What are the reasons to shim the rocker shafts? Is it for adjusting the rocker to valve tip contact point/angle or is it for other reasons? Seems odd that such a small shim could actually help anything.Thanks.
Copper

There suppose tp help correct rocker arm shaft geometry so the rocker arm tip will sweep across the valve tip better.
I like to see around .030 sweep across the valve stem starting on the inside crossing across the center and them return back to its starting point

It is a hack (or a bandaid if you prefer) from the old days. Most likely a waste of time for the parts in the picture since there wouldn't be much point in trying to correct the geometry when you have those loosy goosy stamped rocker arms floating around on the shaft. I'll attach an "old school" picture that makes a little more sense. This was an engine I worked on years ago where the customer wanted to run 906 heads and he wanted to make some RPM. We used a nodular rocker arm and then used blocks to adjust the geometry in order to narrow the wear pattern on the valve tip. It was a bunch of work and didn't really accomplish anything. Once I learned this lesson the hard way I stopped worrying about it but I had to prove to myself that it didn't matter before I could move on.

I built a 440 with Sidewinder Heads and some aftermarket roller rockers. The valve geometry was off way-more than I was comfortable with. Two things in particular bothered me. 1) The sweep of the roller rocker on the valve stem wasn't centered which can cause premature valve guide wear. 2 ) The angle of the adjuster and the push rod when the valve was closed was HORRIBLE. A custom shim / relocation kit from B3 racing was required and completely solved my issues. This shim / relocation kit IS much more aggressive than the thin little shim the TS is showing in his original question, but to the point, YES shims can change the geometry and sweep of the rocker on the valve stem tip. Thin shims are most likely going to be used for minor corrections. My corrections were not minor.


First two photos are BEFORE photos.





Here's what the the corrected geometry with the B3 shim / relocation kit.

hi

those shims are to keep the push rods from compressing the plunger too far into hyd lifters when heads and block have been milled with stock rocker assembly !

.020 " plunger depth is the goal !

If you want to use stock type rocker arms ( I won't ) your going to have to use custom length pushrods to get the preload you want on the lifter cups. Don't be surprised if you need several different length pushrods to get all of the preload exactly the same We use to have to buy and use adjustable pushrods on all our builds that didn't come with adjustable rocker arms on the motor to start with to use solid lifters
The other option is adjustable rockers with the proper length custom pushrod

X2

To lessen valve float for a few more rpm on a hydraulic.

X3

"Correcting" the rocker geometry is a nice idea (and not significant with small errors).
The real argument is not to correct them, but what is correct?
Depends on what you're trying to do.
1. reduce the side thrust vector
2. create the largest "window area under the curve" of valve curtain area
3. move the window closer to a specific point of valve lift
4. move the window closer to a specific point of engine vacuum
5. achieve "mid lift", in which the stem is at 90 degrees to a line through the shaft and roller axle centers @ 50% lift

No, all of these cannot occur simultaneously.

Making the valvetrain stable & not go NUTS is what matters

True, but that includes limits of size, spring pressure, stiffness (resistance to bending), etc.
A perfectly aligned set of shaft rockers on a 1/2" shaft will flop around like a palm tree in a hurricane.

Thanks for posting.
Please note the "crescent" shape of the B3 shims; this is extremely important. A simple curved piece of constant thickness is not only ineffective but fragile. The B3 piece shown is what you want.

& yes I have B3's correction spacers on my junker

B3 kit fixed 3 things for me with Comp promagnums rockers on the trickflow 270's. straightened out the pushrod geometry, centered the roller to valve tip sweep, and put another .030 between the rocker arm and the valve retainer.

The B3 price is not just for small bits of metal, but includes all the R&D he put into it.
You're buying his engineering.