does anyone know what the maximum cam lift is on a stock rocker arm ratio on a pi head before having to change pistons?

 

it has less to do with lift than it has to do with duration. Lift is just one little part of that equation. Call your cam maker. They'll help you out.

 

As stated above, duration has much more of an affect on PTV clearance than lift. Additionally, the LC of the intake lobe also affects PTV. Many people ahve installed the Comp 270s and not had problems. However, the 274s/278s will cause PTV contact. So, it could be assumed that cams with an intake duration/LC similar to the 274s/278s will hit the pistons. Their duration at .050" is 274 and 278.