The next step in the Merlin's final assembly was a major one - the assembly of the cylinder heads and blocks to the crankcase. Because of the modifications made to my liners, the only way this could be done was to slide the pre-assembled head/block pairs down over the pre-installed studs and ring'd pistons in the same way the full-size engines were assembled. The purpose of the previously installed auxiliary head bolts is to hold the head/block pairs tightly together while this particular step is performed.
The Quarter Scale's unmodified stock rods and liners, on the other hand, would have allowed the rods to pass completely through the liners permitting a more piecemeal assembly. In this case, the blocks would be first installed on the crankcase so the rods and pistons can be slipped down through the tops of the blocks to be bolted onto the rod journals. The heads may then be finally installed on the blocks. This assembly method does away with any real need for the auxiliary head bolts which may be why the Quarter Scale's head drawing refers to them as optional.
In my case, assembly began by installing the studs on the decks of the crankcase. Delrin oil seals were then pressed into the bottoms of the blocks using a simple shop-made insertion tool. These oil seals prevent the oil flowing down and around the studs on its way to the sump from leaking out the sides of the engine. After installing the pistons on the connecting rods, the head/block assemblies were lowered down into position over the studs. The tapers turned into the bottoms of the liners were invaluable for guiding the ring'd pistons into their cylinders. Without these tapers, a pair of piston installation tools operated by an extra set of hands would probably have been needed. This assembly method also greatly benefits from the studs being turned down a bit between their threaded ends so there is some 'wiggle room' available to the assemblies while they are being inched down into position. I've included a photo of a full-size engine rebuild going through this very same assembly step.
Tightening down the flange nuts on the tops of the studs not only secured the head/block pairs to the crankcase, but the head/block pairs were pulled together another .002". This crush height collapse was an indication that the liners had been pulled into the heads to complete the combustion chamber seals. The auxiliary head bolts could then be finally torqued. These particular assemblies are probably the most critical in the entire build, and hopefully they won't have to come apart during my lifetime. In a full-size engine rebuild they're separated using an overhead hoist to pull the head/block pair against the weight of a fully loaded crankcase. A similar operation might be performed on the Quarter Scale, but great care would have to be taken to reduce risk of damage to its castings.
After assembly, the friction added by the piston rings increased the rotational torque requirements on the crankshaft to the point where it became very difficult to manually turn the crank using the prop shaft. Of course, this isn't totally fair since the prop shaft's gear reduction nearly doubles the torque required through the prop shaft. At this point, the starter shaft sticking out from the wheel case is the most convenient way to turn the crankshaft, and its operation was verified using an adapter mounted in a battery-powered drill. The crankshaft torque requirement will increase again once the timing chain is installed, and the overhead cams are being driven. When the engine is timed and compression is added, the crank will become very difficult to manually turn. All this is adding to my misgivings about the engine's seemingly fragile starting system. On a more positive note, the connecting rods ended up nicely centered in their pistons, and so the numerous dimension changes that had to be made to accommodate my short crankcase casting seem to have come out OK.
With the cylinder head/block assemblies in place, the intake manifold could be finally installed. Hylomar was used during pre-assembly to seal the manifold's three components together. These components were drilled/tapped almost twenty months ago using fixtures to simulate the engine's final deck heights and angles.
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The manifold gaskets made at the same time to seal the intake manifold to the heads had, over those twenty months, shrunk nearly 5/16" and would no longer fit the heads. Spritzing the gaskets with water and carefully heating them with my wife's laundry iron re-hydrated the material and brought the gaskets back to their original size. A total of 128 SHCS's are used to secure the manifold subassembly to the heads. I was a bit concerned about all these previously match-drilled holes actually lining up during final assembly, especially after my unexpected gasket problem which also changed its thickness. Fortunately, every screw went back into place with no issues.
After the intake manifold was installed, the mounting flanges of the coolant block-off plates and exit fittings were sealed with Permatex Aviation grade sealer. The entire coolant jacket in each head assembly, including its nearly fifty internal o-rings and gaskets, was then leak-checked. Each head jacket was pressurized to 10 psi by injecting compressed air into its coolant entrance fitting while its exit fitting was temporarily plugged. Unfortunately, both head assemblies quickly leaked down. Using a piece of plastic hose as a stethoscope to locate the source of the leaks, I discovered that some of the tapped holes in the lower row of mounting holes for the exhaust tips had broken into the coolant jackets. When the holes were originally drilled, I vaguely recall flagging this as a potential issue during final assembly, but I evidently forgot to record a warning for myself in my notes.
I hadn't planned on installing the exhaust tips until after the spark plug wiring was completed and tested, but I chose to install them now so the leak checks could be completed. A bit of non-hardening thread sealant applied to the ends of all the screws in the lower row of exhaust tip mounting holes in both heads easily sealed those leaks. However, I then discovered a pinhole defect on the outside of the starboard head casting that was evidently deep enough to also penetrate the coolant jacket. My makeshift stethoscope worked much better than I had expected. Although the pinhole was barely visible, the whoosh of air that streamed from it during the test was very pronounced and easily located. I enlarged the pinhole with a small drill and backfilled it with JB Weld. A Mity-Vac was used to pull a vacuum on the coolant jacket to help draw the epoxy down into the defect before it was allowed to cure.
I continued using the stethoscope to probe inside the spark plug ports looking for possible liner leaks inside the combustion chambers. I also probed in and around the gaps between the head/block pairs as well as around all the intake mounting screws for any signs of leaks. Fortunately, I found none. After repairing the pinhole in the starboard casting, the leak down times of both heads had increased to nearly a minute which was essentially the noise floor of my cobbled-up tester.
One piece of advice that I might offer to anyone who gets a chance to build this engine is to plan for a good rotisserie engine stand early in the build. Over time, mine has evolved from being a nice-to-have convenience to a must-have necessity. At this point in the build, the engine weight is some twenty-five pounds, and I continually find myself rotating the engine on its stand to just the right angle so I can best install the next screw. There's no safe bottom nor sides for this engine to rest on without risking damage to some machined part or irreplaceable casting. The only practical way to support this engine is by its motor mounts which are integral to the crankcase casting. Bolting these to an engine stand allows total access to the entire engine during construction. - Terry.
