Leagon's achieves a true horizontal...
Leagon's achieves a true horizontal scribe for marking either the ports'roofs or floors by setting up the manifold on a decking machine andusing a set up in an apparatus like this.
Now, instead of a handful of scribes, you will need a straightedge, apair of calipers (usually at least 12 inches long), and a single scribe.Start by marking your good floor or ceiling with a line all the wayacross the flange of the manifold. Next, scribe the line that marks thegood common wall, again with a straightedge. Both of these scribed linesshould run right along the edge of the walls of the port we'll be usingas a reference port.
With these two locations (for each side of the manifold) marked, you canbegin the real work. Using your calipers, measure each intake port wallon the head and mark that location on the manifold flange. For example,the first dimension you will measure will be the opposite wall of thereference port. Measure that distance with your caliper (if the walls ofthe port are parallel, you know the perpendicular distance because thecaliper will be at its smallest reading), then find that same distanceon the manifold at the corresponding walls and mark it. If you want tofind the point that is the true perpendicular to the good wall, scribean arc with your caliper. The point on the arc farthest from the goodwall is perpendicular to that wall. Always make your measurements alongthe straight walls of the ports--never on the radii that transition fromthe floor or ceiling to the walls.
A milling machine can be used...
A milling machine can be used both to help lay out your port lines aswell as cut out material around the flanges. Here, Jonathan has apointing device in the chuck as he double-checks port locations usingthe instrument panel.
Mark all the port locations, first by measuring on the head and thentransferring those distances to the manifold. Always return to your goodport wall for measuring to another point. This is your constant. Now youcan see why you want to make sure you always remember exactly which portwall is your reference--both on the head and the manifold. When you arefinished, you should have all the port locations from the head mappedout on the manifold flange. Best case, the manifold runners line upclosely with your marks.
Cut and Try
On the top of the manifold,...
On the top of the manifold, you want the opening to match the spacer youwill be using. Notice how the intake here is scalloped. In thissituation, especially if you are using an open spacer, the easiest wayprobably is to scribe the outline of the spacer onto the areas of themanifold that intrude into the flow area.
"Those scribed lines are your road map," says Jonathan Leagon. "When youbegin grinding, you want to work to those lines, but never grind overthem. If you do that, you lose your ability to gauge how parallel youhave the port walls, your maximum size, everything. A good rule of thumbis to have the manifold runner about 0.015-inch smaller than the intakeport all the way around. This is for several reasons: One, it gives youa cushion when you are porting and two, the worst thing you can do ishave the air and fuel hit a wall in the transition from the manifold tothe head, so keeping the manifold part a little bit smaller helps ensurethat doesn't happen."
Now it's almost time to start cutting metal, but before you do, makesure you have a cutter that is the right size. The determining factorhere is the radius that forms the transitions between the floor andceilings and the walls of the intake ports on the head. It is criticalthat the radius of the cutter you use matches the radius in the intakeports. The right cutter will automatically make the right radius in themanifold as you are doing your grinding work. Consider, for example, youare grinding out the floor of the manifold runner. If the cutter is toosmall, if you grind out the floor to your desired location all the wayacross, it will leave that radius too tight. If the cutter is too large,once you have ground out the floor correctly, the cutter has also eatentoo far into the metal that makes up the adjoining walls.