I am a little confused if someone mills their heads and decks their block why would you need a different length pushrod because I would think the rocker arm can be screwed down or up to adjust accordingly instead of using a different length pushrod. Am I not thinking right? Thanks for any help.

 

Yes you can still use a stock pushrod but one that is too long or too short(pushrod) will change the geometry(arc the roller travels in) and move the roller tip away from the center of the valve tip.

 

Here's a kinda simplified way of thinking about it but I think it serves the purpose.

Ok...Think club sandwich...you know the ones you get with a tooth pick in the center to hold it together.

For the example the toothpick and the pushrod are the same.
You only want "X" amount of the toothpick to show out of the top of the sandwich because the the customer hates too much toothpick....

Now you remove some of the middle of the sandwich so now the slices of bread fall towards each other. Now you have more toothpick showing than you previously did.

Basically when the distance between the top of the rocker stud and the lifter is changed due to the removal of deck surface or shaving the head the only static peice that is left is the pushrod...therefore the pushrod must be shorter to keep the same valve train geometry.

 

And if you dont use the correct length pushrod it will wear out the valve guides prematurely since they rockers downward force isnt being applied straight down on the valve.

 

Be especially careful when you remove the middle of the club, because they tend to fall apart when you take the picks out.

The objective is to maintain a 90 deg. angle between the axis of the rr and the valve tip. I.e. if you were to have a straight line from the center of rr fulcrum and the middle of the roller and take a 90 deg turn towards the tip of the valve, the geometry should be ok. You can see where a different sized push rod could change that 90 deg to more or less.

Enjoy your sandwich--and don't swallow the pick.