My next step was to match-machine the heads to the blocks. Earlier, when I surfaced the heads I left them tall so I could finish their surfaces in the same set-up where all the critical drilling and boring will be done. I also thought it best to calculate the static compression ratio before surfacing the heads. The c.r. came out to 7.8 compared with 6 for the full-size engine, which may be a bit high if the supercharger turns out to be functional. Since most of the foundational machining will be finished by the time I get to the supercharger, I should be able to lower the compression by shaving the pistons.
I'll also model a complete operational cylinder soon since I'm planning to increase the size of the water jackets as well as the wall thicknesses of the liners by reducing the diameter of their bores. Although this seems like an inconsequential change, it provides opportunities for valve and connecting rod interferences.
The faces of the heads were mapped to determine the best centerlines for the bores before doing any of the head machining. The cast features in the heads must be machined to match those features already completed on the blocks. For appearance sake I also wanted the outer perimeters of the assembled heads and blocks to align as closely as possible.
The mapping showed I could easily achieve near perfect alignment between the left-side head and block, but there would be a compromise misalignment of .007" on the right head because the cast central stud tube hole in the right block was just too far out of place on its top surface. This small misalignment won't be noticeable because the heads don't actually mount against the blocks. Even though all the cast holes in the heads had to be relocated as mentioned earlier, I was not able to align the already cast central stud hole in the right head to the one in the block without shifting the head .007".
The heads were surfaced to produce an average combustion chamber depth equal to that called out in the drawings before the sealing shoulders on the combustion chambers were bored. It was important to simultaneously keep the intake mounting flanges parallel to these surfaces and also to match their heights on both heads to reduce difficulties later when fitting the intakes. The rest of the interiors of the combustion chambers were left as cast since the valve guide machining will be done later. The Merlin heads are designed for separate valve seats and guides, but I hope to come up with an integral valve cage design. It looks to me like the multi-angle and multi-level contours cast into the heads behind the valves will make it difficult to end up with separate seats and guides which are concentric.
Since the stud tubes penetrate the coolant jackets in both the head and block they must be sealed. I ended up with a slight mismatch, after all, on the troublesome center stud tube hole in the right head; but I believe it can be sealed with gap-filling Loctite.
In order to relocate the pre-cast stud tube holes in the head I fabricated a flat-end drill from a slightly undersize 4-flute carbide end mill. Starting a quarter inch behind the end of the cutter I ground the flutes down slightly in order to clear the hole left behind by the cutting portion of the tool. In the past I've plunged deep holes using unmodified 4-flute end mills, and the results were usually inconsistent. I don't know for sure if my modification helped, but 23 of the 24 relocated holes came out on size and where they were supposed to be. The reamer I used to bring the holes to their finished diameters was dulled rather quickly by investment packed in the coolant passages under the surface of the head. After some research I learned the investment typically used in aluminum casting can be dissolved in water. I soaked the heads in warm water for several minutes, but it didn't seem to have much effect. After the next machining operations on the head I'll try boiling them in water.
I turned partial dummy liners out of Delrin for use as fixtures to assemble the head/block pairs for match drilling and tapping the 24 auxiliary head bolt holes in each head. These liners were turned for press fits in the blocks, and the sealing spigot diameters were turned undersize by .002" just as will be done on the actual liners. The blocks, with their press-in plastic liners, assembled onto the heads perfectly with no alignment issues; but, of course, I'm not yet dealing with the interconnecting tubes.
These plastic liners will protect the sharp sealing edges on the combustion chambers in the heads later while checking the alignments of the 28 fluid tubes running between them. Note how the heads do not sit down against the blocks when assembled but are, instead, clamped against the elevated liner spigots. This gap is what creates the need for all these crazy fluid tubes. My guess is that with the level of horsepower generated by the full-size engine, Rolls-Royce engineers didn't feel that the head gaskets available at the time would be reliable. The high clamping pressures in the sealing corners of the narrow liner spigots may also be the reason that an alloy steel rather than brittle cast iron was specified for the liners. - Terry
I'll also model a complete operational cylinder soon since I'm planning to increase the size of the water jackets as well as the wall thicknesses of the liners by reducing the diameter of their bores. Although this seems like an inconsequential change, it provides opportunities for valve and connecting rod interferences.
The faces of the heads were mapped to determine the best centerlines for the bores before doing any of the head machining. The cast features in the heads must be machined to match those features already completed on the blocks. For appearance sake I also wanted the outer perimeters of the assembled heads and blocks to align as closely as possible.
The mapping showed I could easily achieve near perfect alignment between the left-side head and block, but there would be a compromise misalignment of .007" on the right head because the cast central stud tube hole in the right block was just too far out of place on its top surface. This small misalignment won't be noticeable because the heads don't actually mount against the blocks. Even though all the cast holes in the heads had to be relocated as mentioned earlier, I was not able to align the already cast central stud hole in the right head to the one in the block without shifting the head .007".
The heads were surfaced to produce an average combustion chamber depth equal to that called out in the drawings before the sealing shoulders on the combustion chambers were bored. It was important to simultaneously keep the intake mounting flanges parallel to these surfaces and also to match their heights on both heads to reduce difficulties later when fitting the intakes. The rest of the interiors of the combustion chambers were left as cast since the valve guide machining will be done later. The Merlin heads are designed for separate valve seats and guides, but I hope to come up with an integral valve cage design. It looks to me like the multi-angle and multi-level contours cast into the heads behind the valves will make it difficult to end up with separate seats and guides which are concentric.
Since the stud tubes penetrate the coolant jackets in both the head and block they must be sealed. I ended up with a slight mismatch, after all, on the troublesome center stud tube hole in the right head; but I believe it can be sealed with gap-filling Loctite.
In order to relocate the pre-cast stud tube holes in the head I fabricated a flat-end drill from a slightly undersize 4-flute carbide end mill. Starting a quarter inch behind the end of the cutter I ground the flutes down slightly in order to clear the hole left behind by the cutting portion of the tool. In the past I've plunged deep holes using unmodified 4-flute end mills, and the results were usually inconsistent. I don't know for sure if my modification helped, but 23 of the 24 relocated holes came out on size and where they were supposed to be. The reamer I used to bring the holes to their finished diameters was dulled rather quickly by investment packed in the coolant passages under the surface of the head. After some research I learned the investment typically used in aluminum casting can be dissolved in water. I soaked the heads in warm water for several minutes, but it didn't seem to have much effect. After the next machining operations on the head I'll try boiling them in water.
I turned partial dummy liners out of Delrin for use as fixtures to assemble the head/block pairs for match drilling and tapping the 24 auxiliary head bolt holes in each head. These liners were turned for press fits in the blocks, and the sealing spigot diameters were turned undersize by .002" just as will be done on the actual liners. The blocks, with their press-in plastic liners, assembled onto the heads perfectly with no alignment issues; but, of course, I'm not yet dealing with the interconnecting tubes.
These plastic liners will protect the sharp sealing edges on the combustion chambers in the heads later while checking the alignments of the 28 fluid tubes running between them. Note how the heads do not sit down against the blocks when assembled but are, instead, clamped against the elevated liner spigots. This gap is what creates the need for all these crazy fluid tubes. My guess is that with the level of horsepower generated by the full-size engine, Rolls-Royce engineers didn't feel that the head gaskets available at the time would be reliable. The high clamping pressures in the sealing corners of the narrow liner spigots may also be the reason that an alloy steel rather than brittle cast iron was specified for the liners. - Terry
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