Quarter Scale Merlin V-12

[wirralcnc]

Terry can you post some pics of the 1/8 " scale merlin.
Cabin fever is abit out of my way. Maybe one day I will get over.
Robbie

 

[jschoenly]

Boy - Running the show, I hardly get around to see and talk with many people sadly. I had no idea that Dick's Merlin was there! I have video of this engine running from years ago when we were originally in Lebanon!

 

[jschoenly]

I thought this video was on our Youtube Channel, but I was mistaken. I've added the full run of the 1/8th Scale Rolls Royce Merlin that I have:

https://youtu.be/8yWFs6hybGs

 

[dsage]

Gotta love that wonderful sound. What's the clear tube coming from the back of the engine. Shows a lot of air bubbles passing through. If it's fuel would it explain the rough operation.?

Also I'm surprised you ran that without a shield and with so many people close by especially to the side(s) of the engine. You could have killed someone had the prop decided to fly apart (or any other failure for that matter).

At NAMES they smartened up and now make guys run their engines outside in a loading dock door opening and behind a plexi shield.

Very nice just the same.

Thanks

Sage

 

[jschoenly]

i think the bubbles were fuel and part of the rough first running. This was years ago and people were blocked from the prop line, but yes there should be a shield and a designated area. In recent years if people are running aircraft engines, we setup a space for it specifically. Thanks!

 

[mayhugh1]

Robbie,
Here are some more of the photos that I took at Cabin Fever. As you can see I was more interested in the details at the bottom and rear of the engine. I wish I had gotten some better over-all shots. - Terry

 

[kuhncw]

Terry,

I sent you a PM.

Chuck

 

[kvom]

If superchargers at that scale are ineffective, are you planning to make the internals?

 

[mayhugh1]

Kvom,
Yes, I am. The supercharger in this engine is of a modular design. My plan, once it is finished, is to install it into a test fixture and spin it up to measure any boost it might be capable of producing. At the rpms it will be running at I'll also want to do some stress testing before attaching it to the engine, anyway. - Terry

 

[mayhugh1]

It looks like the wheel case and its components will end up being the most difficult assembly in this build. There are so many shafts, countershafts, and shafts inside other shafts that the interactions among all the various gears make it difficult to come up with a construction sequence. John has patiently answered some of my dumb bunny questions through email, but the documentation could sure use some sub-assembly drawings. I took a couple drawings with me to Cabin Fever so I could study them safely away from the shop and its temptation to start cutting too early.
I eventually decided to continue construction with the supercharger bearing plate. This scratch-machined plate will support the rear bearings for the main rear shaft as well as the countershaft that drives the supercharger. When completed there will be 10 shafts, 15 bearings, and 14 gears packed inside the wheel case in front of this plate, and they will all interact with the two shafts supported by it. So, it's important that its bearings be accurately located with respect to the crankshaft centerline and, eventually, the axes of all the other shafts as well. It didn't seem practical to attempt to match-machine much of this plate to the wheel case since the important features on both parts face one another and are aren't simultaneously accessible.
There are mixtures of spur, bevel, and helical gears inside the wheel case. I had naively planned to make all the gears for this engine. But, after gaining some appreciation for why the wheel case is called what it is, I felt the learning curves for the bevel and crossed-axis helical gears would be best left for another project. Dealing with all the shop-made cutters and the tooth approximations I'd likely have to make while trying to maintain all the interacting center-to-center running distances was just too much to pile on top of what is already ahead. I still plan to machine the spur gears, but I placed an order for the bevel and helical gears from a gear manufacturer in England that was recommended by the documentation. In terms of the distances we travel here in Texas, that gear factory is likely located next door to the original Merlin engine plant.
When I received the involute cutters purchased for this project back in June of last year I spot tested the tooth cutting profiles of four of the cutters by cutting two pairs of test gears and checking their meshes at their theoretical center-to-center running distances. Most gear cutters that I've purchased are imported, and I've found their quality to be very inconsistent. The 32-pitch gears I made were for three of the shafts in the wheel case; but at the time I was only interested in verifying the tooth profiles, and so I didn't complete their machining. The 50/24 tooth gear-pair ran fine with no binding and minimal backlash at its theoretical 1.156" center-to-center distance, and so I put the cutters away for later use. I decided to install the gear-set back into its test fixture to see what kind of errors I'd be willing to accept inside the wheel case. This particular pair ran with noticeable drag but no binding with a spacing of 1.148", and at 1.164" the backlash was too great for my taste. My plan, of course, will be to aim for the theoretical center spacings for the spur gear pairs, but I'll use measured spacings for the bevel and helical gears once I receive and test them. I felt that a +/-.004" error, if necessary, would probably be acceptable on all but the high rpm supercharger gear set.
I needed a consistent way of supporting the wheel case for the numerous machining operations to come; and so I mounted it, front flange down, to a faced 1/2" flat plate. I then machined two opposite ends of this plate parallel with the top surface of the chain cover and the other two ends were machined perpendicular to it. When completed, I had the wheel case mounted to a support plate that was square and aligned to its reference top edge within a few tenths.
The next operation on the wheel case was to drill and tap the seventeen 2-56 mounting holes for the bearing plate. The mounting holes were individually located in the centers of their cast blind bosses on the wheel case mounting flange, and their coordinates were recorded for use in SolidWorks where the bearing plate was laid out. The locations of the rough-cast front bearing pockets for the same two shafts in the wheel case as well as the parallel starter countershaft were also measured in the same set-up. Circular features in these castings have been remarkably accurate. A dial indicator showed these 'rough cast' bearing pockets, were within a thousandth or so of being perfectly circular. They are, of course, intended to be machined to finished dimensions.
Their measured locations, however, disagreed with those in the drawing by about .020". This would be enough to bind the gears on all three shafts and is roughly the same size error I discovered in the wheel case's centerline when I machined the chain cover. This was a big concern because adjusting their locations will affect the gears on the other shafts, and the whole avalanche of possible consequences was too much for me to follow especially since I don't yet have most of the gears to play with.
At first I thought all of this might have been caused by me when I initially squared up the warped wheel case casting. But, further measurements showed a possible interference problem between a large countershaft gear and the wall of the wheel case. The complex shape of the interior walls put my measurements into question, and so I decided to double-check the fit by cutting a simple gear set and installing it on a pair of temporary snug-fitting Delrin shafts that I pressed into the cast bearing pockets. Sure enough, the gear was not centered between the walls of the wheel case and, in fact, was jammed against one side.
In the entire lot of castings this was the first casting error that I had run into that hadn't been flagged in the documentation, and so I spent many hours convincing myself that it was real. I spent even more time experimenting with new locations for the bearing pockets that would eliminate the interference, provide the correct center-to-center running distances for the gears on the three parallel shafts and minimally impact the other shafts. Unfortunately my test gear on the starter countershaft had only .007" clearance to the wheel case, and every fix I came up with moved it even further into the wall. In the end, I could only guess about the ripple-down effects on the various cross-shafts and their gears.
Finally, with a best-effort drawing to work from, I began construction on the bearing plate by chucking a slice of 6061 in my lathe's 3-jaw. Since I was starting with a piece of scrap that was only marginally thicker than the finished dimension of the plate, I cut a plexiglass spacer to help accurately support the material in the chuck. I've found that plexiglass works well for fixtures since it cuts easily on a bandsaw, and its thickness is extremely uniform.
The o.d. and the mating flange of the bearing plate blank were turned for a snug tested-fit to the wheel case bore, and the bearing pocket for the central main shaft was bored. The blank was then flipped around and mounted in a collet chuck where its face was indicated-in before being turned to its finished thickness.
The blank was then secured, rear side up, to a piece of MDF with a close fitting plug pressed into the center bearing pocket so it could be indicated-in on the mill. The clearance holes for the flange mounting bolts were spotted and drilled using the measured wheel case coordinates. Button head screws were added through the flange mounting holes to further secure the perimeter of the blank to the MDF. The blank was shimmed and checked for z-height consistency across its face to insure the bearing pocket for the countershaft was bored perpendicular to the bearing plate. Excess material was then removed from the rear of the plate where a stiffening brace and some inspection holes were machined. The bearing pocket for the countershaft was bored with a boring bar instead of being interpolated. Finally, the 1-72 mounting holes for the two bearing retainers were drilled and tapped.
It was this last operation that turned around and bit me. A mysterious errant move of my Tormach's spindle gouged the newly machined brace on the plate as it moved into position to start drilling the retainer holes. I was able to modify the design of the brace to clean up the damage and re-reference the workpiece, but the source of the hiccup was never found. Unfortunately, I had also managed to place two of the eight bearing retainer holes in the wrong locations thanks to the overly cluttered model I had been using to investigate the bearing relocations.
After assembling a pair of newly designed bearing retainers I discovered that one of the gears in front of the bearing plate will end up so close to one of the .060" thick retainers retainers and its 1-72 button head screws that it will likely rub. So, I redesigned the bearing retainers to use .032" thick brass plate and flat head 0-80's. This meant that I had to re-fixture the bearing plate and re-drill and re-tap it for the new retainers.
In the end, the bearing plate fits snugly into the wheel case, and all seventeen mounting holes line up perfectly. The new bearing retainers hide my drilling errors, or I would have started over on a new plate. However, I think I may have gotten only a taste of things to come with this wheel case. There be dragons hidin' in there. - Terry

 

[Ken I]

I liked your gear mesh test - mental note for future use - thanks.

I presume you are using the axis to determine the best running centre to centre distance via the DRO - cute.

Damn it - your machining ability is downright scary - I just love what you are doing.

Ken

 

[Scott_M]

Hi Terry
As usual ,Beautiful work !
All those gears and shafts are sure to make your head spin.
As far as your errant z move, it may be time to switch to Path Pilot. There are a lot of reports that all the flakey Mach3 issues disappear. I am running it on the SBL but I am still running Mach on the 1100.

Scott

 

[bouch]

Just found this pic on facebook. A little inspiration for you

Been watching this thread, simply amazing stuff you're doing there.

 

[maybach_man]

Absolutely awesome thread...Don't know if its relevant to this but I have a copy of the 800+ Merlin engine overhaul manual, on cd ?

 

[mayhugh1]

I still find the wheel case overwhelming, but its design is beginning to grow on me as I better understand it. I didn't yet intend to do very much on it other than locate its mounting holes on the crankcase so I could complete the crankcase machining. I had planned to be working on the cylinder liners and connecting rods, but with the wheel case design finally coming into focus it's hard to set it aside. At this point I think I understand the starting system well enough to begin working on it.
The Quarter Scale Merlin design provides two methods for starting the engine. A shaft on the starboard side of the wheel case, intended for an electric drill-type starter, is connected to the crankshaft through a 10:1 reduction gear-set. A second shaft at the bottom of the wheel case provides a 5:1 reduction for use with an integral electric starter. Both starter shafts are eventually coupled to the crankshaft through a common one-way clutch. There are no details provided for the electric starter, and so it's design is left as an exercise for the builder. In the full-size Merlin the lower shaft was used for emergency hand-crank starts. It's hard to imagine starting the full-size engine this way, but in a life-or-death situation adrenalin can bring a lot to the party.
The main shaft as well as the countershaft will have their rear bearings located and retained in the bearing plate that was just completed. The starter countershaft, on the other hand, will have its rear bearing supported in a bulkhead bracket inside the wheel case. The notes accompanying the castings mentioned that it had been too difficult to include this bulkhead as part of the wheel case casting as was done in the full-size engine. So, in the Quarter Scale this rear support was designed to be an add-on.
A drawing for a bearing support designed to be inserted through and held inside the electric starter bore at the bottom of the engine was included in the documentation. This is a complex shaped part with contours that are intended to follow the surrounding interior walls of the wheel case for additional rigidity. This support turned out to be another difficult part for me to visualize from the flat three-view drawing that was provided. Its visualization was made even more difficult because the drawing didn't contain some of the projected contours that I needed to create a SoldWorks model. A dimension for one of these contours also seemed questionable because it didn't appear to match the part shown in the drawing. I spent several days wrestling with this bracket and, still not sure I had interpreted the drawing correctly, I decided to machine my best guess.
John lengthened his starter countershaft so a second rear bearing could be included in the bearing plate along with the rear bearings of the other two shafts. I briefly looked at also doing this, but I wasn't clever enough to figure out how to retain the bearing in the available space at the outside edge of the plate.
In order to be able to install and rigidly support the starter countershaft, its front and rear bearings must be located in the wheel case with very close sliding fits. This requires the support bracket to be permanently installed before line-boring the two bearing recesses. In my particular casting the starter countershaft drive gear had to be moved very close (actually into) the starboard interior wall of the wheel case in order to obtain the correct spacing between it and its driven gear on the countershaft. The wheel case's interior wall around this gear had to be machined for clearance before the support bracket could be permanently installed. With a test driven gear installed on a temporary Delrin shaft pressed into the countershaft bearing pocket, the mill dro's were used to position a test drive gear supported on a shaft held in the mill's spindle. A .010" radial clearance was then machined into the interior wall of the wheel case around the drive gear before the support bracket was Loctite'd in place.
Unfortunately, this bracket will complicate the installation of the starter countershaft. As shown in the assembly sketch photo, the drive gear in front of the support bracket is too large to pass through the opening for the bearing. This means the front bearing, the keyed starter gear and shaft, and a pinion depth spacer must all be fed in from the side and built up within the limited space in front this bracket.
After an overnight cure of the Loctite, the line-boring was completed. A bevel was also machined on the rear edge of the bracket bore for clearance to a pinion drive gear that will be installed on the end of the shaft. The final photo shows the completed machining for the starter countershaft. The casting offset which I had been concerned about can also be seen in the photo. I tested the running fits using test gears between the main shaft and countershaft as well as between the countershaft and starter countershaft several times during the machining to make sure the offset actually was a casting error and not my misunderstanding of the drawing. I don't yet have the bevel gears, and so I've only been able to verify the centerline alignments of the two starter shaft bores. - Terry

 

[mayhugh1]

While waiting for the bevel and helical gears to arrive, I continued on with the wheel case machining operations that didn't require knowing any of their spacings. I want to complete all the wheel case machining before starting to make and fit the shafts because most of the bearings inside the wheel case will be open, and I want to avoid getting chips into them.
The first 'easy' operation was facing the two magneto mounting surfaces on either side of the wheel case. These pinwheel-shaped flats must be parallel to the vertical plane of the wheel case. I face-milled their odd shapes under a magnifying glass using the flat end of a small dovetail cutter. With a bit of care I was able to surface the contours right up to the edges of wheel case. The dovetail cutter provided better edge visibility than I would have had using a cylindrical cutter and helped prevent gouges in the surrounding areas. The notes mentioned that Rolls workers performed this particular operation on the full-size engines using manual routers guided by templates. Since the magnetos will eventually be driven from the main shaft through a pair of non-adjustable cross-helical gears, the bores for the shafts will have to wait until after the gears arrive so their center-to-center spacing can be measured.
The next operation was the machining of the supports for the coolant and oil pump shaft. The depth of this shaft's driven pinion gear will be adjustable, and so its machining could be completed without having the actual gear available. This shaft must end up vertical and centered on the axis of the main or crankshaft. Its top bearing will supported by a machined retainer that is threaded into a bulkhead support that was cast into the wheel case. Its 9/16-32 thread is fairly uncommon and required the purchase of an expensive tap that I'll likely never use again. The retainer was machined from stainless steel because the minimum wall thickness between its bearing recess and the outside threaded portion is only .010", and so care will also be required when installing it. The shaft's bottom bearing is retained in a recess located inside a machined adapter for the coolant pump. This retainer is bolted to a machined flat on the bottom of the wheel case. The coolant pump, for which castings were supplied, will eventually be secured to this adapter with a screw pinch clamp.
The rough cast bores for both of the pump shaft retainers were offset by about the same amount that I measured for the countershafts. In my particular warped casting it turned out to be very important to use the supercharger mounting flange to locate the center of the wheel case casting. If the center had been located using any of the other wheel case features including the rough cast main shaft opening it's likely the supercharger mounting screws would have broken out the sides of their bosses.
The oil pumps, which will be located inside the oil pan, will also be driven from this shaft through an idler spur gear. The drive and idler gears are identical and happen to be similar to a gear that I machined several years ago for another project. I had enough left-over 12L14 gear blank material still in my scrap box to slice off two more gears, and after measuring their center spacing I machined the three-piece support bracket for the idler gear. The idler gear bracket will be test fitted on the front of the wheel case's mounting flange after all the wheel case machining is completed, and the fixture plate is removed. This bracket has gone through a complete redesign since Gunnar's engine, but I think John's engine has the same version as mine. In a photo he sent me, though, I noticed he turned his bracket around before mounting it on the wheel case. With as far as I've gotten I can't yet tell if there will be an interference problem with it mounted the way I'm (mis?)interpreting its drawing.
While at Cabin Fever last month I purchased a couple keyway broaches from one of the used tool vendors. Since I'd never seen a keyway broach that didn't require a bushing I had to have the seller explain to me what they were and how they were used. It seemed like these cylindrical bushing-less broaches might solve the non-uniform slot depth problem I encountered earlier when I made the splined prop bushing for this engine. The two tiny broaches (1/16" key slots in 3/16" and 1/4" diameter holes) that I bought seemed pretty pricey at $25 each, but they looked to be in like-new condition. I used the 3/16" broach for the first time to cut the key slot in the pump drive gear, and it seemed to work very well. As it turned out I didn't have a broach in my import set that would fit into a 3/16" diameter hole, and so it was fortunate that I came across this one at the show.
The drawings contain a design for a fuel pump mount on the port side of the wheel case, but there are no details included on the pump itself. I plan to use the same electric fuel pump and recirculation loop that I designed and used on my two radials. My pump will be located external to the engine, and so I didn't include this mount as part of my wheel case.
Finally, as suggested in the wheel case drawing, clearances were machined in the casting around the eventual locations of the large main gear and the pump's driven bevel gear. Since I can't yet verify the resulting clearances, additional material may need to be removed later.
The next step will be to machine the supercharger castings and fit them to the wheel case. Drilling the mounting holes for the supercharger should complete the machining on the wheel case.
I just realized that it's been exactly one year since I began this build. It seems like only yesterday when I was wondering if this project was going to be too much for me. No, wait, ... that really was yesterday. - Terry

 

[mayhugh1]

I've been looking forward to working on the supercharger section since the beginning of this project because of my fond memories of the Paxton superchargers that were Shelby-installed on a few very special first generation Mustangs. When I was younger I owned a '65 fastback with a hi-perf. 289, and I lusted after the bolt-on kit that became available. However, life seemed to always come up with more practical places to spend that kind of money.
The castings associated with the Quarter Scale's similar centrifugal unit include a four-piece housing and an impeller. The impeller will be geared up 10x from the crankshaft for a maximum 36,000 rpm. Whether its design and my machining abilities can produce any useable boost remains to be seen, but it will nevertheless an eye-catching part of the engine.
My plan at this stage in the build is to machine the supercharger castings and complete the rear wheel case machining so the two can be attached. The supercharger internals will be added later.
A recess must be machined into the front half of the housing to accept the mating flange previously machined onto the rear of the wheel case. The two will be secured together with a handful of SHCS's. If everything fits the way it should, a drive gear on the wheel case countershaft will perfectly mesh with a driven gear on the supercharger's impeller.
To begin the machining I prepared the front half of the housing so the recess could be turned on the lathe. A surface plate check of the casting showed no significant warpage, and so its clean-up started with a visit to the mill. The half-housing was shimmed with its rear-side down against the mill table so its frontside (the side that will eventually bolt against the wheel case) could be faced. The casting was then flipped over, and a pair of custom sawed wood clamps secured its irregular perimeter to the table while the rear was machined flat. When this operation was completed the casting was moved to the lathe's faceplate.
With the wheel case side of the housing facing toward the tailstock, a tight-fitting plug located the center hole of the casting to the bored center of the faceplate. My hope was that a dial indicator would show that the area to be turned was reasonably concentric with the center hole of the casting. Fortunately it was, and this saved some tedious set-up and more custom clamps. The housing was then secured to the faceplate with a circular array of screws. The locations of the holes for these screws were selected so they could later be used to secure the internal diffuser ring.
The mounting face was skimmed, and the mating recess to the wheel case was turned. The goal was to exactly match the i.d. of this recess to the o.d. of the shoulder on the wheel case. As I approached the final i.d., I trial-fitted the two parts together after each .001" (diameter) pass. After going .004" beyond where I thought the parts should have fit together (the i.d. of the shallow recess was difficult to consistently measure), I discovered the wheel case flange was not perfectly circular.
By design, this flange is not continuous around the entire rear end of the wheel case but is interrupted just behind the starter countershaft. One of the ends of this flange had moved slightly - probably due to internal stresses relieved by all the machining done earlier in this area of the casting. A small segment of this flange created enough interference to prevent the two parts from fitting together properly. After filing away a bit of material I then had a sloppy fitting housing with about .003" excess clearance. I finished the lathe work by boring the center of the housing for the impeller bearing retainer. After removing the housing from the faceplate I used a sharp pointed scribe to manually upset the metal around the o.d. of the wheel case flange with an array of 20 prick marks. This appeared to remove the slop; and measurements showed that, with the housing resting on the rear of the wheel case, the impeller bore was concentric with the installed wheel case bearing plate to within .002".
The next step was to drill and tap the holes for the 21 fasteners used to secure the front half of the supercharger housing to the rear of the wheel case. There's barely enough room for 1-72 SHCS's, and even then two locations required using a stud because of limited access. The real challenge was to align the drilling of the 21 pairs of holes in the two parts since they could be neither match nor transfer drilled.
Matching x-y coordinates between the two parts as I did with the bearing plate seemed risky because I didn't have an independent reference to orientate the supercharger housing. The notes provide a design for a single-hole drill jig that uses the outside contour of each boss to locate its center hole on each part. The problems with this approach were the o.d.'s of my castings do not perfectly match each other nor are they truly circular. My solution was to reference the holes to the machined flange or recess on each part with a tiny washer that matched the radii of the bosses. I placed this washer against the machined flange (or recess) to set the distance between the center of the hole and the center of the casting. I made sure it was also concentric with the boss in order to set its radial angle. A spindle microscope was used to position the spindle over the center of the washer for each spotting and drilling operation.
The accuracy of this scheme, as well as the one recommended on the drawing, depends upon the centers of the screw bosses of both parts lying on identical radial lines. This appeared to be a reasonable assumption, but there was a bit of uncertainty near the top of my wheel case where its rough cast o.d. did not exactly match that of the supercharger housing. Using this washer method I drilled and tapped the 21 holes in the front half of the supercharger housing, and I clearance-drilled their mating holes in the wheel case. The screws are threaded-in toward the rear of the engine just to make things a little more difficult.
During trial assembly, I found the radial locations of the bosses on the two parts evidently did not match up as well as I thought over some 60 degrees at the top of the wheel case. The distortion in this casting, that I've been continually dealing with, required the elongation of several of the mounting holes in this area. I was eventually able to get all screws to freely thread-in without any break-outs, but the result wasn't at all pretty. It won't be visible when the two parts are assembled, and the casting mismatch can only be seen from underneath the engine, but it's a crappy result of poor planning. In retrospect, I probably could have done much better by matching x-y coordinates between the two parts.
With the front-half of the supercharger bolted to the wheel case I checked the flatness of the supercharger's previously machined rear surface, and it was within a few tenths. I then brought it to its finished height while it was still mounted to the wheel case. I also re-checked the concentricity of the center bore of the housing with that of the installed wheel case bearing plate and found them to now be within a thousandth.
The bevel and helical gears have arrived, but before returning to the wheel case I'm going to continue machining the supercharger castings. The wheel case, mounted to its fixture plate, has proved to be a convenient machining fixture for them, and so it feels best to continue on - Terry

 

[kvom]

I learn something from every one of your posts. The modeling clay is one such lesson.

 

[mayhugh1]

The machining of the supercharger castings continued on with the rear-half of the housing. This casting was badly warped and had to be annealed and straightened before any machining could be started. After correcting it using the same heat-and-bend technique described much earlier in this build, its front and rear surfaces were cleaned up and made parallel. The part was allowed to rest for a couple days before finally finishing its faces.
With the front half still bolted to the wheel case on its fixture plate, a tight-fitting center plug was turned to help locate the front and rear halves of the housing to each other. The holes for the 37 screws used to secure the two halves together were then match-drilled and tapped. This turned into several hours of mind-numbing work, but this time all the holes ended up exactly in their correct locations. With such a large number of closely-spaced fasteners, there may not be need for a gasket or sealer between the housing halves. A light test showed no sign of leakage between the flanges.
The casting for the rear cover required only a simple facing operation so it could be attached to the rear half of the housing. A little planning helped distribute the material removal between it and the rear half-housing so its bottom flange matched the top flange on the carburetor casting that was supplied.
According to the notes, the carburetor casting was inspired by the Stromberg PD-18 two barrel updraft unit used on the late model Merlins. A design for a functional, but maybe untested, carburetor using this casting was promised in the early documentation. It appears that the project was abandoned before anything actually materialized because no drawings were supplied. The notes mentioned that the designers had also considered using a Honda GX-120 carb on this engine, but at the end of the day the carburetion was left as an exercise for the builder.
I faced both ends of the carb casting and drilled and tapped the top flange, but I haven't yet decided how or even if it will be used. I did some online research on the GX-120, and it certainly does look interesting for use on a model aero engine. It has a float, a .456" venturi, and a form factor that would look very much at home on the rear of a Hodgson-type radial. New ones are available from Amazon for about $15.
The last casting in the supercharger group, besides than the impeller, is an elbow outlet. It's used to route a length of straight pipe that will carry the air/fuel mixture from the supercharger to the intake manifold log located between the heads. For my particular assembly I determined a flange angle of 19.4 degrees would be required to locate the pipe on the engine's centerline. After machining the supercharger flange to this angle and cleaning up the elbow flange, the mounting holes were drilled and tapped. A length of scrap plastic rod was turned to simulate the pipe and to keep its trajectory on the engine's centerline while the holes were transfer-drilled. The timing chain housing was temporarily installed on the wheel case to help with this centering. A few of the drilling operations were complicated by the fact that some of the holes weren't accessible with the elbow in place. One of the elbow bosses under the pipe, in fact, had to be cut down with a slitting saw in order to make room for a nut and stud. The actual pipe will be o-ringed at both ends so there will be a bit of wiggle room if, during final assembly, it's found that the flange angle should actually have been a more likely 20 degrees.
With the elbow temporarily but firmly secured to its own fixture plate, its i.d. was skimmed with a boring bar to provide a smooth sealing surface for the o-ring. A similar operation was repeated on the inlet to the intake manifold log.
These last steps concluded the machining of the supercharger housing which, incidentally, has added another hundred 1-72 SHCS's to the engine. Since the cross-helical gear set for the magnetos has finally arrived, I should be able to determine the spacing required for its proper mesh so I can complete the wheel case machining. I'll then be able to start working on the numerous shafts, gears, and bearings inside the wheel case. - Terry

 

[10K Pete]

Man, that thing has more screws..... !!! Lookin' good!!

Pete