Thanks all for your kind comments ...
In order to wrap up the coolant system components that are actually mounted on the engine, a means must be provided to return the coolant from the outlets on the fronts of the heads to an external off-board radiator. In the Merlin's aero applications the full-size engines used a header tank for this.
For me, the header tank was one of the most difficult to understand (and ugliest) components on the Merlin. The Quarter Scale documentation contained little information about configuring a coolant system for the model and nothing at all about this tank. Dynamotive's Youtube video shows a scratch-built tank installed on its prototype, but this was a late attempt to solve the engine's overheating problems.
The full-scale tank, hidden beneath a plane's cowling, took on a number of different shapes depending upon the model of the engine (there were 57 variants) and the airframe in which it was installed. On an engine stand it would have been situated well below the prop wash and appear to provide little cooling benefit.
A revelation concerning its real purpose occurred when I came across a re-builder's online photo showing its internals. The tank wasn't at all what I had expected and was filled mostly with air. Its actual purpose was to isolate a pair of large diameter (expansion) coolant return lines from the engine's heat so they could de-aerate the coolant before it was returned to the radiator.
Since I plan to display and (hopefully) run the Quarter Scale on a test stand, I chose to fabricate a functional header that was more appropriate for that set-up. The engines I've seen displayed in simulated Spitfire or P-51 mounts are very impressive, but they greatly limit access to areas of the engine that will likely require some fiddling to get it running for the first time. These mounts would certainly demand a header tank, though, that was more reminiscent of the one in the first photo.
I started fabrication of my header by forming a length of 3/4" diameter stainless tubing into a 200 degree five inch diameter bend. The 3/4" die set that I own for my tube bender happened to be very close to the required diameter, but the last 20 degrees of the bend had to be muscled in using a vise and a large clamp. In order to prevent the tubing from deforming, I filled the starting workpiece with Cerrobend.
I've learned through experience that a lot of Cerrobend headaches can be avoided if the time and effort are taken to use it properly. In order to avoid overheating the metal, it was melted in a beaker sitting in a pan of boiling water on our kitchen stove. While the metal was being heated, one end of the tube was tightly corked and filled with ordinary cooking oil. The oil is really necessary to keep bits of Cerrobend from later sticking to the interior of the tube, and one of the reasons for not overheating it with a torch is to prevent scorching the protective oil. The oil was poured out of the tube just before filling it with the molten metal. The other end of the tube was then quickly corked so the workpiece could be plunged into a sink filled with ice water. The fast chill added some ductility to the Cerrobend. After the bends were completed the tube, with its two open ends pointing up, was re-heated in a large pan of boiling water. The Cerrobend poured out cleanly with no 'cling-ons' and was reclaimed in a scrapped muffin mold.
Machining began by notching the tube for a pair of inlet fittings that will eventually connect the header to the engine's outlet fittings through a pair of short pieces of flexible hose. These fittings were lathe-turned from 303 stainless and then tack-welded to the tubes. This particular stainless alloy isn't really weldable, and so the fittings were soldered to the tubes. I used a 96% Sn, 4% Ag solder alloy available from TM Technologies (kit #ABS-0065 includes the flux)
https://www.tinmantech.com/html/soldering.php
I previously used this particular product, recommended to me by Petertha, to fabricate the fuel inlet tubes for my 18 cylinder radial.
I wanted the tube and its fittings to appear as a single sculpted part, and so I buttered the solder onto the assembly as though I was doing auto body lead-work. The next several hours were spent with files and emory paper metal-finishing the result. It's less frustrating to perform the initial shaping with a file that doesn't tend to load up with the soft solder. I have two round chainsaw files that work nicely, and one of them was used for most of the work.
A pair of outlet fittings was next machined for the bottom ends of the header. Each of these two piece fittings included a hose barb that was permanently threaded into a lathe-turned 45 degree elbow which, in turn, was joined to the header tube with 620 Loctite.
The flexible hose couplers would not, by themselves, provide adequate support for the header tank against the engine's vibration. And so, two band clamps were formed from .010" stainless shim stock and added to the lower ends of the header. These bands secure the header to bolts already present on the front of the prop drive cover.
The color of the Ag/Sn solder wasn't a perfect match to the stainless steel and was something of an annoyance for an assembly that was going to be in full view at the top of the engine. While trying to decide whether or not to paint the header, I noticed a slight shadow under one of the front fillets that I thought had been previously polished out. This made me suspicious of my soldering that, up to that point, I had been so happy with. So, I decided to pressure test the header. The disappointing result was a very slight leak at the edge of the fillet. This indicated that the solder had evidently not wetted the seam which was located a good eighth inch under beneath the fillet, and so there was likely contamination as well.
Merely re-heating the joint would probably not have been a reliable fix because there was no way to clean and re-flux the affected area. So, I unsoldered the whole assembly, ground away the weld tacks, and separated the parts so I could start over. After removing all traces of the soft solder, I brazed the pieces together using a gap-filling (35% Ag, 26% Cu, 21% Zn, 18% Cd) brazing alloy available from McMaster Carr. Large thick fillets can be obtained with this particular alloy, and they flow out more controllably doing away with the need for soft solder 'buttering.' The final metal finishing was made more difficult by the harder filler, but the assembly didn't leak when It was completed.
The now yellowish fillets left no doubt about whether to paint the header, and so I used the remainder of the matte black Gun-Kote purchased earlier for the valve covers. The full-scale header tanks were typically painted white in order to reduce the absorption of the engine heat surrounding them. In my case, the coolant will likely be hotter than the immediate surrounding area, and so the black paint seemed more appropriate. - Terry