Metal Mule 2-into-1 exhaust system for Yamaha XT660R Review - £400

For some unknown reason Yamaha decided to run two pipes under the XT660R and into two heavy cans at the back. The MM two into one system fixes that by re-running the pipe round the right-hand side of the bike. Sounds good, but it’s not all it’s cracked up to be

adventure motorcycle exhaust


  • Price: £400 new (can be bought second hand now for much cheaper)
  • Time tested: 7 years
  • Testing conditions: UK-Middle East and RTW ride
  • Durability: 2
  • Ease of use: 1
  • Value for money: 2
  • Contact:


The system replaces the two individual header pipes and runs them into eachother on the right-hand side of the engine. The pack comes in three pieces: the main pipe, attachment pipe and the can. You also get a carbon fibre heat protection guard and two clips to hold your exhaust can in place.

What’s good?

Firstly, the system does away with the pipes being run underneath the bike. Secondly, it shaves 10kg off the bike’s weight by ditching the twin exhausts and thirdly, it sounds amazing. But unfortunately, the good stuff stops there.

What’s not?

Take a look at the brilliant XT660 Forum and you’ll see plenty of posts describing how difficult it is to fit the system. It is a sweaty, sweary pain. It’s tricky getting the pipes to sit in a way so that they don’t rest on the oil filter housing, or push up against the frame, or rub on the underside of the airbox. It’s a very tight squeeze through the frame and a little too close for my liking.

The major problem though is clearance. The end of the pipe is very close to the swingarm, any big compressions and the pipe will knock and rub on the top of the swingarm. The underside of the exhaust has also taken a battering from the brake calliper protector.

metalmule motorcycle exhaust

The heat shield is also problematic, it’s held on with three screw heads, and after the first day of ownership one of the heads had burned a hole in my motorcycle trousers. I decided to grind them off with an angle grinder and heat wrap the exhaust (you can check out how to heat wrap your exhaust in our Travel Tips section here. This also solved the problem of the very hot right leg.

The exhaust sat on my bike without complaint for the first six years, enduring a six-month long ride from the UK to the Middle East and back, off-roading and a few crashes. However, on the most recent trip it has literally fallen to pieces. Six days after setting off for a round-the-world ride with a pillion, the exhaust casing separated from the pipe and had to be welded back together. A month after that the baffle decided to detach itself from the exhaust and made a run for it on the motorway. As we couldn’t find it we had to make a new baffle and weld it into the exhaust.

metal mule two into one motorcycle exhaust



The Metal Mule system solves the pipes being run under the bike issue but throws up many more instead. If I could go back in time, a much, much easier fix would be to simply by a low-slung sump guard to protect the pipes.

Total score: 415

Please feel free to ask any questions about this review in the comments below or send us an email. If you’d like to see something specific reviewed then just let us know. Don’t forget to subscribe and we’ll send you an email once a month with a review, blog and travel tips round-up!

Subscribe to our blog

Here's a bunch of reasons why you just gotta!

  • You'll get updates on our adventures, travel advice, tips, tricks and destination guides
  • We'll only email you once a month
  • Your info will never be shared with anyone and you won't receive spam
  • It's easy peasy to unsubscribe
  • You'll love it, we promise!