As title says, is there some way to make this fork lock completely? Mine has around 1cm of travel even when locked, and it annoys me.

is the fork at the right pressure setting? My Reba did this when I let the fork pressure get too low. Inflate appropriately, and you should have no problem.

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I own a lot of Treks.

or fill it with a little more oil on the motion control cartridge!

The pressure is correct, but it still moves a cm or so when "fully" locked. Damond, will this affect dampening somehow?

Most RockShock forks have a small amount of dampening when locked whereas Fox forks tend to totally lock. I have 2 of each and there are advantages to both. The newer Fox have a threshold so you can make them like the RockShock. I tend to prefer the Fox on Single Speeds since I am standing and mashing more and the RockShock's on everything else. The totally locked out Fox's do seem to cost you time if you hit a root or rock on the way up a hill since they basically lift the whole bike instead of just the wheel.

That said, I have recently started running a Sid RTC3 on my fastest single speed (Air9C) and I am consistently faster on it in the middle position and not locking it out. It seems like it is more efficient (less small bump compliance, still not a horrible ride but peddling on flats doesn't move the fork much if any) in this setting and yet still fairly efficient in climbing. Also, it still takes the big hits very well since I generally use almost all the travel according to the rubber o-ring during a lap. I had read a review (I think it was Guitar Ted on 29inches.com) where the reviewer said that he had found riding the RTC3 on the middle setting was perfect for 95% of all riding. My initial reaction was "no way". After spending a month trying to make a remote for my RTC3 I gave up and tried it and I was shocked at how fast I was lapping. This has made me think that in general, we are using shocks that do not have enough dampening and that is costing us efficiency. I am running my RTC3 with 5 psi more in the neg air (2012 model)which firms them up a little more than even pressure. On a 9 mile loop I am consistently 3 tenths mph faster with this fork and I had the same bike with the same setup with Fox Float 29 with FIT and Kishima before. I moved the Fox to my Jet9.

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For certain parts stiffer is more important than lighter.