Looking for an XT660R master cylinder as I've recently fitted 660R wheels front and back on my 660Z, my spokes on an "08 had rusted badly and it was cheaper buying second hand wheels than having mine rebuilt.
Still have master cylinder mounted and both calipers with the right hand side cable tied and lockwired with a worn pad between the good ones to maintain a decent lever ratio.
Would like to get rid of the right hand caliper and lose some more weight from the front end, always found the brake on my R more than adequate.

Pete.

What stops you buyng a shorter banjo bolt that converts the line feed from double to single?..ie one from an R...

You generally find that the bore sizes between master cylinders differs between those for single disc and twin disc.
If I use the master cylinder off the 660Z with a single disc setup the resulting amount of pressure you have to use at the lever becomes excessive and the resulting loss of feel at the lever usually described as feeling wooden.
Having built and repaired many race bikes in my lifetime I have experienced this problem many times.
The bore in master cylinders differs in size from bike to bike, usually dependant on single or twin disc.

Pete.

fair enough.

I do a large number of miles on my TDR250 which still has the standard 320mm disc up front with a blue spot caliper from an R6 and a mastercylinder from a Yamaha WR125, the feel and stopping power is awesome and two fingers are more than enough to put you over the bars if you wished.
The standard master cylinder and the blue spot still make a good brake but the effort and the feel is pants.
Hope this explains my thinking.
If we were a little nearer, you would be more than welcome to have a spin on the TDR and see what can be achieved with a caliper or master cylinder change.
Pete.

http://www.ebay.co.uk/itm/YAMAHA-XT6...8AAOSwmoVZykUW