Cirrus Construction Log: Cylinder Head - Part 1

Created: September 2006

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Work starts with a nice lump of 2024-T3 allow, sawn from a 1" thick bar once intended to Thorp T-18 wing mounts. After sawing, the blank was brought to the precise dimensions using a fly-cutter. Only one of the faces was fly-cut. This would become the side that mates with the cylinders and great care was taken to preserve it in all subsequent operations.

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The mill vice jaws are carefully squared by traversing a DTI over the rear jaw until the error is as close to zero as obtainable. With the blank on parallels, an edge finder is used to locate the edges. The DRO is then set so zero-zero is in the precise center of the block. Using offsets derived from a CAD drawing, the 10 hold-down holes are drilled. The location of the head cavities are center drilled (carefully as the recess is only 0.090" deep), along with the location of the valves. The latter being deeply center drilled.

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The head cavities and valve recesses are all done on the faceplate, centering each job using a wobbler and DTI in the precisely positioned center-drilled locations. This is not as tedious as it seems. First the head cavities are bored. This removes most but not all of the valve guide center drillings.

This shot shows a dead-center in the tailstock being used to push a center-drill point against the face-plate, thus holding and aligning the head blank while the clamp nuts are lightly tightened. This gets the part within a few thou of the correct location, requiring only a few light taps of the plastic mallet to get the DTI running true to less than 0.001".

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Once centered, the valve guide is drilled through, reamed, counter-bored, and finally a narrow 45° valve seat cut. Doing this all at the same set-up assures the best possible concentricity. After the rude shock of pricing a 7/32" ball-nose slot-drill, the end of an old 7/32" twist drill was rounded and relieved on the Quorn. The valve guide bush hole allowed the modified drill to drill cleanly with no entry chatter (I was surprised). Photo #15 shows the 45° valve seats being cut using the set-over compound slide. Photo #16 shows the finished job. A trial assembly of some valves and guides shows an excellent seal between the seats and valve heads.

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Here the recesses and entry holes for the Inlet Manifold are cut and drilled. Later, angled passages will be drilled from these blind entry holes to for bifurcated passages to pairs of inlet valve cavities.

On the opposite face of the head, holes are blind drilled, counter-bored and tapped for the glow plugs. These holes do not intersect the head cavities; they end above and off-center from each head and will be connected later by a small, angled passage. After center-drilling, the tip of the tap-size drill is touched to a steel rule of known thickness. The hole is drilled to the required depth, plus the rule thickness.

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The block goes back on parallels and is again centered with an edge finder. A step is now milled on the top of the head leaving a 0.400" wide fence on the exhaust manifold side. The undetected error is locked in forever. Next (Photo #21), holes for the rocker pivot posts are drilled, counter-bored and tapped for the attachment screws. Still using the same set-up, all the cylinder hold-down stud holes are counter-bored 3/16". This required a long-series cutter as seen in Photo #22.

The head is flipped cavity side up and re-located with the edge-finder. The location of the passage from cavity to glow-plug is gently marked with a 1/8" slot drill. The head is then angled and the tip of a twist drill aligned in a recess, then replaced by a center drill to start the hole. The drill is replaced and drilled through to the bottom of the plug cavity. The hole is finally cleaned up with a slot-drill. Two slope to the right and two to the left. Great care was taken to get them right! Note the protractor used to set the block at the intersect angle in the vice jaws.

Similarly, the head is clamped at an angle to finish the inlet passages. The blind entry holes drilled in Photo #18 are 1/4". With the head at 30°, if a 7/32" slot-drill is centered in this hole, the passage can be drilled without touching the edges of the 1/4" hole so they intersect the inlet cavities radially. Amazing, and very satisfying to look at afterwards.




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