Valve Train Geometry on OEM Fixed Height Rocker Shaft

[Polysphere]

I thought it best to post in this section of the site with the logic that there may be more members with similar valve trains to what I have

I’m working on a Dodge Polysphere 318 and am in the process of final assembly and valve train checks now that my rocker arms have arrived. In looking at and measuring rocker arm, valve stem and push rod geometry relationships, I’m very close to the recommended 90-degree rocker arm pivot shaft centerline to rocker arm shoe (as it sits on the valve stem at mid-lift. (OEM forged non-roller rocker arms)).

What I see in the rocker shoe travel across the valve stem tip however is a wide sweep and in one direction only versus travelling out to just past ½ of the valve stem and then returning (from a fully closed valve to fully open). The travel pattern looks like the “Incorrect Rocker Arm Geometry” diagram below I borrowed from a 2006 Motortrend article. Disregard the measurement numbers and roller rockers shown.

VTG Picture.png

If I’m reading this correctly, it seems to indicate my rocker arm shaft is too low (although new it’s at a fixed factory height, non-removable mounts). Although I have new valves and heavier valve springs, I’ve made no changes to the OEM valve stem height. The rockers have been bushed, new adjusters added and the contact pads resurfaced (.005-.010” removed). All this seems to indicate the rocker sweep was this way from the factory. If correct is this pattern typical of factory valve train designs using steel or iron rockers? It would seem that the ideal relationship between components would be the same whether roller or non-roller rockers.

[Walter R. Malik]

I thought it best to post in this section of the site with the logic that there may be more members with similar valve trains to what I have

I’m working on a Dodge Polysphere 318 and am in the process of final assembly and valve train checks now that my rocker arms have arrived. In looking at and measuring rocker arm, valve stem and push rod geometry relationships, I’m very close to the recommended 90-degree rocker arm pivot shaft centerline to rocker arm shoe (as it sits on the valve stem at mid-lift. (OEM forged non-roller rocker arms)).

What I see in the rocker shoe travel across the valve stem tip however is a wide sweep and in one direction only versus travelling out to just past ½ of the valve stem and then returning (from a fully closed valve to fully open). The travel pattern looks like the “Incorrect Rocker Arm Geometry” diagram below I borrowed from a 2006 Motortrend article. Disregard the measurement numbers and roller rockers shown.

VTG Picture.png

If I’m reading this correctly, it seems to indicate my rocker arm shaft is too low (although new it’s at a fixed factory height, non-removable mounts). Although I have new valves and heavier valve springs, I’ve made no changes to the OEM valve stem height. The rockers have been bushed, new adjusters added and the contact pads resurfaced (.005-.010” removed). All this seems to indicate the rocker sweep was this way from the factory. If correct is this pattern typical of factory valve train designs using steel or iron rockers? It would seem that the ideal relationship between components would be the same whether roller or non-roller rockers.

Correct geometry is normally determined by whoever manufactured the valve train pieces.
The Geometry you mention, (low pivot geometry), is what JESEL recommends for their stuff.

Factory rockers did not have rollers on the tip so, their sweep was almost all the way across the valve tip; starting on the inside and rocking toward the outside as valve lift increases to minimize tip scrubbing with a radius tip rocker arm, (not like a roller as rollers shouldn't scrub).
In the instance you mention, getting the pushrod side correct by ultimately using a different valve tip height, will be paramount.

ALL Chrysler Corp. divisions had their own different Poly engines.
At one time, Chrysler, Dodge, DeSoto and Plymouth all were different.

[Polysphere]

Thanks for your feedback Walter. I have a few questions and comments:

- You mentioned Jesel recommending the low pivot geometry for their parts. I'll see what reference material I can find on the web to understand what their specific recommendations are.

- Your description of factory rocker sweep is what I expected since even though I have new and different pieces in my build, all of the geometry of the original design has been maintained. So I'm seeing the same pattern now as to what the rockers had from the factory (although due to a slightly larger camshaft, sweep travel is slightly different). With this in mind, it seems I have a "factory similar" VTG that will function but that is not optimum. is this a correct statement? Are their some formulas you can point me to for optimizing rocker tip to stem height ? I have some older rockers I can experiment with by removing a little of the pad at a time to measure the change at the stem (The pads are massively thick and I have a no issues with
rocker to retainer clearance).

- Can you point me to a reference for the correct push rod to rocker geometry at mid-lift?

[Walter R. Malik]

Thanks for your feedback Walter. I have a few questions and comments:

- You mentioned Jesel recommending the low pivot geometry for their parts. I'll see what reference material I can find on the web to understand what their specific recommendations are.

- Your description of factory rocker sweep is what I expected since even though I have new and different pieces in my build, all of the geometry of the original design has been maintained. So I'm seeing the same pattern now as to what the rockers had from the factory (although due to a slightly larger camshaft, sweep travel is slightly different). With this in mind, it seems I have a "factory similar" VTG that will function but that is not optimum. is this a correct statement? Are their some formulas you can point me to for optimizing rocker tip to stem height ? I have some older rockers I can experiment with by removing a little of the pad at a time to measure the change at the stem (The pads are massively thick and I have a no issues with
rocker to retainer clearance).

- Can you point me to a reference for the correct push rod to rocker geometry at mid-lift?

Just a little bit at a time, trial and error is the only way I know about.

It has been my experience that the pushrod cup with those heads should be 90 degrees to the adjustment ball at about 2/3 valve lift.

[Polysphere]

Understood Walter. I'll check that pushrod geometry today after I confirm the current contact pattern.

[PackardV8]

Also, don't get caught inside the box thinking the rocker shaft pedestals can only be what they were OEM. For almost sixty years, when changing valve stem heights or changing lifter cup height, we've been machining off the bottoms of the pedestals or shimming under the bottoms to achieve the geometry we wanted. The bad news is custom pushrods have doubled and now redoubled in cost and lead times.

[Polysphere]

Thanks Jack. Understood. Unfortunately my shaft mounts are "fixed" or non-adjustable as they are. I envy the shaft rocker designs that are able to be adjusted (I have to slide my rocker shafts in from the end of the heads). Had I envisioned I would have geometry issues as I am (and should have), I would have machined down or off the OEM pedestals and made my own that would have been height-adjustable. Even though it's somewhat painful to disassemble things and take that path now, I may still do so after I take measurements and run a few tests on the current geometry to see how much adjustment is needed.

[Polysphere]

An interesting series of quick checks today revealed a few things. First is that both intake and rocker sweep patterns are not what I believed I was seeing by eye previously as I rotated the crankshaft and tried to watch rocker travel. Below are a series of 3 pictures each for the 1.6 ratio exhaust and 1.5 ratio Intake rockers.

Exhaust:

1st picture shows the push rod being very close to coaxial with the push rod adjuster @ 1/2 lift
2nd picture shows the rocker to valve stem @ 1/2 lift
3rd picture shows the sweep pattern. Note the (roughly) .020" blue stripe at the edge before the sweep begins.

20230419_132329.jpg

20230419_132308.jpg

20230419_124300.jpg

Intake:

1st picture is meant to show the push rod being off-axis with the push rod adjuster @ 1/2 lift
2nd picture shows the rocker to valve stem @ 1/2 lift (difficult to show clearly due to the distance)
3rd picture shows the sweep pattern. I ran this test twice since I did not see any blue margin at the beginning but I'm not 100 % sure due to the blue being too thick and possibly not fully dry at the time. The blue was getting lifted rather that worn to some extent.

[BillK]

I have only done one Poly engine but I do not remember having any issues with the valve train. Mine was also a Dodge. Do you know what your valve stem height is ?

Is this just a stock engine or a performance deal ? A lot of people don't realize how terrible some of the machining and dimensions were back in that time frame.

Just thinking out loud

[Polysphere]

Thanks Bill. I didn't measure the factory valve stem height but I do know the factory spring installed height was 1.700". I'm at 1.650" with the aftermarket springs. I'll check on the OEM tip height to see what I can find. This is a mild performance engine but will run a supercharger @ 6 PSI. Most if not all descriptions and pictures I've found show a stock valve train to have the same rocker motion across the valve tip as to what I'm seeing. Although my sweep pattern is offset.

Poly 318 Rocker Motion.JPG

If I use my current 11/32" valve stem diameter and assume the leading edge of my current sweep pattern stops at the middle of the valve stem, there is .172" of the valve tip that remains. It appears to center my rocker sweep on the valve I need to move the pattern by .086". Now I have to experiment with rocker heights to see what that equates to.

[BillK]

My AERA software says the tip hgt is 1.800-1.830 Thats what I would have used when I did the heads. I seem to remember having to put seats in them.

Here is the finished product. I have a picture of the truck somewhere, it is huge.

318 poly engine.JPG

[Polysphere]

Thanks Bill. I appreciate your help. Can you elaborate on the AERA software?

Yes, since I sent my heads out to a head shop, I just provided the parts I wanted him to use and gave instructions on porting. I should have taken measurements ahead of that.

That Poly looks great.

[BillK]

AERA has an engine specification software package called Prosis Pro. Depending on what type of engines you are working on I cant imagine having a machine shop and not being an AERA member and using Prosis.

https://www.aera.org/

[Polysphere]

That sure sounds like an invaluable must-have tool for an automotive machine shop.

Well I broke out my depth micrometer and took a few measurements from the valve stem tip to the spring seat and found I'm anywhere from .075" to .105" over the spec, depending on which end of the spec you want to use. So this is the apparent cause of my rocker contact pattern problem. Unfortunately everything has to come apart now to correct the stem height. I hadn't gotten around to checking the stem height previously because I thought for sure, the cylinder head shop had met the OEM spec.

Thanks all for your help with this issue.. One quick question, is there an absolute limit or "rule of thumb" for the least amount of valve stem to have protruding above the retainer and locks as long as the rocker can reach the stem without contacting the retainer or locks?

[BillK]

That sure sounds like an invaluable must-have tool for an automotive
Thanks all for your help with this issue.. One quick question, is there an absolute limit or "rule of thumb" for the least amount of valve stem to have protruding above the retainer and locks as long as the rocker can reach the stem without contacting the retainer or locks?

I think it depends on the rocker shape. As long as it does not hit the retainer or the locks it should not be an issue but . . . . .

If you are asking this because you are thinking about grinding the tips of the valves to get your stem hgt I dont think that is a very good idea. I doubt that the hardening on the tips is deep enough to survive that. The correct way to fix it is to put seats in the heads and get the stem heights correct.

Prosis shows the stock spring installed hgt to be 1.685 but that would also be with the stock retainers.