Here are the details Frank McGurk published regarding modifying this head which could represent a benchmark:
“This latter operation was done on the intake ports by using a 1 1/2 inch diameter shell reamer piloted into the valve guide bores and enlarging the valve pockets to the point where the ports change direction. Next, a 1 9/16 inch diameter piloted shell reamer was inserted halfway into the valve pockets and the ridges between the two reamed diameters were removed by blending the surfaces together with a hand grinder. A 70 degree piloted hand reamer was then used to increase the minor diameter of the valve seat to 1 ¾ inches. The 1 ½ inch reamer was also used to enlarge the port openings on the side of the head to the point where the ports change direction. At the juncture, the hand grinder was again used to blend the surfaces together. The exhaust valve pockets were enlarged by using a 1 3/8 inch diameter piloted shell reamer. The exhaust port openings were not enlarged appreciably, but were merely cleaned up with the grinder. The 70 degree hand reamer was used to enlarge the minor diameters of the exhaust seats to a consistent 1 3/8 inches. Next the “barnacles” were removed from the surface of the combustion chambers which were then ground to a smooth finish, the hand grinder used for both operations. Finally the intake valve seats were ground to 30 degrees and were narrowed from the top to a width of 1/16 of an inch by using a 10 degree valve seat grinding stone. Similarly the exhaust valve seats were ground to an angle of 45 degrees and were also narrowed to a width of 1/16 inch.”
He goes on to say .030” was milled from the head and the same amount was removed from the top of the valve stems. These improvements were documented as providing a power increase of 8.9 percent and a torque increase of 4 percent. Compression improvement was 16.9 percent due to head milling. I would be curious to know if those values would be repeated based on the difference between the stock port configuration and what Mr. McGurk did and how much further performance improvement beyond that is possible with the tools and methods CNC Dude will use.
I intend to do this work on a basic 3 axis vertical CNC mill and plan on posting the toolpath so that other Inliners can get this done in most average machine shops and not just the few with 5 axis capability. That’s the beauty of CNC in that one tool can be made to behave like a whole collection of shell reamers and generate smooth transitions.
Digitizing determined that the intake valves are set in the head at an angle of 5 degrees so I needed to come up with a way to hold the head so that the valve guides would be parallel to the machine spindle. The valve cover bolt holes measured 12.766” front to back and 5.562” from side to side and I decided to use them to mount sine blocks. I drilled and counterbored a pair of 1” keystocks and milled them at 5 degrees with a difference of .450” in height. Now I’m ready when CNC Dude has data on where and how much to cut. Here’s a couple pic’s of the blocks:
http://s176.photobucket.com/albums/w186/curt160/848%20head/?action=view¤t=000_2420.jpghttp://s176.photobucket.com/albums/w186/curt160/848%20head/?action=view¤t=000_2421.jpg
