MEGASGAS: 700cc Two-Stroke Supermoto!

GasGas “EC700” Supermoto from Sib Mustard…  

For most of us, the Honda CR500 and Kawasaki KX500 represent something akin to the apex predators of the two-stroke dirt bike world — 500cc smokers infamous for their light-switch powerbands, violent soundtracks, and lethal reputations.

Of course, when speaking of such insane machines, any rational hot-rodder has to ask: what if you could…go bigger? After all, for some speed freaks, “fast enough” just doesn’t exist. In the words of Hunter S. Thompson:

“On my tombstone they will carve, IT NEVER GOT FAST ENOUGH FOR ME.” -Hunter S. Thompson, “Song of the Sausage Creature”

GasGas 700cc

Enter our new friend Sebastian “Sib” (@sib_mustard) of Germany’s HAISL — a bike-builder and two-stroke lover who’s earned a large following on social media for crazy customs like the 700cc two-stroke supermoto you see here.

GasGas 700cc

The bike began life as a GasGas EC300, a lightweight two-stroke designed to tackle the gnarly terrain of hard enduro.

“This is the lightest enduro bike I have personally ever ridden… On our scales, amazingly the EC300 weighed in at 106kg [234 lbs] with ½ a tank of fuel in it; even lighter than the spec sheet.” –Planet Knox

But Sib had other plans for the powerplant, modifying the frame to accept the 699cc liquid-cooled two-stroke Mega engine from Austrian company Rübig Motortechnik — an engine designed for sidecar motocross!

This required not only frame modifications, but a custom swingarm, rear shock mount, exhaust, and more.

GasGas 700cc

Sib says he wanted to build a “wolf in sheep’s clothing,” preserving much of the traditional GasGas design language but with a black and copper color palette inspired by the Cupra R.

Aptly enough, Sib has nicknamed this monster the MEGASGAS:

“I wanted to keep the original GASGAS design, hence the large MEGASGAS lettering, but with steampunk elements because it fits well into the color concept.”

GasGas 700cc

Though Sib hasn’t had a chance to get the MEGASGAS on a dyno yet, he says the engine is rated at 84 hp. While you might expect such a big engine to add a ton of weight to the bike, Sib says the Mega engine is only 10 kg (22 lbs) heavier than the original GasGas engine, meaning the bike weighs 115-120 kg (255-265 lbs) — about the same as a CRF450X!

GasGas 700cc

As you might expect, the vibration is enormous — especially since the engine has no balancer shaft — but comfort was never the name of the game when dealing with big-bore two-strokes.


All in all, this is one of the wildest smokers we’ve seen, and Sib is just getting started.

“My next project is already being planned and I will attempt a 2-stroke turbo build.”

We can’t wait, man! Below, talk to Sib for more details on this “EC700” MEGASGAS.

GasGas “EC700” Supermoto: Builder Interview

700cc Supermoto

• Please tell us a bit about yourself, your history with motorcycles, and your workshop.

I’m Sib, and I’ve always loved working on bikes. When I first got my driving license, I also worked on cars a lot. At some point I got back into two-wheeled enduro sports and, through social media, into supermoto sports. I don’t have a workshop where I build bikes to sell; this is my workshop where I only build bikes for my content on social media. Many of my followers only know me through my crazy and detailed builds.

700cc Supermoto

• What’s the make, model, and year of the donor bike / engine?

It is/was a 2021 GASGAS EC300.

• Why was this bike built?

It’s my content on social media, and I often try to make builds that have little precedent or don’t exist. My next project is already being planned and I will attempt a 2-stroke turbo build.

700cc Supermoto

• What was the design concept and what influenced the build?

In terms of color, I was inspired by Nissan Cupra (black and copper). I wanted to keep the original GASGAS design, hence the large MEGASGAS lettering, but with steampunk elements because it fits well into the color concept.

700cc Supermoto

• What custom work was done to the bike?

Mainly modifying the frame so that the 700cc engine fits in. I particularly wanted to preserve the original look of the GasGas (wolf in sheep’s clothing). The damper mount of the special swingarm was also adjusted with special CNC work.


• Does the bike have a nickname?

Yes, MEGASGAS, because it is a GASGAS and the 700cc engine manufacturer is called MEGA.

700cc Supermoto

• Any idea of the weight or horsepower?

It’s just the heavy engine that makes the difference. Otherwise everything is the same as with an original 300 GASGAS. I haven’t weighed it but it’s only 10kg heavier. So around 115-120 kg.

I haven’t been on the dyno with this machine yet, but according to the engine manufacturer, this engine should produce 84 hp.

GasGas 700cc

• Can you tell us what it’s like to ride this 700cc smoker?

It is definitely not a touring bike. This engine comes from sidecar racing and is an old concept, so it does not have a balancer shaft. The vibration is enormous. You don’t want to ride it for a long time.

• Is there anyone you’d like to thank?

My main sponsor HAISL, the other parts sponsors and my followers, who make financing my projects much easier.

GasGas 700cc

Build Photos

GasGas 700cc

Follow the Builder

Instagram: @sib_mustard
Youtube: @sib_mustard
Main Sponsor: | @haisl_official




  1. That engine looks like yet another iteration of the old Maico 700

  2. This is absolutely stupendous. I have never seen a 2-stroke cylinder that big. Wow. Super nice finish. Surprised that it seems pretty docile at idle.

  3. It’s so beautiful, powerful and most important is that it’s the builders art..

  4. Nice work but not really a wolf in sheep’s clothing with that big bore pipe it gives the game away to a smokes eye..if you like upside down power bands get a big boar.good that pipe

  5. Mouhamed bhd

    I want to have one it’s a dream coming true were I can buy the bike .

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