Shims can be a bandaid fix if the plunger in the lifters bottoms out on the cam base circle from decking the block and (or) milling the heads. It can cause the lifter plunger to bottom out and hold a valve slighty open. It happened on my friends car years ago when he started his fresh 383 and it was missing on 3 cylinders. The hydraulic lifter is made to have around .100 to .150 of plunger travel to keep the rocker right at the valve tip and adjusted at all times. If a valve job is done and the valves sink say .020 then the lifter plunger will bleed down or pump up to get the lash at zero. But if the plunger moves all the way down and bottoms out it can now hold the valve open. I like to set my lifter preload about .020. I actually watch and measure the plunger travel as I tighten the rocker shafts. I had to either get adjustable rockers or adjustable pushrods or shims on the 383 that was in my boys Dart. I had milled the heads and the lifter plungers were bottoming out. I was very short on cash so I used the rocker shaft shims. They were in the car for 7 years and worked fine. We raced the car many times and they did the job just fine.

Do you need rocker shims ???
Thats up to you. I would always check my lifter plunger travel and if they bottom out you need to fix it one way or another. The shims are a cheap fix as the best way is to get adjustable rockers and the right lenth pushrods. Just assemble your eng and check the lifter plunger travel. If it does not bottom out and you have some room for movement then you will be ok. Ron