thread on my rocker studs. It’s not hitting the valve retainer anywhere. Currently with the 9.1in pushrods, The path is .030 away from the center of the valve stem but I am only getting .375 in of valve lift while I should be getting .521. Should I get 1.65 ratio rockers instead of the 1.5s that I have now?

What!?
Are these fully roller rockers?

If so do you have the rocker trunon facing the wrong way with the flat side facing the poly lock?

Yes they are roller rockers, and the flat side of the trunion is facing the polylock.

FWIW Once you get your geometry set you should be able to fit under stockers, I have all this going on and they fit with 2 VC gaskets

I measured again and I do in fact have the correct amount of valve lift. I am just going to run it. I attached a pic of the wear pattern. I am going to order a slightly shorter polylock and with thick valve cover gaskets the covers do fit! So problem solved. Thanks guys for the input.

NO! You do NOT have the problem solved!

Your issue is the way your positioning the flat side of the trunion facing the poly lock.

The side of the trunion with circular depression in it is where the poly lock goes into.

The proper ploy locks will fit in that depression.

Is the plunger in the hydraulic lifter at the bottom or is the clearance reduced when checking the push rod length?

Just to say, and I know how many feel about this, but the half-lift method is the 'proper' way to achieve proper geometry/pushrod length.

https://www.youtube.com/watch?v=o5is9BsH5OU

You don't have to use 2 VC gaskets, just get the double thick ones if you need more room. No mess gluing them together, etc.

Double check trunnion orientation.



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Yes I have the polylocks sitting in the circular depression, I thought that was what you meant by “flat side”. I have some thick cork gaskets that I will use on my vc and they fit good.

I had to get longer Rocker studs for my #64 heads. I belive I have 9.4 or 9.5" pushrods as well. Valves are longer. Dave at SD Perf installed longer valves for greater lift. Thus requiring longer pushrods and rocker studs. ARP sells them in all sorts of lengths. I found my pushrod length using the half lift method, then seen how short my studs were. Then ordered of ARP the ones that would ensure they worked with the polylocks

The lifter is pumped up and has some springiness to it. I checked the pushrod length using the geometry method and came up with 5.21 turns on the polylock and got a length of 9.1 in for my pushrods.

My contact patch was much larger when I got the contact area in the middle with a 9.5in adjustable pushrod. I really don’t want to have longer rocker studs because that would mean tall valve covers and thus modifications to my a/c bracket and power brake booster.

It seems that the overall reach of the rocker arms is not optimal for the geometry of Pontiac heads. I looked in the instruction manual for these rockers from Comp and they only list Chevy and ford applications, no Pontiac engines. However, they were provided to me by butler and there are obviously others who have had success with these. There are a million factors that can determine these things I guess.

They are probably 1.6 Ford rockers .

There is no right or wrong rocker geometry. The narrow-est mark in the centre of the valve tip is very popular but it takes no account of what is happening on the prod side. What happens on the valve side in terms of lift & area under the curve ...... is times one. On the prod side it is times 1.5 [ for a 1.5 rocker ]. David Vizard's BB Chev book has a chart of various rockers he tested. Lift at TDC & max lift was tested as well as the overall ratio. The numbers are all over the place & show you can spend a lot of time to gain nothing except a warm & fuzzy feeling.

Put a .050 lash cap on and try it again. You have the right retainers

This is not true, there is definitely a right & wrong. There may be a little wiggle room due to the fact that pushrods generally come in .5 increments, but if you're more outside of that, it's 'wrong'. Or at least not 'optimum'.

The problem with the Comp roller tip arms is really more about the varying ratio than the usual regurgitated rocker ball issue, which some say they have corrected (to a degree, pun intended).

Just because Butler offers them they may not be 'optimum' for YOUR engine. Lower lift cams and less aggressive profiles won't be impacted as much as the opposite. So what 'works' on one engine may not work with another. And vendors may offer 'budget' alternatives to not chase customers away.

Do it right, do it once.

These would be a much better option, and still on the 'budget' side:

https://butlerperformance.com/i-3164...tegory:1459691

Get what you pay for.

Been there done that, but never tried those Butler stainless ones. I only buy one now, they last forever, and there's no question:

https://butlerperformance.com/i-2445...tegory:1459691

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As a test, check patterns on ALL the rockers. Then you will see how far out of whack (or not) they are.

Also, it is possible the intakes may require a diff length of rod than the exhausts, which is basically a factor of valve availability and the fact that some exhaust seats may have had to be cut diff due to valve recession.



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HWY,

So why is it 'wrong' If the roller is more to the 'outside'? Or more to the 'inside'?
What does it change? The actual ratio is changing during the cycle of opening & the valve. Smokey Yunick explains this well in his book.
In the usual attempt to get a narrow centered pattern on the valve tip, you need to change prod length. So what will THAT do?

The now defunct PE magazine did a test on different length prods with different rockers, testing how TOTAL valve lift was affected. Three prod lengths were tried & three rockers: 1.5 & 1.65 stamped & 1.65 roller tip
Advertised lift was 0.587". The measured lift varied from 0.521" to 0.592". The longer prod gave the highest lift. So working to get a centered pattern on the valve tip might require a shorter prod...& give less lift.
Crane brought out quick-lift rockers that had faster off the seat lift & they did this by lowering the prod seat.
Rocker geometry is complex & there is no right or wrong way.

In general there’s no doubt that the more spring pressure you need to run then getting the pattern as centered as you can needs to also be considered with how long will the motor need to run with a less then a optimal pattern.

In a drag motor faster valve guide ware can be tolerated more then a endurance motor which to me includes a street motor that the owner wants to have live for 30k plus miles.

I agree with much of what you said, but disagree that there 'is no right or wrong'. You can't just throw any length pushrod on there and expect it to work properly, or there would be no reason to check it.

Rocker ratios for most arms are all over the board, especially on the cheap ones. Centering the pattern may do exactly what you describe, it may require a shorter pushrod, but not give max lift. That's one reason why you calc it at half lift. The pattern may be to one side or the other, but the geometry would be correct.

In the example you provided for testing, the longest pushrod was probably the one closest to the 'correct' geometry.


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True. Side loads matter at higher spring psi, and using the half lift method usually does center the pattern.

Pushrods also have more deflection as spring psi's rise.

The more lobe lift there is, the more likely the pattern width is wider.

Depending on the lobe design, the half lift point may be lower or higher on the ramp. But it will never be abstract.



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I have to comment on this. Last year I got all swept up in this mid-lift geometry stuff by some guy...I forget his name. Sounded really good on paper, and from a mechanical/physics standpoint, it made sense. So we attempted to dial in the pushrod length by that method. Total fail. The valve tip pattern was offset similar to your pictures or worse. So we went back to the "witness mark centered on the valve tip". We used a large caliper and experimented at 0.050" even increments since that's how most (not custom) push rods are mass produced. Once we found the one that made the best pattern, that was it.

I'm using the short style pre-67 valve covers with Crower Enduro SS roller rockers with the shorty "Sure Locks". Also using the thickened 1/4" valve cover gasket from BOP Engineering or Tin Indian....I forget. I have clearance.

The reason why the mid-lift does not work as advertised is because the rockers we buy are not designed specifically with mid-lift geometry in mind. They work great out of the box using the witness mark method.

Isn't the lifter suppose to be as solid as possible for push-rod measurement? Are you checking pushrod length with the double springs installed or a valve checking spring?

Another thing that was mentioned was the stud length. You want to make sure you have good thread depth into the poly-locks. I believe a bit longer rocker stud is not gonna cause your poly-locks to sit higher. you will just end up with the same height poly-lock, but have more thread engagement on the poly-lock.

I have replaced them when the studs are too short. ARP makes different lengths. A friend of mine broke a poly lock because studs were to short.

Also mentioned, make sure those rocker bodies are not touching the retainers at any point in the lift.

I also don't like your pattern on valve stem as it could be centered better.
From what I understand, inboard is better than outboard as it puts less wear on the valve stem. Centered and narrow is your friend.

I also like this guys video.
https://www.youtube.com/watch?v=o5is9BsH5OU

I used a valve checking spring during the half lift test. I thought the lifters should be lashed a 3/4 turn like usual when testing. But still the geometry is not to be desired. I will contact butler today and order another set of rockers to try and find a better design. I’m looking at the Pontiac specific designs they have. I will probably return the comps as I want the geometry to be perfect for the amount of money I am spending on this motor. The rocker never touches the retainer. I hope seperate prod lengths are not required as it’s a pretty basic cam and motor setup. I have the butler #8022 cam. I think I will try different rockers first before longer studs because then I would run into issues with the valve covers again due to a longer prod of 9.5 in measured by the contact area in the center of the valve stem. That may be the final solution but it seems pretty long for a Pontiac and the wear pattern was larger than the 9.1in.

Good that you used a checking spring.

Not meaning to tell you what to do, but seeing you have spent a bunch of money anyway. Get yourself a set of 1.5 Crower Stainless rockers. Yes the studs might be ok with different rockers. Another nice set of rockers is the Harland sharpe. I have run the Crower rockers for years with no issues.

What lifters have you gone with?

I have the comp cams evo hyd roller lifters.

The crowers are a little pricey, anybody have any experience with the butler stainless pro series rockers? They are on sale right now and look pretty similar to the crowers. If the crowers really are the best I might just have to go with them.

All I run is the Crowers, (they are the best before going into shaft-mount), except on the Olds/Bernard-head stroker, I use the Bernard moded HS ones. I run the Crowers on a solid roller and one with EOVs, never had an issue with the half lift method.

When you use the half lift method, type of lifter doesn't matter, you don't compress it at all when you're measuring. So you don't need a light spring, at all. The cam is on base circle, and the cam never moves, so the valve never moves.

You don't calc the lash into the measurement, regardless of HFT, HR, or solid.

I'm not sure how a Crower rocker wouldn't be designed for using the half lift method, the rocker doesn't change anything in relation to the trunnion and the roller tip.

The rocker tip travels in a slight arch, and if you have the geometry set at half lift, the pattern will be as narrow as physically possible. It might be a hair towards the stud, but with deflection, it cancels that out.

If you try to get the pattern more away from the stud by using the pattern, the pattern will be wider.


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Whenever I did get the pattern centered on the valve stem, it was much wider, so I think I will probably send back the comp rockers and get the crowers.

Those Butler rockers are re-branded PRWs:

https://www.summitracing.com/parts/p...1/make/pontiac


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That's my-go to vid for half-lift method references, posted that link in post #10.

If you look at that vid, he never turns the crank/cam, so it doesn't matter if you have a checker spring on there or not.


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Two things to add here:

1. I used a set of CompCams Pontiac rockers a few years ago. The pushrod length required to get the geometry I wanted, was longer than expected. I have a set of Crower rockers sitting around, which have test-fit about the same as Harland Sharp and PRW rockers on various motors - so I tested them against the Comp rockers. The Comp rockers required a pushrod 0.300" longer than the Crower.

2. I recently built a motor with a set of (owner supplied) PRW aluminum rockers. Using the "half lift" method as well as my version of the "centered on the stem" measuring, I got the same pushrod length. Speedmaster heads, PRW "1.65" aluminum rockers.

EDIT: NO IDEA why the photo is sideways on here, it is not that way on my computer.

I know this is not the race section so this info might not matter much to some, but here is why patterns produced by the same ratio rockers can be different with the same length push rods being used.

These charts were made by David Vizard using BBC rockers.

Note in particular how the initial ratio can be by very different even though the advertised rocker ratio is the same!

This is also why some motors running the same ratio rockers can be down or up by 8 to 10 hp.

It needs a light spring cause he’s opening the valve to half lift and checking for 90 degrees at that point. He used a light spring in the video.

Here ya go.

One thing about the "half lift" instructions going around, that bothers me - where to draw the line.

Everyone is drawing the line to the center of the tip's fulcrum. A physics teacher taught me to consider absurdities, when "mentally" looking at such things. The center of the tip's fulcrum will vary according to the diameter of the roller - a 1" roller tip (absurdity, I know) would create a crazy looking valvetrain.

The contact point, the tangential point of contact between the valve stem and the roller - that seems like to where the line SHOULD be drawn, to me.

When using the half lift method, does the cam need to be rotated so the lifter is at the half lift point on the cam?

I may be wrong, but "half lift" is lobe lift times rocker arm ratio divided in two is X.
Place rocker 90° to the valve stem.
Adjust the rocker nut half the amount of X.
Adjust the push rod for zero play.
(With hydraulic lifter add .050" to the final length of the pushrod)

To check, turn the crank, take measurement at valve tip for X and the rocker will be 90° to the valve stem.

Jobs done!

No, the cam should be on the base of the lobe.

One does not account for the preload/valve adjustment when calculating pushrod length using the half lift method.

Watch the video all the way through to understand it:

https://www.youtube.com/watch?v=o5is9BsH5OU

If one is unsure if the lifter plunger isn't full at top of (HYD) lifter, you can shine a light down there and actually see it. It should be seated against the snap ring.

If you can't see it, use a cheap borescope, like one of these:

https://www.amazon.com/gp/aw/d/B0DK9...9-da0da9323246



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He specifically says the lifter is on the base circle. He never moves the crank/cam.

The valve is completely seated.


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Some rocker ratios are different, but what this is showing is that the tolerances of the rockers are all over the place. They can vary from rocker to rocker in the same set even.

Some manufacturers choose to manipulate the rocker ratio in diff ways, like as an example, one may choose to move the cup, some may choose to move the tip roller centerline. This can change the initial ratio. Or better to think as 'the arc' of the rate.

Variance for TDC valve lift seems to be kind of low, like in the .001-.004 range.

'Advertised' rocker ratio is basically only to the .1 range. Like a 1.7 advertised rocker can vary from 1.700-1.799 .

So a better test would be to compare all the rockers of a SET of the same. That would show you the variance of tolerance across the same rockers.

If all the rockers in a set are the same ratio, or even varying .001-.004, the amount of performance difference will be very minor.

Not sure how this applies to the OP of 'pattern vs half-lift methods', but still interesting.


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Can someone post a link to the Dave V. test?


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In this pic from the vid, you can see the valve is not compressed at all:


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If your asking about what I posted up it was out of his second book on Big Block Chevys, so there no link to post.

Hey guys,

Thanks for all the responses as they have been helpful. I decided to try a different set of rocker arms instead of my comp cams ultra gold. I am going to return them and I already have a set of Harland Sharpes on the way. I will post updates when they get here but I theorize that I will get some different results. I most likely will need different pushrods but we will see. They are the same ratio, that being 1.5.

Near the 7:09 mark, he rotates the poly-lock 7.1 turns, which moves the rocker off the established 90 degrees. At 8:04, he says you lower (unseat) the valve to re-establish 90 degrees. I believe the point of the light spring is so you can verify your measurements as he does at end of the video when he rotates the cam to the half lift point to illustrate the rocker is at 90 degrees to the valve.