SB Rocker Arm Geometry

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340sFastback

340sFastback

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Heads are new Edelbrock and rockers are new Comp Cams Pro Magnum. The roller tip starts in the middle of the valve stem (slightly toward the rocker shaft) and rolls away from the rocker shaft toward the outter side of the valve stem. Does not look like an ideal scrub pattern. Anyone ever seen a case like this and know if it can be improved? I would like the scrub pattern more centered on the valve tip if possible.

RockerScrub.jpg


ScrubClosed.jpg


ScrubFullOpen.jpg
 
Here are some pictures taken outside in natural light. The first one is with the valve fully open with the retainer up against the valve seal. The second one is with the valve closed. The roller tip does sit very slightly offset toward the rocker shaft when the valve is closed but not enough to get a centered scrub pattern.

ScrubFullyOpen2.jpg


ScrubClosed2.jpg
 
I had the same pattern when I had some Comp Cams rockers but mine were even worse than yours. Mine nearly rolled off the stem. Yours isn't perfect but it's not terrible either. Since their rollers they won't apply any side load to the valves like they would if they were non rollers. The roller tip is actually too low so the only way I know to correct it is to either run longer valves or you could cheat and run lash caps. Of course if your retainers aren't designed for lash caps they'd have to be changed also. Money-money-money. I tried a set of stock rockers on my heads and they lined up perfectly unlike the Comp Cams so I'm not sure what the deal is with CC but it appears they are made a little too long. But like I said yours aren't too far off to worry about it but that's JMHO.
 
Fishy I saw an old post of yours describing the same problem you had with the comp rockers. One possible solution, if I want to perfect the scrub pattern, is to have the rocker shaft stands milled slightly (.030" - .060") and that would move the scrub pattern back toward the shaft allowing it to be centered on the valve stem. I do think I could run it as is because the roller tip won't put a side load on the guides like you say. A centered scrub pattern is more critical on non-roller tip rocker arms.

I'll keep looking into it because if I can get it corrected via milling of the stands I might go that route. That will require a shop that really knows what they are doing not to mention one I can trust with my Stage 2 ported heads and expensive rocker arms.
 
Yeah your right. Milling the stands would help. That's one I hadn't considered. The only thing is it might cause you to need shorter pushrods if you have to take alot off. I know what you mean about finding a shop that'll do it right especially when your dealing with high dollar heads like that. I don't have any around here that I'd trust.

BTW: What were the flow numbers on your heads and who did them?
 
i'm thinkin' you need to raise the shaft to move the rocker tip in towards the center, your pretty close now........... or am i missing it?i
've got some shims for the old style LA head if you want to "borrow them"
....the pattern in the center is a good thing..and we're assuming the correct length pushrods
DvorakMachine has an tech article about this
let me know
 
from "Dvorak's papers"..
improper geometry will look like the rocker tip is scrubbing toward the exhaust manifold side of the valve tip for the entire lift cycle OR PULLING back on the tip of the valve toward the shaft side during the entire cycle.

IT IS IMPERATIVE THAT THE TIP OF THE ROCKER CHANGES DIRECTIONS AS IT "SCRUBS" ACROSS THE VALVE TIP.

A roller tip rocker is okay if the roller center is on the center of the valve tip at half lift in most cases. Raise (with shims) or lower (by machining the pedestals or the shaft stands until you can accomplish that position. But if you observe the movement across the valve tip it should do the same as described for the stationary tip rocker. And that is start contact to the shaft side of the valve while the valve is closed, move across the tip just to center or slightly past, and then return to the original position during full lift. Then back to center at one half closed and back again to the original position when the valve is closed.
,,,see his website for more indepth stuff..
 
mrbill00 said:
i'm thinkin' you need to raise the shaft to move the rocker tip in towards the center, your pretty close now........... or am i missing it?i
've got some shims for the old style LA head if you want to "borrow them"
....the pattern in the center is a good thing..and we're assuming the correct length pushrods
DvorakMachine has an tech article about this
let me know
Click to expand...

Raising the shaft will only make it worse. I had the exact same problem he has and tried installing shims and it moved the roller closer to the end of the tip and nearly rolled off at full lift. You really have to study it good to see what's happening. It's because the angle of the rocker shaft stands aren't perpendicular to the valves that throws a person off in thinking it needs to go up to move the tip closer to the shaft when in reality it's just the opposite. Only lowering the shaft (by cutting down the stands) or raising the valve tip height will move the roller closer to center. I've been through it just last year.

mrbill00 said:
from "Dvorak's papers"..
improper geometry will look like the rocker tip is scrubbing toward the exhaust manifold side of the valve tip for the entire lift cycle OR PULLING back on the tip of the valve toward the shaft side during the entire cycle.

IT IS IMPERATIVE THAT THE TIP OF THE ROCKER CHANGES DIRECTIONS AS IT "SCRUBS" ACROSS THE VALVE TIP.

A roller tip rocker is okay if the roller center is on the center of the valve tip at half lift in most cases. Raise (with shims) or lower (by machining the pedestals or the shaft stands until you can accomplish that position. But if you observe the movement across the valve tip it should do the same as described for the stationary tip rocker. And that is start contact to the shaft side of the valve while the valve is closed, move across the tip just to center or slightly past, and then return to the original position during full lift. Then back to center at one half closed and back again to the original position when the valve is closed.
,,,see his website for more indepth stuff..
Click to expand...

Dvorak machine does have some good info about valve train geometry but I think when they say "IT IS IMPERATIVE THAT THE TIP OF THE ROCKER CHANGES DIRECTIONS AS IT "SCRUBS" ACCROSS THE VALVE TIP their referring mainly to non roller tip rockers. Standard non roller tip rockers would create alot of side loading when the geometry isn't right. With roller tips almost no side loading is present. Also on a Mopar pushrod length has absolutely nothing to do with rocker arm geometry because the rockers are mounted on a shaft that doesn't change height. Unlike a Chevy whereby the rocker changes height with just adjusting the valves the pushrod is the main factor in correct geometry.
 
I'll probably leave it as is for now and mock it up with the cam when I get the block back. The cam won't open up the valve as much so the roller tip won't go over toward the exhaust port as much as its doing now. The cam is a Crane hydraulic retrofit part # 699641.
 
That sounds like the thing to do. It probably won't be any problem at all since it doesn't lift as far. If your like me it's those visual things that bug me especially when I've spent alot of money on something.
 
fishy68 said:
Great pic. The angle lines show why if you raise the shaft it moves it over towards the exhaust side of the head and vice-versa.
Click to expand...

I was thinking the same thing. This is very demonstrative of the problem. Most informative. Thanks guys.

Ray
 
the pic with the lines helped me "get it".. THXS..so the solution is too lower the shaft or raise the valve tip.

lash caps would be the lesser cost and effort?

and most a Dvorak's stuff is aimed at stock style rockers not roller stuff..keep us posted
 
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