It's time to work on
porting and polishing the 12 valve Cummins cylinder head. I had a cylinder head failure on another truck so i had to borrow the cylinder head from my Junker Drag Truck to get the other truck back on the road. This head was sourced used from Ebay for around $210 shipped to my door. It has a few minor cracks but actually looks to be in better shape than my original Junker Drag head. Since this head is new to me, I started out checking for flatness with a machinist's straightedge and found it to have 0.003" variance from one end to the other which is well within the factory service manual specification of 0.012" or so.
Here's a picture of the stock exhaust port.
Step one is the remove material from the short side of the port. There is a water jacket about 3/8" below the surface so you can't go too extreme.
In this picture you can also see that the corner is worked over and the right side to help widen the port on the short side of the turn out of the bowl.
Now reaching in there deeper to help blend the transition
Now down near the valve guide there are some sharp corners from the factory machining: I smoothed these over but didn't really try to remove a lot of material, just round off the edges and clean up any large rough spots in the casting.
I smoothed out the sharp edges but didn't really try to remove a lot of material, just round off the edges and clean up any large rough spots in the casting.
I also removed material around the throat of the valve as seen in this picture. You can see the somewhat shiney 45* valve seat, then there is a 60* cut, then a large thick taper back to the casted port. It's good idea to leave most of the 60* cut below the valve seat in-tact and symmetrical with the valve seat edge above.
A couple more pictures of the stock exhaust port to show the valve seat, 60* cut, and throat.
After (1) 60 grit sand paper roll to clean up the rough carbide cuts:
Next I reached in deep with the short carbide burr and started rounding the edge of the valve stem boss area.
Now it's time to switch over to a long carbide burr than can reach in through the exhaust manifold port to finish the short side radius blending and clean up the casting flash in the port runner itself. I started by tracing a gasket's inside edge with a sharpie marker. Almost zero gain is picked up on a mild port and polish job by opening up the runner and gasket matching the port. This traced line just gave me a nice reference point to make sure the head port is equal to or slightly smaller than the manifold so I don't end up with a transition lip between the head and the manifold.
From the exhaust port side, it's easy to touch up the area around the valve guide and finish blending. You'll also notice that more material was removed from the short side of the radius into the runner.
A few rough spots in the the casting in the runner that need to be cleaned up
All roughed-in, now it just needs some sandpaper roll work to blend my rough cuts and it will be finished.
