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  #181  
Old 1 Week Ago
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billeuze billeuze is offline
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Quote:
Originally Posted by Maxeffort View Post
The only caveat is the Shovelhead rocker system may not have been designed to be 90* to the valve at mid lift. (Modern rocker systems are, but not always the old stuff).
I would be still be looking at that, but also considering what part of the rocker pad is contacting the valve and the contact pattern on the valve tip.
well yes. I am looking at that now and am starting to think that the shovelhead rockers were not designed to be 90* to the valve at mid lift. So, the question is do I stick with the design that has seemed to work alright for the past 50 years, or convert to the more modern design of 90* at half lift which in theory should work better, but... who knows for sure.

Reasons for thinking the shovel is not designed for 90* at half lift:

According to the article you linked a few posts back, "The principles for cam technology and specifically rocker arm geometry that would soon come along in 1980, but spawned in 1973, have not changed to this day". So the principle of 90* at half lift wasn't a thing back when the shovel rocker box was designed (or copied from the ironhead which was designed even earlier)

One would imagine that the pad of the rocker arm that pushed on the valve stem would be designed such that the center of the pad would make contact at 1/2 lift. But look at these photos. a line through the center of the shaft does not hit the center of the pad. And the line that does hit the center of the pad intersects the shaft well below the center.


Also, the factory specs for valve stem protrusion are 1.600 - 1.645 (measured from cylinder head boss). My valve stem protrusion measures 1.675. It is longer than factory so it stands to reason that I would have to shim the rocker boxes to maintain factory geometry. And going by the visuals of trying to get the center of the pad on the valve stem at 1/2 lift, this makes sense.

But, as I now know, having the center of the pad on the valve stem is NOT 90* to the valve stem.

So I made a line on the rocker shaft as close as I could get it to where the 90* line to the center of the shaft would intersect the body of the rocker shaft. Its not perfect , but close. Then with the rockers assembled in my inspection rocker box, lay a straghtedge across the top of the valve spring collar and rotate the crank till the straghtege lines up with the mark. something like this but not easy to get in a photo:


The above photo is from earlier with the evo rockers. But same idea. Anyway, I don't have great accuracy with exactly at what degree of lift the top of the valve stem 90* line intersects my centerline mark, but as close I can measure it is:
when using the .075 shims, .145" less than mid lift.
when using no shims, .085" less than mid lift.

This tells me that to get what is nowadays considered correct geometry (perpendicular to valve stem at 1/2 lift) I need to shave the rocker box or use lash caps. Or, to get back to what was the original shovelhead geometry design, I would need to go the other direction and shim the rocker box.

I am leaning towards using lash caps to get correct (in nowadays thinking) geometry. But I am open to ideas if anyone knows of a good reason to stick with the original design
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  #182  
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needspeed needspeed is offline
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Originally Posted by billeuze View Post
I am leaning towards using lash caps to get correct (in nowadays thinking) geometry. .
Looking at a computer screen it's hard to see where you're at with your goal of achieving 90° at half lift. But your stem height is already .030 over the max given in the FSM. Adding a .060 lash cap would then put it at .090 over. That seems to be going in the wrong direction, though as mentioned, maybe shovelheads never had the 90° at half lift relationship?

The extra stem length may be why the pic with the box raised .075 looks so good in post #166.

One of the points made by the author in the link given in post #167 is that the stem height is the reference point. It makes sense, that's where it all starts. Also I wonder if setting the CLOSED position of a non roller rocker is easier to measure? (diagram #2)

By the way, what is the stock shovelhead valve lift?
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  #183  
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I’d be checking the contact pattern of the rocker arm to valve tip at different rocker box height.
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  #184  
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stock shovelhead valve lift .390.less than panhead at .412.
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  #185  
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Quote:
Originally Posted by needspeed View Post
Looking at a computer screen it's hard to see where you're at with your goal of achieving 90° at half lift. But your stem height is already .030 over the max given in the FSM. Adding a .060 lash cap would then put it at .090 over. That seems to be going in the wrong direction, though as mentioned, maybe shovelheads never had the 90° at half lift relationship?
Hmm, I thought maybe lash caps would be available in a wider selection of thicknesses. But All what I can find are .06 and .08, nothing lower than that. But yes my thought was that if the shovelhead was not designed with 1/2 lift geometry, then maybe leaving the rocker in its stock position would move it closer to half lift geometry when using longer valves
Quote:
One of the points made by the author in the link given in post #167 is that the stem height is the reference point. It makes sense, that's where it all starts. Also I wonder if setting the CLOSED position of a non roller rocker is easier to measure? (diagram #2)
Now that I've researched some more, I don't think its easier to measure on a non roller rocker. with a roller rocker, the 90° line is the one from the axis of the roller through the axis of the rocker shaft. For a shoe type rocker the line is drawn from the pad of the shoe through to the center of rotation of the arc. The problem is that the center of rotation is not the center of the rocker shaft. All this time I was drawing the line from the shoe pad to the center of the shaft. I understand now why that is wrong. In that article a method is described how to find this center of rotation but I don't understand what he is saying (putting an upside down stud in a vice...). But I did find another article that does explain how to find this center of rotation: http://bullfire.net/TR6/TR6-19/TR6-19.html says "that the rotation center is directly below the rocker shaft axis by the radius of the shoe". That sounds like it might work. I'll have to make some paper cutouts to test it then work out a jig to mark it on my rocker arms.
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  #186  
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Quote:
Originally Posted by Maxeffort View Post
I’d be checking the contact pattern of the rocker arm to valve tip at different rocker box height.
OK, I went and got some prusian blue. Since doing this I leaned that is is probably better to put the blue on the the valve stem. But I didn't know that so I put it on the rocker pads. It still gives me an idea. Instead of using my chopped up inspection rocker box, I used one of my real ones. The exhaust valve looks about what I expected:

with gasket only:


shimmed +.075":


shimmed =.155":


But, on the intake side, it doesn't look very good. It looks as if the rocker pad is hanging over the edge of the valve stem when using the higher shims:

with gasket only:


shimmed +.075":


shimmed =.155":


Now of these rockers I am using are very worn. there are rather deep pockets of wear so they don't make full contact with the valve stems. I have some new ones on the way. Once I get them I'll check their contact pattern.

Also, I discovered that there is quite variation in pad design. I have these worn out shovel rockers and I have 2 sets of evo rockers and they are all have quite a different pad profile. It may be that one of them will work better than the others. I guess I'll do the prussion blue on each of them to see test. This is what I mean that they all have different profiles (the furthest right is the shovelhead rocker:
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  #187  
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the center of rotation is not the center of the rocker shaft
That's wrong.
He's trying to describe how the geo changes as the rocker rotates. The line is not through the shaft center and the radius because a different part of the radius is contacting as the rocker pivots. This changes the geo and the ratio, but the rotation is always through the shaft center.

Pallet (radius) rockers need some scrub to develop hydraulic pressure between the pallet and the stem tip (like a journal rotating in a bearing). Engines designed for long mileage at low RPM (truck) have different scrub geo than race engines.
Roller tip rockers attempt to get mid-lift with minimal scrub but (again, again) it's not an adjustment, it a permanent part of the rocker design.
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