WHHIPSNAKE: 75cc Puch Cylinder Project

Hey Gang, Like Alex from 2StrokeStuffing, Kilian Münscher, Doug Binder, Jurrian Mulder & Graham, I am also working on a custom-cast cylinder project. This is not my first, however. I cast a cylinder back in 2009 for the Decepticons ‘No Rules Race’, which was limited to 50cc & built around my Case Reed-Inducted Puch ZA50 motor. -But that’s a story for another time.

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 :)

Hit the Books

Sorry I haven't posted anything in awhile. Notably, there has been some progress with the casting stuff, but I have yet to cast anything since that time. Early last month, I took some time to cook up some aluminum and pour a few molds. Phil came over to watch the process and help out a bit. The results were pretty discouraging, even though I had done things as best as I knew how. The pictures show some copied wax positives and the resultant castings, as well as an original mold and its almost "ok" result. Watching clips on youtube, it seems every yokel with a sandbox and a blowtorch can cast aluminum accurately. So that made me really question what was going on.

Thusly, I took a trip to the Seattle Central Library to spend some quality time getting to better know what the ding-dang is going on with my aluminum. This is the part of the movie where there is a montage of me pouring over books, writing equations on a chalk-board, maybe pouring fluid in a beaker and taking notes? It concludes with the camera fading away as I am asleep on top a pile of papers at a table. Yep. Most of the information was the typical facts and figures and numbers that I already know, but one book actually had a troubleshooting section. That proved really helpful as it identified my problems exactly and also offered new information I had not yet encountered. So hopefully, the quality of my results will improve hereafter.

We're going to get off topic a minute here -

I also took a trip back to Ohio for Christmas. After some flights getting bounced around, Thanks to my cheapo standby status, I finally wound up in Huron. We hung out with Liz's nice family for a bit, then down to Columbus to hang out with my folks for a few days. I got to run into a few old friends and even go to see my elusive sister for thirty seconds.
In other news, I've been trolling around the Moped Army forums looking for cylinder bodies that I can use to copy in preparation for a mass-produced cylinder project. With the cylinder port-arrangement insert that I have, case-inducted cylinders are the natural candidates (Batavus, Vespa, Peugeot, Derbi). Thus far, I have no specimens suitable for replication other than the flat reed and pyramid reed Derbi jugs that I got from Vic of the Puddle Cutters. My original intent was to avoid the marques of machine that already have a good selection of performance parts available. Seeing the cylinders first-hand, its now easy to see the tremendous potential a derbi motor can have, and how much more work would be required to match it with another brand of machine. As an example, the spigot diameter is quite large (54mm) which means a determined tuner could build a 90cc Derbi without having to touch the cases, aisde from porting. The cylinder studs are spaced so that with some machining to the case mouth, a 100cc motor is plausible. If last year's Polini Cup is any indication, I'd say it's only a matter of time before these things are reality. (maybe already are?) I'm not a Derbi guy, so i don't really know.

Look for updates in the near future regarding casting progress and a well-equipped metal shop I might get to play around in. Weee!

Dark Crystals

So as of my last post, things seemed to be ramping up quickly towards producing some side-draught daelim / tomos / polini intakes, and then suddenly nothing. I remade some wax positives and invested them properly in plaster, which is what failed the first time. Everything was ready to pour and then ...nothing. This was caused by a cross section of me being broke, bad weather, and some otherwise good developments. I've managed to pick up a freelance design gig with a local company. They currently import, sell & distribute electric bicycle and scooter components and are currently developing a hybrid human-electric vehicle for use in local delivery routes for small parcels. Liz calls them space cars. I've been developing vehicle concept sketches in conjunction with the larger engineering efforts. Its a pretty awesome project, and I'm pleased to be a part of it. Here are a few of the rejected ideas from the first round of sketching:

(I'm not sure what my disclosure agreement will allow me to show, so this image may disappear.)

The success with the blue goo capturing useful geometry off the intake and Naz's MHR cylinder has made me 'git rill' about producing cylinders. The biggest obstacle is keeping costs down. Pistons are really difficult to produce because of the tight geometric, mechanical, and metallurgical tolerances they must adhere to. As such, making my own pistons -even though Burt Munro did it- is beyond my capability for now. So I'll have to buy any piston that goes into a kit I make, the kit will also need some light machining, and then be nikisil plated.

Just between the nikisil plating and piston cost, we're talking about $235 per unit before I even account for machining/honing fees or my own materials, nevermind my own time and trouble. If I were to order enough jugs plated and enough pistons, that number might drop to $110 per unit, but that requires me to have enough cash in fist to make such investments. These numbers bummed me out pretty hard for a minute & made me research the process of nikisil / nicom plating in general; which I've always viewed as an esoteric black magic, wrought with bloody ceremonies and bejeweled cloaks.

Turns out, you can leave the chickens at home, as it's basically the same as nickel plating (electroplating) with some secret sauce. The secret sauce is in part composed of silicon-carbide, which is not terribly expensive if you can find it in the right purity and grit. So if I set up my own electro-plating rig, presumably in the kitchen next to the crock-pot and toaster, I can get the costs down tremendously. This does create another problem, though, as the only way to be sure that the nikisil formula is correct is to produce an experimental series of cylinders, build some motors and test them 'til failure. Unless someone can think of a DIY way of analyzing the chemical composition of existing cylinder liners? hrrmm.... I do have a furnace...



If your weekend isn't set yet, Liz and I will be at Urban Craft Uprising this Saturday and Sunday. Liz will be selling her wares (Blue Diamond Stamps) and I will be eating cookies and drinking beer. My otherwise great cat, Jack, barfed on my clean laundry last night so I should probably go deal with that now. Bad Kitty!!

Mass Production / Massive Potential

So my recent dickering around with the blue goo has produced flexible master molds I can use to make copies of the shapes modeled. The Peugeot / Tomos / Polini reed side draft intake mold isn't quite perfect yet, but I decided to have a go anyway using candlewax -which is part of the problem. The potpourri fragrances, however, are an aromatherapy solution. Sooner or later, I'll man up to some microcyrstalline wax for some super serious molding. I'll also switch up to a Urethane-based master mold, which is tougher for repeat usage, and has better rigidity qualities for accuracy.

I also copied Naz's MHR scooter cylinder, though reproducing this will be slightly more involved. The master here, is of just the internal port structure and port layout. It would need to be adjusted for the specific stroke and crown heights to go into another motor, I would also have to construct a mold of the exterior... so yes, we're still aways off yet. But! We are getting closer to a nightmarish post-apocalyptic world where 20hp Jawa 207's and Batavus HS-50's rule the streets with an iron fist and the Gilardoni Peugeots, Hobbits, Derbis rock back and forth in the corner crying. Keep the dream alive, buddy! Don't stop believin'.