Ever since I started goofing with mopeds twenty plus years ago, the Gilardoni 75cc cylinder was the peak of off-the-shelf Puch performance. My second ever build in 2002-2003 was a Gilardoni Puch ZA50. Watching the parts availability flourish over the years from the ‘tractors & volkswagens’ state of things when I started, I’ve been waiting for the Gila’s successor. But it never came. Yes, a Polini ported just so, or a EuroKit / 80 Metra with the right mix of parts will beat a Gila. And of course the watercooled conversions are plain bonkers. But the Gila is still the gatekeeper. So I decided to design and craft a direct replacement for the Gila that outperforms it in every metric. Sure, I do love learning stuff & gaining skills. I have a stubborn DIY, Burt Munro style ethic, but that’s not my goal. The Goal is to beat the Gila. To make a cylinder that slaps onto a bike already set-up for a Gila with power output much more like the AutoScoot, Kart & Dirtbike engines that share many of the same limitations but are significantly punchier than classic mopeds. I recognize that a go-for-broke approach is dumb, and needs to be moderated against the tunability and transmission liabilities of a 40+ year old moped -and yet- I still feel the Gila’s 11-13hp leaves opportunity on the table.
So, I did a buncha of math. Primarily using A. Graham Bell’s Two-Stroke Performance Tuning (2nd Edition) formulas and parameters. & Honestly, I can’t recommend that book any higher. -But also reverse ‘engineering’ cylinders with performance I’m trying to duplicate: Honda NSR 80, Kawasaki KX85, Malossi MHR series for Dio / Zip / NRG, Aprilia RSA125, Cristofolini, etc.. I landed on some porting numbers I feel are a good starting point.
I then designed a 3D model of the cylinder. Compared to the Gila, I added additional cooling fins, a larger intake system, a larger transfer face and an exhaust flange 1mm wider. The internal port flow is comparable to other modern cylinders. There was a strong emphasis on maximizing port area as well as preferable radial & axial port angles.
Most of the guys previously mentioned are using the lost-PLA method, which (trust me here) is much simpler than lost-wax casting. That process is ^ everything above, but then 3D printing the cylinder with internal structures modelled, adding gates, sprues & runners and then investing the model assembly into plaster. I chose not to go that route because of three reasons: texture, drift & scalability. 3D prints all have a texture, even at a high quality level and I needed the internal face of the transfer ports to have a different texture. Every 3D print I’ve ever worked with, had at least a tiny bit of drift -as in not precisely the shape that was modelled. Since the port features are so small, I needed to really sweat it with my caliper micrometer. Which is not something you can do to a fully formed 3D print. Scalability was also a thought. From experience I knew that it was going to take a few tries to get it perfect and the vendor cost and wait times favored lost-wax casting. Having said all that, there are many penalties to lost-wax and it kinda comes down to individual preference. I trusted my gut.
So I 3D printed the internals of the cylinder as a positive shape (known as a buck in mold-making), I then did some bodywork & sanding to get them exactly right. After that, I created a Silicone RTV mold of these multiple internal parts. This mold is used to cast the internals in plaster. These plaster parts assemble in a modular manner that allows me to make changes without having to redesign / remodel / reprint / remold the entire assembly. This is the pre-fabbed core of the cylinder.
I 3D printed & made a mold of the external cylinder form. The external form has registrations that match the registrations on the pre-fabbed core, so it plugs into the empty mold while waiting for the wax. The external mold was a multi-part assembly to allow for the wax positive to be removed without damage. The mold material is Smooth-On silicone rtv (Mold Max 25) and sometimes TAP Plastic RTV 30.
After that, I had my wax cylinder loaded with a pre-fabbed plaster core. I then had to attach the sprue, runners, gates, vents, etc… and figure out the best location for these elements as well as a plaster mix that is durable enough but also fluid enough during investing. Pure plaster of paris is way too brittle and will not survive the burnout process, too much sand will cause the investment to freeze quickly and leave voids. I’ll share the recipe if you’re curious.
I tried a variety of gate, runner & sprue locations -all with mixed results. The textbook method is a sprue that runs alongside the mold down to a gate at the very bottom of the mold, feeding the aluminum up through the bottom. Looking at youtube, some manufacturers simply pour through the center of the cylinder body. Thus began my campaign of trial and error. Every attempt saw some experimentation with wax type, gate & vent arrangement, and burnout procedure. Across 2019-2020 I attempted around a 12-15 molds & pours with only 3 quality results. Of those 3, one was perfect except it had a tiny imperfection in exactly the wrong spot *rage scream at the sky* and the other two were machinable but very ugly and likely had internal voids. Locating the gates, sprues & runners is tricky but likely not the source of the many failures.
The real culprit here is the burnout process. Once you’ve invested your mold assembly into plaster and the plaster sets, you’ll heat the mold to get the wax (or PLA) out to create the void for the aluminum. Proper burnout is ESSENTIAL. Plaster at the microscopic level is quite porous. While heating the Plaster up, the wax or PLA will start to liquify and soak the plaster. So even if you heat the mold up-side down, to get the wax or PLA to flow out, it’s still soaking into the plaster. If not all of the wax or plaster is eliminated from the plaster during burnout, the hot aluminum entering the mold will cause it to ignite, blowing gas into the void and screw up your casting.
Trust me, I know from experience.
Ideally, you’ll need an electric oven / kiln that can get up to and sustain 800f degrees for days. Professional sculptors I know will burnout 3-4 days prior to pouring aluminum. This would also probably be in a facility that won’t burn your house down. The other thing is that the mold cannot cool down after burnout while waiting for the aluminum. So if you only have the one kiln or furnace, etc.. you're screwed. Also, you can’t burnout the mold one weekend then do the pour the next. Thems the breaks. I tried many different strategies to cheat this and none really worked.
So I decided to get rid of the wax.
Last winter, in the smoldering aftermath of my most-recent casting failure -and considering the pro & cons of a tricky sandcast system, or a highly expensive tool-steel die-mold process (which is what the factories use), I decided to use the idea of my internal parts plaster mold and expand it to make a multi-part matrix of plaster cores that together, form one fully detailed waxless mold assembly. Without any wax, the prep for the mold is as simple as preheating it to a temperature to drive any moisture out.
But that meant I had to design a system of plaster chunks that could fit together without altering the original dimensions of the cylinder. So I designed that. It’s an assembly of nine parts (plus the internals) that use the same registrations as the original design. I also tweaked some of the dimensions of the fins & intake tract as I saw opportunities for improvement after the original series.
So I 3D printed all the mold parts, did bodywork & modifications to make them more accurate. I then began the process of making molds for all of the 3D prints. That & living life took about a year. At some point I started calling it the WHHIPSNAKE! because of a funny video about an over-featured piece of equipment. I just recently finished the molds and began pulling plaster cores out last night. They look pretty good & I think it’s gonna work. So that’s everything up to this point, I’ll drop updates as they come up :)
1 comment:
This is simply awesome!!
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