While procrastinating over the intake manifold drilling I tried to keep the project moving along by working on the valve covers. This project has taught me to spot a problematic casting even from a distance, and my experience tells me the valve covers will be the last ones to require straightening. Besides having a twist along their major axis, their thin half-shell shape had to be spread to fit completely across the heads. Compared with the intake castings they weren't difficult to correct, but they were also about 3/32" too short for my heads which wasn't correctable. I couldn't figure out a way to safely stretch them, and so I settled for an alignment of their fronts with the fronts of the heads since this was the most prominent area of the engine. This decision left a gap at the rear of each head; but, even worse, the threaded holes for the cover's rear mounting bolts almost completely missed the rear mounting flange on the head. This was disappointing, but the defective threads will eventually end up hidden under the valve covers. The increasing front-to-rear mismatch of the cover's mounting hole bosses with those in the heads tells me that maybe the valve cover shrinkage wasn't fully compensated. The surfaces of the valve cover flanges were lapped instead of milled because the castings were too flexible to be safely supported for machining.
I've noticed my head castings seem to be different from those in the photos on Gunnar Sorenson's website as well as those on the Quarter Scale Merlin website. There is a series of holes in the top surfaces of my heads that were probably used for core supports. I hope my heads aren't an early obsolete version, because measurements I made show the required surfacing operations for the camshaft support blocks will leave the top surfaces dangerously thin. I guess it's possible that the mismatch that I encountered between the valve covers and the heads was actually the fault of my heads and not the valve covers.
The valve covers were match-drilled to the heads using 2-56's, and I decided to also use these smaller bolts for the intake manifold instead of continuing with the 3-48's. This gave me a opportunity to experiment with bolt hole clearances, but I eventually ended up again with a .004" diametral clearance which meant that the head mounting holes needed to be drilled and tapped with a positional accuracy of +/-.002". This was one of the reasons I'd been putting off the manifold drilling.
After finishing the valve covers, I finally began the tedious process of drilling the mounting holes for the intake manifold. I match-drilled the side flanges of the assembled intake manifold to the heads using strips of gasket material sandwiched between the two to account for the thicknesses of the intake manifold gaskets. There will eventually be a total of eight manifold gaskets. Six of them will used in the assembly of the manifold itself, and one will be used between each head and the manifold. I purchased a sheet of 1/64" fiber gasket material with an actual measured thickness of .013" to use for the temporary spacers and then, later, for the gaskets themselves. After drilling and tapping all the mounting holes in the 60 degree valley, any other gasket material that deviates more than .002" from this value may no longer work.
Eighteen of the seventy manifold mounting bolts went through the manifold's plenum, and they had to be carefully drilled from both sides of the manifold before being reamed to their final diameters in three steps for the 1-1/4" 2-56 mounting bolts. The reaming was complicated by the rounded interior surfaces of the plenum that continually pushed the reamers off trajectory. All these new long bolts were slightly bent, which isn't at all unusual, and this took up most of the their clearance allowance. After match-drilling and tapping all the manifold mounting holes in the heads, I performed a test to see how critical my hole alignments really were. I increased the effective gasket thickness by .004" by adding a sheet of paper to the strips of temporary gasket material between the manifold and heads. Sure enough, the additional .004" material shifted the manifold holes with respect to the threaded holes in the heads so that only a few of the bolts could be inserted. When the actual cylinder liners and liner collars are machined later, their dimensions will have to closely match the temporary Delrin parts used to fit the manifold.
In total, 180 hole pairs were match-drilled and tapped in order to install the manifold and valve covers onto the heads. I ceremoniously retired the single 2-56 tap that did all the work by grinding its end off so it can someday takes its place as a piece of shafting or a cutting tool in another project. - Terry
I've noticed my head castings seem to be different from those in the photos on Gunnar Sorenson's website as well as those on the Quarter Scale Merlin website. There is a series of holes in the top surfaces of my heads that were probably used for core supports. I hope my heads aren't an early obsolete version, because measurements I made show the required surfacing operations for the camshaft support blocks will leave the top surfaces dangerously thin. I guess it's possible that the mismatch that I encountered between the valve covers and the heads was actually the fault of my heads and not the valve covers.
The valve covers were match-drilled to the heads using 2-56's, and I decided to also use these smaller bolts for the intake manifold instead of continuing with the 3-48's. This gave me a opportunity to experiment with bolt hole clearances, but I eventually ended up again with a .004" diametral clearance which meant that the head mounting holes needed to be drilled and tapped with a positional accuracy of +/-.002". This was one of the reasons I'd been putting off the manifold drilling.
After finishing the valve covers, I finally began the tedious process of drilling the mounting holes for the intake manifold. I match-drilled the side flanges of the assembled intake manifold to the heads using strips of gasket material sandwiched between the two to account for the thicknesses of the intake manifold gaskets. There will eventually be a total of eight manifold gaskets. Six of them will used in the assembly of the manifold itself, and one will be used between each head and the manifold. I purchased a sheet of 1/64" fiber gasket material with an actual measured thickness of .013" to use for the temporary spacers and then, later, for the gaskets themselves. After drilling and tapping all the mounting holes in the 60 degree valley, any other gasket material that deviates more than .002" from this value may no longer work.
Eighteen of the seventy manifold mounting bolts went through the manifold's plenum, and they had to be carefully drilled from both sides of the manifold before being reamed to their final diameters in three steps for the 1-1/4" 2-56 mounting bolts. The reaming was complicated by the rounded interior surfaces of the plenum that continually pushed the reamers off trajectory. All these new long bolts were slightly bent, which isn't at all unusual, and this took up most of the their clearance allowance. After match-drilling and tapping all the manifold mounting holes in the heads, I performed a test to see how critical my hole alignments really were. I increased the effective gasket thickness by .004" by adding a sheet of paper to the strips of temporary gasket material between the manifold and heads. Sure enough, the additional .004" material shifted the manifold holes with respect to the threaded holes in the heads so that only a few of the bolts could be inserted. When the actual cylinder liners and liner collars are machined later, their dimensions will have to closely match the temporary Delrin parts used to fit the manifold.
In total, 180 hole pairs were match-drilled and tapped in order to install the manifold and valve covers onto the heads. I ceremoniously retired the single 2-56 tap that did all the work by grinding its end off so it can someday takes its place as a piece of shafting or a cutting tool in another project. - Terry
