Another Radial - this time 18 Cylinders

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Project of the Month Winner
HMEM Supporting Member
Oct 24, 2007
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Central Texas
I was so happy with how my 9 cylinder radial turned out that I've decided to try my hand at an 18 cylinder twin. I came across a beautiful set of photos from another pair of builders that has inspired me to build something similar:
Their project seems to be a heavily modifed Hodgson built several years ago. I'm going to attempt something similar. I've studied their very thorough set of construction photos; and I believe that using them and the H-9 info that I already own I can come up with a good-looking, and hopefully, working 18 cylinder model. Basically, it will be two 9 cylinder crankcases mounted back-to-back. I very much like their head and cylinder design because I think their combo looks realistic, and the heads will not require so many different machining setups as the original Hodgson design. Even though I already have all the fixtures I used in my 9 cylinder model, I'm not sure I have the needed enthusiasm to build another 20 of them without some significant simplifications. I like the looks of the open pushrods so I don't plan to use their pushrod tubes, but I will do something similar for the rocker arm boxes/supports. I don't have the crankshaft figured out yet, and so I will wait until I get the crankcase and bearings completed so I have some parts in my hands to help visualize the assembly process.
The crankcase will consist of 4 sections connected with tie bolts:
1) rear cover containing the carb and a pair of distributors
2) rear cylinder section containing the rear 9 cylinders and the fuel distribution plenum
3) front cylinder section containing the front 9 cylinders, and
4) front cover.
It is critical that these sections be carefully machined since when stacked they will register the 4 bearings that support the built-up crankshaft and my goal is only a .00075" (diameter) oil film clearance between the bearings and the crank journals.
Construction started in early July. My plan was to attempt to reach, with lots of photos along the way, what I felt was the most difficult milestone in this project - laying a freely turning crankshaft inside the crankcase. If I was successful in doing this, my plan was to then start a build thread which included all the work that went into getting the project to this point, and then continue the build thread in real time after that point.
Here we go.
I'm starting construction with the front crankcase section which is very similar to the crankcase in my 9 cylinder. I still have a 5 inch diameter length of "6061" that I purchased for 30 cents per pound some 15 years ago from a local scrap yard. It is stamped with markings from a Chinese foundry but it turns very nicely. This is the same material I used for my 9 cylinder. The recesses which will eventually register the bronze crankshaft bushings are the most critical turning operations and they are all be turned in the same setup on my Enco lathe. I’m using Korloy inserts for the turning operations. These inserts are designed for aluminum, are razor sharp with lots of rake, and give a polished finish right off the machine. The sequence of photos should be self-explanatory. The finished turned part is then moved to the mill and the end holes are then drilled. A vertical rotary table is used to cut the flats and bores for the cylinders as well as the tappet bushing bores on my Tormach. I was able to use the CAD/CAM that I had developed for my 9 cylinder for this part since it is identical to the one in my 9 cylinder model. The 4-40 cylinder mounting holes (72 of them) are then drilled and hand tapped. I'm happy to report that I got through the whole process without breaking a single tap. - Terry










This is amazing stuff, one day maybe but I really don't think I would have the staying power for so many repetitions, I will be following along.

The next part is the rear crankcase section. Portions of it are similar to the front section, but much busier. It will contain two of the three bronze bushings that support the crank and will support the rear nine cylinders. It will house the cam ring assembly for the rear 9 cylinders, the oil pump, the impeller, and the air-fuel plenum for all 18 cylinders. It is critical that concentricity be maintained between this section and the front section so there will be no bind in the crankshaft. Nine 6-32 3 inch long SHCS pass through the front section to secure it to this section and concentricity between the two is maintained to a few tenths with a registration boss and recess scheme that also minimizes the chances for oil leaks. There are no gaskets between any of the 4 crankcase sections. Sealing is done with well finished machined surfaces and lots of screws. After completion, I was able to verify the concentricity between the two sctions to a few tenths.
On the nine cylinder engine the air intake tubes were rather simple single pipes running from the crankcase plenum to each cylibder head. On this engine the intake pipes are one-into-two's in order to feed both rows of cylinders from a common plenum pipe. This complication requires a different method for sealing the pipes to the crankcase in order to avoid interference with the very busy rear end of the crankcase. Brass bushings will be machined to compress o-rings around the intake pipes in order to seal them into the rear portion of this section. I expect the fabrication of the y-pipe assemblies will be extremely difficult later on especially since I will want to make them from stainless tubing as I did for my 9 cylinder. All the machining is completed on this section after moving from the lathe to the mill except for a few features primarily related to the oil pump and sump. The machining of these features will define the bottom of this crankcase section and so they will be done later after checking the fit of the crankshaft in the completed crankcase just in case there is a "preferred" orientation for smoothest crankshaft rotation. The lathe machining of this section filled my 30 gallon garbage can two times with swarf
Another 72 4-40 holes were hand-tapped not including some three dozen miscellaneous holes of various other sizes without incident. At this point I'm just about tapped out. - Terry









The next part doesn't require a lot a hole tapping and should be a nice break from the previous two crankcase sections. The rear cover on my 9 cylinder radial was a casting that I purchased with the original planset. I spent a good bit of time cleaning up unused features on this casting and polishing the outer surfaces. Since the air/fuel mixture flows through this section from the carb on its way to the diffuser, a machined air guide was also inserted internally to smoothly direct this flow. This rear section also houses the distributor. In this 18 cylinder twin, I have no casting and so I'm on my own. My plan is to machine the air guide integral to the rear section out of a single chunk of metal. I have a forged aluminum piston blank that I acquired many years ago from my favorite recycling yard that I will use as the starting workpiece. Another wrinkle in this particular engine is that two distributors are required. I plan to use the distributor design that I developed for my 9 cylinder and so I have proven dimensions from that part to work from. I was able to identify the helical gear set used to drive the distributor from the Chaos Industries Photobucket photos. I will purchase these rather than attempt to machine them myself as they would be pretty significant projects in themselves, and right now I'm focused on getting the crank into the crankcase. In my 9 cylinder I was able to time the distributor by pulling the rear cover back to un-mesh the gears in order to rotate the distributor shaft. This technique isn't applicable here as I would likely lose the timing on one distributor while setting it on the other. So I will mill a clearance pocket in the air guide around each distributor gear so that each distributor can be independently lifted with the rear cover installed in order to un-mesh the gears. The machining begins on the lathe by turning the internal air guide contour and the registration boss to the rear crankcase section . This same contour will be used later to machine the close fitting impeller. The rear cover is then moved to the mill where the pockets for the distributor gears are the milled. The rear cover is then flipped over where the real fun begins. I decided to do two rough waterline passes in order to remove as much material as possible from the part before doing the finishing pass. The reason for this is that I plan to use a new ball cutter that I have no experience with on my Tormach. For the finishing pass I need a 2 inch long spherical cutter with a diameter less than 5/16". For me this is typically a recipe for chatter and poor surface finish. The body of this cutter is tapered for maximum strength and worked well even though I still had to modify it prevent rubbing on the finished part. The part came out beautiful and I was able to polish the outer surface to a brilliant sheen with only a fine Scotchbrite pad. This is probably the most complicated part I've ever made on my Tormach since it contained a number of very complex fillets. It taxed my CAD/CAM software, my computer, and my own abilities. The runtime on the exterior surface was about three hours. I checked the resulting gear mesh with a dummy distributor shaft. It turned smoothly with no noticeable backlash. I was also able to verify the length of the rear crankshaft section in my CAD model. - Terry








The final crankcase section is the front cover. It covers the cam ring for the front row of cylinders. It also contains a ball bearing that supports the front of the crankshaft, and so it is again important to maintain concentricity with the other three sections. It is fastened to the front crankcase section with eighteen 6-32 SHCS. Nine of these are the 3 inch tie bolts that hold the front three crankcase sections together. The starting workpiece is a sawed disk from the same 5 inch diameter chunk of scrap that I used for the other sections. All the machining was done on my 9x20 Wabeco lathe using matching spline contours on both the interior and exterior surfaces. The finished part was moved to the mill in order to drill the 18 mounting holes. An end mill was plunged into the part to create interpolated counterbores for the heads of the SHCS. The thru holes for the bolts were then spotted and drilled. I got so busy controlling the swarf coming off the part while in the lathe that I forgot to take more photos and the ones I did take were a bit blurry. - Terry





I just found that you're doubling the ante on that 9 cylinder radial you just finished. That sounds quite ambitious to me. I will be following along.
The crankcase houses three bronze bearings in which the crankshaft will run. These are a light press fits into recesses turned earlier in the front and rear crankcase sections in order to their concentricity. All three bearings were turned from 3-1/2" diameter SAE660 drops which were about one inch thick. Most of this expensive material went into the swarf bucket. Since I have a length of precision ground rod that is .5952" in diameter that I can use later to check the alignment of my bearings and crankcase sections, I decided to make all the crankshaft journals this same diameter and to bore the i.d.'s of all three bronze bearings to .5960". This will leave a running diametrical clearance of .0008" between the bronze bearings and the crank journals for an oil film. This is the same thing I did in my 9 cylinder engine and seemed to work out well. The front and rear crankcase bearings are identical, and they also contain the bearing surfaces for the cam rings as well as the jack shafts needed to drive them. The center bearing will eventually hold the oil pump and and so an o-ring groove is cut into the o.d. of the crankshaft bore to control leakage. As with the crankcase sections no machining is yet done on the bearings which will define their rotational position in their respective crankcase position. This will be done later when the fit of the crankshaft is verified just in case there is a preferred orientation of one of the bearings for the "free-est" rotation. Some other openings may be needed to help with final assembly but I haven't thought things through that far ahead. The Chaos Industries guys did a lot of machining on their bearings and in the end they looked like 9-legged spiders. This was required for assembly of their engine because of their pushrod tubes. It looks cool and I just might do it also. It would allow me to extend the lifter bushings a bit into the interior of the crankcase and partially eliminate the pesky oil leakage around the lower pushrods where they exit the crankcase. - Terry





Looks great! Whats next a 36 cylinder radial?;)

That thing is going to sound great when finished, I can't wait!

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At this point in the build I have just under 300 hours invested in the crankcase. The four crankcase sections and their bearings are essentially finished and, when assembled, my test rod turns freely with no sign of binding as I hoped. I've also been able to check the mesh of my distributor drive gear with dummy distributor shafts in the rear crankcase cover and can feel no sign of binding or backlash. Considering the alignment requirements for these seven parts to achieve this with less than .0008" diametrical bearing clearance, it's pretty obvious that luck has been on my side so far. When I started the crankcase I promised myself that I'd walk away from this project with no regrets if I'm not happy with the crankshaft fit. It's just a personal issue with me but without a near perfect foundation to build upon, I feel it would be impossible for me to keep enough interest in the project over the next year or so to slog through the huge number of parts that are going to be required to finish the engine. So, my next goal is to come up with a crankshaft which turns just as freely as my test bar, is easy to assemble/disassemble, and is robust enough to handle the torque of the twin. I don't consider this project to even be real until the crank is spinning freely in place. The remainder of the work, although tedious, will consist of smaller, less complicated parts and assemblies that should be within my capabilities since I've already built similar parts for my 9 cylinder. If I'm not successful with the crankshaft then the crankcase will just become a neat conversation piece on my desk. - Terry




That is some beautiful work you are doing, look forward to hearing it run
The construction of the crankshaft in the Chaos Industries engine seems to follow what I expect is the recommendation of the Hodgson twin planset since it is very similar to the one in the 9 cylinder planset. It is a built-up of nine individual pieces held together with taper pins. It requires a separate precision jig to be constructed for drilling the holes for the taper pins while the crankshaft parts are held in proper alignment. In my 9 cylinder engine Iwas able to significantly reduce the number of parts by constructing the crankshaft from a solid piece of steel and then cutting it in half after the crankpin and alignment pin holes were drilled. I then used pinch bolts to secure the two crank cheeks to the crank pin. This 3 piece approach eliminated the need for the alignment fixture, the need to broach two precisely placed square holes, and pretty much guaranteed a precise alignment in the crankcase. Taper pins work best when the parts aren't likely to have to be disassembled multiple times. In the Hodgson approach the pins are bradded over at final assembly with their limited access inside the crankcase making it difficult to later remove the crank without chance of damage. I followed the saga of Ken-ichi Tsuzuki as he completed 4 crankshafts before getting an acceptable result for his 9 cylinders gone. Many but not all of his issues were related to the taper pin construction. I'm sure many others have successfully built this engine according to the original planset; and I'm not criticizing the original design, but it just isn't for me. My background includes many years of designing easily maintained oilfield equipment, and so I looked for another approach. I like to verify the fit and operation of each sub-assembly as I go along on a large project and leave final assembly to just finally assembling proper fitting parts and tested assemblies. This means that I'm likely to take the crank in and out of the engine many times before final assembly, and so my crank needs to be easily taken apart and put back together in precise alignment every time. The crank cheek pinch bolts worked well on my 9 cylinder. I estimated the torque produced by that engine to be less than 5 ft-lbs. Experiments showed that a steel 6-32 SHCS torqued to 20 in-lbs in the 1/4" steel crank cheek would grip a .375" shoulder on the crank pin and maintain a holding torque of 15 to 20 ft-lbs. This gave me a factor of 3-4 margin and it still seems to be working well. This was repeatable so long as the bore in the crank cheek is no more than a few tenths over the diameter of the crank pin shoulder. If the bore is a full thousandth over the diameter of the shoulder then the measured holding torque drop to almost half. In addition the actual location of the pin in the cheek can shift a bit from its desired location. I expect the twin will generate twice this torque and even though on paper I still have margin, it is uncomfortably small for me. The solution I eventually came up with was to add a dowel pin key to positively lock each cheek to its corresponding crank pin shoulder. I decided to make the crank shaft in two separate assemblies -a front section with and a rear section. Each section will be turned from a solid steel round. The crank pin hole and a separate alignment pin hole will then be drilled. The two halves are then separated by sawing them in half and then the cheek contours are milled. When the crank pins are keyed to the cheeks using an alignment pin in the alignment holes, I will have two easily separated crankshaft assemblies that should fit perfectly between their bronze bearings.However there still remains the issue of tying these two assemblies together while maintaining precise alignment between them. I decided on the square drive approach. I own the square broach recommended in the 9 cylinder planset but found that it was very difficult to press the broach through a quarter inch of steel while keeping the hole perpendicular to the workpiece. This is because the broach is very long and thin and it tends to bend to one side or another during the pressing operation. The adapter recommended to keep the broach perpendicular costs some $500 and so is not an option for me. So I decided to mill a square recess in the front of the rear crank section and to mill the square boss on the rear of the front crank section. I also decided to go for a light press fit between these two sub assemblies and to not tie them together with any type of fastener. The two crank sections are captured between their bronze bearings and will not move axially. When the engine is disassembled these two sections can then be easily separated. If I fail to get the square recess in the exact center of the rear crank section I'll have the option of opening it up a few tenths before scrapping the rear section. The pressurized oiling system will keep some oil in the interface to minimize metal-to-metal contact should there be any relative motion between the two section because of misalignment. The photos below show some of my test pieces. The next post will detail my process for machining the entire crankshaft. - Terry


Wow, impressive. I was happy to see these new posts & will certainly enjoy another of your projects!

Can you elaborate on how you are centering & clamping the crankcase to (I assume its) an upright rotary table for milling operations?

One pic looks like a wooden plug/plate inside the CC. Is this connected to a threaded stud through the RT hole & thats how its clamped down? How about centering - somehow clock runout with a dial by gently bumping it into position or something?

The other pic shows some hexagon thingy's in the T-slot grooves. Are those eccentric clamps or something?

Re the radial holes for cylinder mounting, whats the plan there? Threaded studs that screw in & the cylinders jugs are then clamped down with nuts? Are they blind tapped or all the way through?


It's nice to hear from you again. The crankcase section is actually centered on the rotary with a tooling button that I turned. One side of the button is fit to the center hole of the rotary and the other is a snug fit to a recess in the crank section. The piece of wood is just a large washer to prevent marring the crank section when the nut on the threaded rod is drawn down to hold the workpiece tight against the rotary.
In the second photo I am using eccentric clamps to do the centering as I had a job on my lathe that I didn't want to disturb and so I couldn't turn another tooling button. A threaded rod is used to draw the workpiece tight against the rotary.
The 4-40 threaded holes that will hold the cylinders to the base are blind holes. During final assembly I will Loctite socket head grub screws for use as studs in these holes. The cylinders will be then secured with small pattern washers, lock washers, and nuts. - Terry
My crankshaft starts out as two sawed lengths of 2 inch diameter cold rolled 12L14. One of these lengths will become the front section and one will become the rear section. I rough turned both blanks to about .050" over their eventual final dimensions on my Enco lathe since the metal removal rate of this lathe is much better than my 9x20 . The disadvantage of my design is that a lot of material is wasted to create the two blanks. I then attempted to stress relieve the blanks some at 600F for 6 hours and allowed them to cool in the oven overnight. Although some will argue I didn't use a high enough temperature, I felt that anything was better than nothing. I then used my 9x20 lathe to turn the shaft features to final size including the bearings and gear blanks and then drilled the axial oil passages and PCV vent. The diameter of the bearings was turned to match that of my test rod. The cheek disk was left .050" over. The parts were then moved to a 4-jaw chuck on the horizontal 4th axis of my mill. Here I cut the square boss on the rear of the front section. I was able to reduce the TIR of the shaft to a tenth or so in the 4 jaw to insure the square boss ends up in the center of the shaft. I cut the integral CAM drive gears on the front and rear sections using a custom 3/8" diameter gear cutter that I made for the crankshaft in my 9 cylinder. A keyway was cut on the front of the front section for the prop hub and the cross drills for the oil passages were completed. The front section was threaded with a left hand 3/8-24 thread for the prop spinner. At this point the sections are ready to have their crank and alignment pin holes drilled just before being sawed apart. -Terry










Hey terry I read your thread on your last one and that sir was amazing. I can't wait to see this one running.your skills as a machinist are amazing and the work is fantastic. Can't wait to see the finished product.
All the best James.
Unreal....great quality, amazing planning. A lot of repetition in the little monster. ;)
___>Be watching.
Hi Terry

Wow amazing progress so far! I'm really enjoying the CNC aspect of your work; The crank case assembly is a work of art!

Thanks for posting.

The two crankshaft sections are next inserted into a sacrificial block that has been skimmed flat on the mill table. The sacrificial block has been bored with two holes the same diameter as the bronze crank bearings so each crank section can be clamped flat on the sacrificial block with the shafts extending safely through the block. I machined each crank section in its own position in the jig. A 3/8" hole for the crank pin shoulder is then drilled and reamed through each disk. A 3/16" hole is also drilled and reamed through the disk and this will be used later to align the cheek halves when they are keyed to the crank pin. The drilled/reamed holes are continued into the fixture block for dowel pins which will later locate the cheeks in proper position while their profiles are being milled. Holes for a pair of .063" drive pins are drilled into the rear of the front crankshaft section. These pins lock a gear to the crankshaft that will drive the oil pumps. The square recess is milled into the front end of the rear crank section. Holes are plunged into the four corners of the recess with an .093" end mill to eliminate corner chatter later when the recess is finished. A 1/4" diameter end mill is used to rough out the recess followed by an .093" cutter used to mill the linear edges of the recess. Light cuts are taken and the dimensions of the recess are continuously checked until a very light press fit with the front crank section is achieved. After the crank and alignment pin holes are reamed, the section halves are separated by sawing them apart on a bandsaw. The cheeks are also manually roughed out on a vertical bandsaw before moving them back to the mill for machining their finished profiles. The parts are placed back onto the fixture and registered into position using dowels in the crank and/or alignment pin holes. These dowels have been thru-drilled so the parts can be held down to the fixture with threaded fasteners. When the cheek profiles are completed the crank sections are ready to be keyed to their crank pins. The crank pins are turned from drill rod but will not be hardened. Their lengths are adjusted for .0015" thrust clearance between the bronze bearings of each crank section. The shoulder diameters are turned for 2-3 tenths clearance fit in the cheek bores. The pins are then marked to associate them to a particular cheek for consistent assembly/disassembly later on. The parts are then moved to a final setup in the mill where they will be keyed and the radial oil passages drilled. - Terry











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