SBM Edelbrock/Speedmaster Head Data

There is no real 'need' to tube the head bolt holes. This casting will approach 300 cfm without the tube. That's enough flow to push the limits of a stock block anyway. But, in my experience, to get over 300 cfm more area is needed at the apex of the short turn. Tubing the head bolt hole allows for another 0.040" to 0.060" of width right where it's needed most.

There is a second benefit to tubing the hole, especially if porting by hand. It gives a reference point for width measurements. For each valve size (2.02, 2.055 and 2.08), PBR had width and height (area) measurements at the apex that he wanted to hit. Vertically he would reference off the deck. It's easy to get a digital caliper into the port from the chamber side and take a deck to port floor measurement. Horizontally, a reference point is needed. If the head bolt hole is tubed, you can grind all the way over to the brass tube and then use snap gages from there.