The wrist pins were machined from a length of 5/16" diameter O1 drill rod. I used the same drill, reamer, and holder to machine the holes for the wrist pins in both the pistons and the rods so their diameters would all be identical. From a couple different lots of drill rod that I had on hand, I selected a length that was a close slip fit in the bores. The wrist pins float and can move side-to-side in their cylinders, and so the liners need to be protected from them. The centers of the pins were drilled out, and a soft aluminum rivet was pressed into each end in order to buffer them from the liners. I also drilled tiny air escape holes through the centers of the rivets before they were installed.
The pins were hardened before the rivets were pressed in place. I've created extra work for myself in the past by hardening cylindrical parts after they had already been carefully fitted to their mating bores. So, I ran an experiment on a trial pin before launching the dozen or so parts I'll eventually need. After all its machining was completed, the diameter of the annealed wrist pin measured .3126" at room temperature. The pin was then placed in a triple-folded stainless steel envelope filled with argon and heated for 45 minutes at 1475F. After quenching, and a return to room temperature, the pin's scale-free diameter had increased by a whopping six tenths to .3132" and would no longer fit any of the bores. After a one hour temper at 450F and an overnight return to room temperature, the pin diameter remained at .3132". I had to polish a half thousandth off the pin's diameter to re-fit it. I searched through my stock of drill rod once more and found another length that measured .3122" that, in its annealed state, had a loose fit in the bores. I used it to make the rest of the wrist pins. After hardening, I still had to polish off a couple tenths, but polishing left the pins fitting perfectly and with nice bearing surfaces.
The amount that heat treatment affected these particular pins was dependent on a number of factors including their size, form, material and, most likely, a number of factors involved with the quench. Looking back on my notes for the 18 cylinder radial I built, I remembered that its O1 tappets had grown only three tenths from an identical heat treatment.
Assembling the heads to the cylinder blocks and then installing the combinations onto the crankcase will be a complicated step that I hope to perform only once. The original Quarter Scale design would have allowed the rods with their attached pistons to be installed down through the tops of the cylinder blocks before the heads are installed, which might have made the process a bit easier. Because of my liner modifications, though, the rods' big ends will no longer fit through the liners. Instead, each cylinder block will now have to be inserted down over 14 studs and all six rods and pistons simultaneously after they've been pre-assembled to the crankshaft. An online ancient Rolls factory video shows the full-scale engines also being assembled this way.
My plan is to prepare everything I possibly can ahead of time to make this assembly go smoothly, including locating and clearing any binds or interferences beforehand. I've had a major concern about potential errors between the axes of the rods and cylinders that could cause piston scuffing or, even worse, binding. The Merlin's design which separated the cylinder blocks from the crankcase is a result of reliability issues discovered during testing of its original uni-block design. There was no time in its wartime production schedule to fix the problems associated with the original design, and so the Rolls engineers decided to break up the block into three separate castings. This decision added considerable complexity to the engine's manufacturing, but it evidently solved the reliability issues.
In the Quarter Scale, this three casting combination resulted in some dozen machining operations in as many different setups that affected the alignments of the rods in their cylinder bores. This issue first became a real concern of mine over a year ago while working on one of the machining setups for the crankcase. When I realized how errors from that setup, as well as from so many other set-ups to come, could stack up to affect this alignment, my hiatal hernia woke up.
With the rods, pistons, and wrist pins completed, I could finally begin checking the rod alignments. An important first test was to install a pair of rods and their liners, one opposing pair at a time, so I could individually check the smoothness of motion of the reciprocating piston pair as the crank was rotated. For this test I torqued the main bearing caps to their final values, and I also installed the main cap cross studs which were made much earlier during the crankcase machining. In order to secure the liners in their registered positions on the crankcase, a couple simple Delrin fixtures were constructed. The same pair of fixtures were usable in all six cylinder locations. To my great relief, I couldn't detect any rod issues in any of the cylinders.
The installed rods were checked by carefully watching the un-ring'd pistons for any twist or side-to-side rocking motion while feeling for any bind in the crank was as it was slowly rotated. Then, with the pistons at various depths in their cylinders, the rods were moved back and forth axially on their journals to the limits of their clearances while the small ends were watched for the same motion on their wrist pins. These tests weren't quantitative and they didn't include the cylinder blocks, but I came away feeling a lot better about the crankcase and crankshaft machining. A similar test will be repeated later with all the rods, pistons, and cylinder blocks installed before the heads are added.
The two head/block assemblies will eventually be secured to the crankcase with twenty-eight 8-32 studs. I was lucky to find a package of 50 four-inch long studs on a Cabin Fever vendor's table earlier this year. The packaging claimed these commercially made studs were fabricated from 60 kpsi steel, and their threads rolled. In addition to being shortened a bit, the center three inches of each stud had to be reduced to 1/8" diameter in between its threaded ends. The clearance this operation provides will allow the head/block assemblies to more easily slide down over them during assembly and reduce the difficulties involved with trying to simultaneously register all six liners and fourteen oil seals to the crankcase. It was possible to turn down only about a half inch at a time on each stud before repositioning it in the lathe's chuck. This tedious operation on the whole lot of studs required several days, including lots of procrastination time, and was one of the least fun parts of the build.
All the rods were finally assembled to their journals using full length steel rod bolts and the custom washers made earlier. Before installing them, I drilled holes through each rod bearing pair so that pressurized oil from the rod journals could better reach the blade rods. I drilled a single hole in the fork cap bearing shell near the journal hole and a pair of escape holes through the opposite bearing half. After digesting everyone's comments and doing some more research, I think these holes and the use of 7075 for the blade rod is a good compromise and should probably have been part of the original Quarter Scale design. During my research I found an online drawing showing similar oil holes in the full-size Merlin's bearings. Finally, the cylinder block studs were threaded into place in order to verify their fits in the cylinder blocks. - Terry
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