Shimano SLX M666 disc brake review

Top notch stoppers

BikeRadar score4.5/5

Shimano’s SLX M666 brake packs all the best bits of the latest Japanese anchor generation into a remarkably affordable, top performance package.

The new double-barreled reservoir and master cylinder design has a subtle two-tone finish and the short deeply crooked lever gets tool-free reach adjust. There’s no Free Stroke bite point adjust but that’s a rather underwhelming and stiff feature anyway. 

The easy-fit hinged clamp with hidden safety catch is all present and correct, and syringe based bleeding is easy through the top cap. You get the same Ice Technologies finned heat dissipating pad technology of XTR too. 

This stops the mineral oil internals overheating, even when hammered relentlessly, and performance is superbly consistent. We’ve had no reliability issues with any of the new generation Shimano brakes, even in 18-month-old long-term or regularly crashed sets. You can even run them on cooking oil in an emergency.

Separate part selling means you can choose to run standard single piece steel rotors (£24.99), alloy centre spider rotors (£36.99) in conventional six-bolt or easy-to-fit/remove Center Lock splined versions, or trade up to the heat eating steel and alloy Ice Tech rotors (£44.99). Alternatively if you’re upgrading existing brakes you can buy the fully bled brake kit for just £89.99, making high weight the only potential gripe.

This article was originally published in Mountain Biking UK magazine, available on Apple Newsstand and Zinio.

Guy Kesteven

Freelance Writer, UK
Guy started filling his brain with cycle stats and steaming up bike shop windows back in 1980. He worked the other side of those windows from '89 while getting a degree in “describing broken things covered in mud" (archaeology). Dug historical holes in the ground through the early '90s, then became a pro bike tester in '97. Guy has ridden thousands of bikes and even more components the world over since then and can remember them all in vivid, haunting detail. Can't remember where the car keys are, though.
  • Age: 45
  • Height: 180cm / 5' 11"
  • Weight: 68kg / 150lb
  • Waist: 76cm / 30in
  • Chest: 91cm / 36in
  • Discipline: Strict sadomasochist
  • Preferred Terrain: Technical off-piste singletrack and twisted back roads. Up, down, along — so long as it's faster than the last time he did it he's happy.
  • Current Bikes: An ever changing herd of test machines from Tri bikes to fat bikes and everything in between.
  • Dream Bike: His Nicolai Helius AM custom tandem
  • Beer of Choice: Theakston's Old Peculier (not Peculiar)
  • Location: Yorkshire, UK

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