Does anyone know where I can get one ? Prowled fleabay and all I can see is twin systems
Xt660r

Metal mule do a complete 2:1 system for the R. My friend has one. It sounds really nice.

MTC http://www.mtcperformanceproducts.co...st-28030-p.asp

There are essentially four 2-1 options for your XT660R. Here they are listed roughly in order of cost:

MTC - Cheapest option, using the stock low headers and a link pipe to allow a single can (and can be purchased with a forum discount):

http://www.xt660.com/showthread.php?t=22657

Creptus - Custom built high level headers and single can:

http://www.xt660.com/showthread.php?t=23542

http://www.creptus.cz/index.php/en/

Metal Mule (made by Scorpion) - High level headers and single can:

http://metalmule.com/by-product/exhaust-systems/yamaha-xt660r-xt660x-replacement-exhaust-system.html

OTR - High level headers and single can (the headers can also be bought separately so you can fit a choice of cans, or source one from elsewhere):

http://www.off-the-road.de/en/XT-660R-X/Exhausts/OTR-SR-2in1-system-e-marked.html

http://www.off-the-road.de/en/XT-660R-X/Exhausts/OTR-2in1-headpipe-system-XT-660.html

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I went the 2 into 1 route last year,I already run twin Akra's 1 and 2 filter holes in the airbox and a PC it runs well really well!! but hey you have keep looking so I bought a second hand system looked good and make's the bike look 100lbs lighter where the shock came was how it transform the engine and power characteristics.

The reality is the only way to find out and try one maybe going from a stock system to a 2 into 1 maybe a plus but after my twin Akra's it was a no from me but!!! if you have an R and will be doing the off roading thing that characteristic might float you boat.

I have got Metal Mule one, investigated for several months (back then MTC did not produce 2in1 system) and I believe MM is the best price/quality one.
If you ca afford - go for Akrapovic the finest on the market.
Anyway the weight saving is HUGE. The whole MM 2 in 1 weighs almost the same as one of the OE pipes. Saves around 10 kilos. And one should not forget the power gain. The sound is wonderful with dB killer on, without it I do not know yet
Whatever you choose, do not forget to remove/block the AIS, there is a very good thread here somewhere.
Last but not least you can search ebay for used piece but now as the season is on it will be difficult task.
Good luck

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that is how i feel about my XTR
Mods so far:
Hyperpro front springs; Hyperpro rear shock spring; modified air box; LED tail light; LED indicators + new relay; YZ pegs; Lucas aluminum bar; brisk spark plug; snorkel removed; DNA stage 1; second hand DynoJet O2 mod; crash bars; front and rear steel braided brake lines; soft EBC brake pads; mitas 07 tyres, 2in1 Metal Mule exhaust, Blocked AIS, tall OEM screen, OTR bash plate

Sorted I think
Been in contact with Mtc for a 2-1 and think I'm going for it