[2 stroke] Damion2T's 62.5cc R&D project

Hello everybody.  I'm happy to have this as my first post in the pit-lane forums!

I am a racing enthusiast and a 2-stroke dreamer.  I would've been perfectly happy riding whatever 2-strokes the factories produced for the rest of my life but since that's not happening anymore, what choice do I have but to make my own 2-stroke engine?  It's something I've always wanted to do from an R&D standpoint and now that I am fortunate enough to work for a prototyping and fabrication firm, I believe that dream can become a reality.

I am still relatively new to most of the equipment in this shop but the CAD/CAM software is basically the same for almost everything.  I've been playing around with designing a cylinder just to get better at making complicated geometry but there's no reason I can't have one of the CNC's mill out a plug for casting later on.  Here's my progress so far:

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Please keep in mind that this is very early in the design stage.  I've only played around with it for a few days during my lunch breaks.  You can see the exhaust ports (pink), rear transfer ports (orange), and a single front transfer port (green).  Of course, it's missing the transfer on the other side and the center intake port.  I'm not happy with some of the geometry so I'm still playing around with it.  I'll be adding an exhaust valve at some point in the development but probably not for the first casting.  I'm basically posting here because I welcome comments regarding design improvements, criticism, and discussion of manufacturing & prototyping techniques in general.

The engine I'm trying to design is a water-cooled, 62.5cc 2-stroke single with the design based loosely on the later RSAs.  Why 62.5cc???  From a development standpoint, smaller is better as it's less expensive.  I've always liked the rear-rotary inlet port and since I doubt I can make a better RSA engine during my lunch breaks, I wouldn't mind making a 1/2 size version of it.  If I can develop the 62 cylinder to have good output, it's not out of the question to produce a 125 twin ...but that's a long way off.

My first guess at sizing was a 44.5 x 40mm bore-stroke but only because 65cc dirt bike pistons are that size and readily available.  I plan on using as many off-the-shelf parts as I can to keep costs down (at least until I'm able to exceed the performance factor of those parts).  At this stage of development I'm mainly focused on getting a good starting point for port heights and shapes.  I know many of you have much more experience than me in this area and I welcome your input!  I plan on seeing this to completion and should it be successful, I'll consider a small production run.  But for now, just a dream...

Italjet Moto Srl made a 125cc twin called the Formula. It was a very high build quality engine. Pistons will still be available. You may even wish to copy the bore x stroke or even use the crankshaft.

You should read all of the Jan and Frits posts. There are hundreds and it will take you a long time, but please don't insult Jan, Frits, or the other members by asking questions that are contained in the earlier posts. That forum has become almost un-readbale with lazy idiots asking about squish clearance and spark plugs. Read the entire legacy Jan and Frits have left and respect their large intellectual investment in the forum.

Also, would you tell us what sort of work your intended engine will be doing? How many gears you have, what sort of selection of ratios are available, etc. It will help members to understand how to help you better

Good luck !!

Sure.  My intended use would be exclusively for racing.  I'm thinking that if I get a good cylinder with 62.5cc, it shouldn't be incredibly difficult to over- or under-bore it to 50cc/70cc/80cc so it could be used for different classes.  I fully intend to develop the engine with different porting, exhaust pipes, cylinder configurations, etc.  I don't think development ever stops, but that's why I want to do it.  I'm tired of just tuning existing engines with increasingly hard-to-find aftermarket parts.  Real development is doing something new.  Who knows?  Maybe it will never fire-up.  Maybe I'll take the national title.  At least I won't die with the regret that I never tried.

I have an extensive range of CNC, EDM, Laser-cutters, and such at my disposal so this definitely won't be a garage build.  Of course, things like cylinders would still have to be cast and then machined but CNC'ing the ports should allow for some amount of repeatability for control testing.  I plan on bench testing the cylinder on a pretty standard 85cc dirt bike case.  Unfortunately, I don't have access to flowmeters or any engine test equipment at work but Continental & Synerject are right down the road from me so if this gets serious, outsourcing for test tuning is not out of the question.

Engine will be a pretty standard 6-speed.  I plan on machining a custom bottom-end once the cylinder can pre produced reliably.  Removable gearbox (I was going to copy the Rotax ratios) but that's further on in the development.  I'm really more concerned with the way air flows through the cylinder.  I figure that's the key to unlocking the maximum power out of the engine.  I'm sure it will spend a lot of time on the bench dyno before ever turning a sprocket in anger.

Also, I would never dream of insulting the legends of two-stroke.  Some of these guys have lived the life I dream of so I have much respect for what they have done (or continue to do).  I will try my best to post intelligent questions.  For example, why does the RSA exhaust port flange out on the sides?  I've never owned a "real" GP bike before so I don't know how the exhaust pipe mates to the cylinder.  Is the shape beneficial to flow from the secondary exhaust ports?  How much does the exhaust valve close when fully closed?  Cylinder scavenging techniques with port arrangement?  Turbulence from rotary valve positioning?  I'm sure I can figure out what's the best kind of oil on my own but please forgive me if I don't know anything.  I am here to engage in constructive discussion, after all.

Ultimately, I want this to be an entire bike project.  I would like to do a small production run of built bikes for racing - something like Metrakit used to do with complete miniGP bikes.  I don't like being the only one on the grid and I certainly won't have any fun if I just show-up to destroy whatever 16-year-old that races with a modified dirt bike on slicks. This will be something fun but also serious. I'm anxious to start milling this big block of aluminum so I want to start with a good initial design - it will take some time. I'll be sure to post plenty of pictures as it comes along.

You are lucky that a racebike with 6-speeds will likely lend itself very well to the port time areas outlined by Frits and Jan on their RSA.
The answer to your exhaust port question lies within that forum, and I think its best that you read all of the context around that explanation rather than me relay it in brief here.
A guy you might want to connect with is Stefano, SB06 I think is his username. He has essentially already done what you propose for his company SG Racing in Italy.
Lastly, by way of encouragement, there is all the information you require already in the four RSA threads to get you designing and prototyping. Most people don't get far beyond the beginning of the designing phase. I hope you produce a finished cylinder and we can all enjoy it.
All the best and happy reading

Hello Damion, welcome to Pit-Lane.
Your plans sound very interesting. I hope you'll keep us informed of your proceedings, including the inevitable failures, because we all can learn much more from mistakes than from successes.
One remark struck me: you plan to build a short-stroke engine because of piston availability. With a short-stroke two-stroke you are starting with a considerable disadvantage. There must be suitable pistons around, for example here: [Vous devez être inscrit et connecté pour voir ce lien]

Hello Frits, a 43 x 43 should be better ou worse ?

Frits Overmars a écrit:

I hope you'll keep us informed of your proceedings, including the inevitable failures

Yes.  I know all too well about the success rate of prototypes.  I'm sure there will be plenty of revisions as time progresses.  As far as the piston size, I hadn't really considered other options.  I now see there are plenty of 43mm pistons available for retired 60cc dirt bikes.  I suppose a more square bore/stroke ratio would produce better figures.  Well, on to design number 2!

I've been spending most of my free time trying to re-read all of the RSA thread but there is a lot of great information to absorb.  I originally read what had progressed up to about a year ago but I just skimmed through it.  Now that I am a bit more serious about this project, I am trying to take my time and make notes of important details.  I'm sure I will have questions about the "why" of how things work the way they do when I get done reading it and many other great threads in this forum.

I'm hoping to have a new design done for the ports in the next few days (if time allows).  After that, I'll design coolant passages and I'll start considering what bottom-end it should mate-up to.  More to come soon!

Nice read, keep the good work.
What is the gudgeon pin size that you want to use?
If its 12mm there is a ton of pistons to choose from, there is pistons from 40 to 45mm from vertex to fit in the yamaha dt50lc, honda nsr 50, and a lot more, its just that none have a central pin for the ring, its always offset to the left or the right, but they cost around 30-40€ and the vertex ones are pretty light(never weighted one..).

Hello again.  I've been a little busy at work lately but I still made a little progress on the cylinder design.  Here are the port shapes I'm working with right now:
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(red = exhaust; green & blue = transfer ports)

I've also started shaping the main exhaust port's inside run.
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I'm not entirely sure if I should have the center of the exhaust port split.  It seems like a rather large area for the piston rings to bulge into but I guess only testing will tell.  I've been looking at all sorts of cylinders lately and studying they're port design in relation to their power output.  I already have several variations of the initial design I'd like to test but I'll stick with this one to start.  I'm also finding it difficult to shape some of the surfaces with the software I have at work.  I might remake it in another software suite.  More to come soon.  I just wanted to give an update.

Hello Damion !!

Why , Mr Frits , do you not promote your two opposite exhaust ports?
It would be very interesting!

nice looking work.

i do question the reason for doing.

WHY do I question ?

because much of the same has been done and is READILY AVAILABLE.

if i was to use my time in a similar effort, i would focus on producing an UPGRADED cylinder for one of the popular brands of VINTAGE dirt bikes .

make something new & improved that nobody has !

you could produce a modern reed valve cylinder for a 250 cc Bultaco Pursang which would utilize an improved con-rod length.

something like that would be adaptable for American flat track as well.

or, one of the popular mid-70's Yamaha 250's....there were thousands of those things made and one improved type of cylinder could be sold to any number of racers / restorers today.

maybe you could study an old CZ 250. one of those friggin' cylinders weighs a " ton ". Nikasil solves that problem.

you have to realize that many of those things are modified with reed valve set-ups. a modern, purpose built cylinder with integral reeds ( not a bolt-on block ) would be much better.

all in all, every area is only a niche market and i'm only throwing out ideas.

if you wanted to stay with a smaller cylinder, let's say within the 100 cc category, build an improved jug for a Kawasaki G31M. you might say those have a cult following, and decent versions of that bike bring " good dollars ."

in the same area, you could produce that type of cylinder adaptable to subsequent versions of Kawa's 100 cc Enduro / trail bike. those had larger transfer areas, but the cylinders used very mild porting.

again, only my opinion, but bottom line = pick & choose something that is UNIQUE, therefore marketable.

with your own special cylinder, it follows that a matching head would be needed.

Only one doubt, what are the transfer timings being used?
Your transfer ports seem very high, stealing you precious blow-down area.

ambike a écrit:

much of the same has been done and is READILY AVAILABLE.

I don't know if I entirely agree with that but the point of this project is that I will be making a prototype GP engine.  I want to show-up at a track on something unique that displays my talents (or shortcomings) as a designer.  At the very least, I want to fail (while at the same time knowing it was my best attempt).  If it makes you happy, one of the newest arrivals in my garage is a '73 Suzuki 125 of some sort.  I plan on restoring it over the winter but it will remain with it's "classic" engine configuration.

senso a écrit:

what are the transfer timings being used?

I'm afraid this computer doesn't have Photoshop (or any decent picture software) so here is a very crude drawing of the port map:

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The horizontal lines show the top and bottom of the ports.  I'm going with the 43,43 bore x stroke.  Not too sure about the ports, really.  I bought a dead Suzuki RM60 just to examine the port timings (since it also uses a 43mm piston).  The stroke I'm trying to achieve is a bit more so I just stretched the RM60's timing a little.  I know many of you might be scratching your heads as to this approach.  Please remember, the purpose of this exercise is to make me better at drawing geometry for work.  I'd really like it to also become an engine so I will try but I'm not an engineer - just a rather crafty machinist.  I'm mostly going on the thoughts of "Yeah, sure, I think that should work."  My plans are scribbles on cocktail napkins.  I have books and papers on two-stroke theory to read through but not much time to do that.  However, if anyone wants to throw some formulas up here, I'll do my best to include some calculations as this project progresses.

Here's some of the progress I've made on the design:

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You can now see the secondary exhaust ports (orange) and center intake port (dark blue) along with some of the cylinder's outline starting to take shape.  I didn't previously realize how little clearance the different parts of the cylinder have to each other.  I know there's not much to show in my work so far but I've been spending a lot of time just looking at the different parts of the cylinder - like how the water jacket passes around exhaust port and how much room is left for the studs that pass through?  Some of my co-workers must think I'm crazy when I sit there, staring down the bore for ten minutes at a time.  There's a lot to be said about trying to copy the shapes from the RSW/RSA's drawings (or any other engine) too.  So far, I've been trying to scale everything down while keeping as mathematically correct as possible.  Not easy given the software I'm using.  The interface is very crude and there is no "undo" button.  That last bit cost me half a day's worth of work after moving a section of geometry in the wrong axis.  I had both transfers almost completed!  Luckily, I seem to have convinced the higher powers at work that I should be using better software so a third revision of my design will be from scratch again in a week or two.  I have some more to share but I'll save that for next time.  Meanwhile, if you have any thoughts on port timing, I'd love to hear them.

As Frits and Jan say that incoming charge is the most important factor in cooling the piston, I think it is essential in any design that the bottom of the A and B ports is level with the piston top in BDC. In your drawing, A and B's bottom have different heights. I would make them the same.

I thought they mentioned the A, B ports are slightly higher than piston at BDC?