I received and bench tested the replacement motor for my lathe, and it seems to run as it should right out of the box. The cooling fan on my older motor comes on immediately when the unit is energized, but on this newer version it evidently switches on only when the motor reaches an elevated operating temperature. This might be one of the 'improvements' made by Wabeco to reduce the amount of swarf blown through the fan to accumulate over the motor. With my new cabinet ventilation system, I'd rather have the improved electronic component reliability expected with a continuously running fan. A rule of thumb I used during my real working days was that electronic component lifetime generally doubles for every 10C that its operating temperature is reduced. Of course, fans have their own reliability issues; but they're typically cheaper and easier to replace. My current plan is to limp along with the old motor for a little while longer - at least until after the holidays. I guess I'm still hoping to stumble upon a fix before completely giving up on it.
The next step in the build was machining the 56 stud and coolant transfer tubes that run between the cylinder blocks and heads. The coolant tubes were trivial - just 28 short lengths of metal tubing that connect the coolant passages around the cylinder liners to the main passage in the head. These .160" long 5/32" o.d. tubes were parted off in the lathe from a length of thin wall aluminum tubing.
The 28 stud tubes, on the other hand, are a bit more complicated. The long studs that will tie the heads and cylinder blocks to the crankcase pass through the head coolant passages, and so the head stud holes are sealed by flanged metal sleeves. These stud tubes double as conduits for top-end waste oil to return to the crankcase. Openings in the tops of the tubes above the coolant seals allow waste oil to enter and trickle down and around the studs to the crankcase. These openings are really only required on the outside seven stud tubes in each head since the lower sides of the heads are where oil will tend to accumulate.
The first photo shows two possible designs for these tubes. The Quarter Scale documentation provides the design on the left which is made from a length of 7/32" o.d. thin wall aluminum tubing with one end spun to form the flange. The notes call for the bottom of this flange to be coated with a 'suitable' sealant. A steel slotted washer provides a durable surface for tightening the stud nut as well as an entry slot for the oil.
I tried my hand at lathe-spinning this flange on some test parts and was surprised at how easy it was to do using a hand-held sharpened wooden dowel. An issue I ran into, though, was an inevitable radius left in the corner underneath the flange that prevented it from sitting down flat over the sharp corners that I'd left on the reamed stud holes in the heads. I experimented with deforming the radius using a press and a scrap block, but even with re-annealing I could see tiny cracks in the stretched metal. Since I didn't want to radius the corners on the already completed heads I decided to re-design the stud tubes.
I machined my single-piece design shown on the right side of the photo from stainless steel. The tube's o.d. was turned for a close slip fit in the head, but a slight taper on its bottom end provides a couple thousandths clearance to aid assembly with the cylinder block. A .020" wide groove, machined in the bottom surface of the flange, will be filled with the Permatex sealant recommended earlier by Pete10K to form the upper seal. The tube's wall thickness ended up at just over .020".
The Merlin heads were not designed to fit down against the top surfaces of the cylinder blocks. Instead, shoulders inside the combustion chambers will be sealed to pressed-in liners which protrude slightly above the decks of the cylinder blocks. These liners create a .050" gap between the top surface of the cylinder block and the bottom surface of the head. All 28 tubes must also be sealed inside this space between the block and the head. Although the full-size engine used custom pocketed seals, the Quarter Scale uses simple o-rings placed around the metal tubes bridging this gap. There are no machined pockets for these o-rings in either of the two surfaces, and so the .070" thick o-rings will be compressed by about .020" within the gap.
A challenging part of the assembly is to engage all 28 o-ringed tubes in both the head and block while simultaneously engaging the liners with the sealing shoulders in the combustion chambers. Once this is done the head and block pairs are bolted together with an additional 24 head bolts (whose holes must also align) in order to form a standalone subassembly. This subassembly will be much easier to deal with than the individual heads and blocks when it's time to slip them down over the studs and the ringed-pistons.
Back in April when I drilled all these tube holes I realized it wasn't possible to match drill any of them. They all, including the combustion chambers, had to be referenced to a common datum on each pair of castings. Some of the holes even had to be 'moved' because they had been pre-cast in the wrong locations for my 'short' crankcase. To make things even more challenging, the holes for the coolant transfer tubes were reamed for light press fits in the heads and close slip fits in the blocks.
I was thrilled and totally surprised when the long awaited trial assemblies of each head/block pair with its 28 tubes and 6 temporary Delrin separators went together per plan without having to enlarge any of the holes. I wasn't even sure that over the ten inches I had to work the DRO's on my mill were up for the task. For me, this milestone was even more significant than getting the crankshaft laid in. But, of course, I don't yet know the subassemblies are leak-free.
Inconsistencies in the head counterbore depths, though, came around to bite me while trial fitting the stud tubes just as the inconsistent counterbores for the valve guide flanges affected the valve fitting. I had to re-machine most of the stud tube lengths and create four different groups of parts based on length to accommodate the range of head thicknesses I inadvertently created for myself. It seems that just as with Jerry Howell's IC engines there are very few quick-and-dirty machining steps allowable in the Merlin. - Terry