Ordered a their "Thunder Shark Pro Racing 66cc/80cc Bicycle Engine Kit - 5.0 HP" kit. Unfortunately, the "Boost Expansion Chamber Muffler" head does not fit with the "Shark Racing CNC Cylinder Head". Also, the sprocket and the chain don't mesh! I was terribly disappointed as I bought a kit thinking my build would be easy. Upon contacting gasbike - it took days to get a response. When I finally was able to get a hold of someone, they said it couldn't be an issue b/c they sell hundreds of these kits. They also told me to look at the picture on the website as it shows the muffler with that head. Ironically when looking at the pic - it's a different head!!! They did offer to do an exchange, so I shipped my parts back. I was told when I sent a return shipping tracking number they would ship the new parts. After sending my tracking number and inquiring about it - I had one of their salespersons call and yell at me for sending too many emails - when I onlly sent one! This has been the worst customer experience I have ever had to deal with and have yet to receive anything! It's been 4 weeks! Be forewarned, not all the kit parts fit!!! AND If you have any issues, you're on your own!!! As of right now I've LOST OVER $150 inparts! ... and I'm still waiting. KNOW THAT gasbike.net AND kingsmotorbikes.com are the SAME company!!! RUN FROM BOTH! Do yourself a favor, simply call their phone number. You won't get an answer and after about 5 minutes it will simply hang up. Save yourself the hassle - go somewhere else - it's not worth the money you might save - assuming you don't have any issues. User's recommendation: RUN from gasbike.net AND kingsmotorbikes.com.