RobbiRobbi

I had one of the mounting screws at the bottom of my rear rack somehow work its way loose yesterday, and I’m going to the hardware store later to get a replacement screw and washer. When I looked closely at the mounting on the other side, I saw that I addition to the washer, the bike shop person who installed it years ago added a knurled nut that appears to be a valve stem nut. I don't know whether or not the same thing was on the other side.



Note that it’s not acting as a spacer between the rack and the frame. It’s sitting on the outside of the rack serving no purpose I can tell except making the mounting screw not reach all the way through the frame eyelet. I thought possibly there was paint or something on the eyelet threads preventing it from screwing all the way in, and they did this instead of properly clearing the hole. But when I tried putting the screw back without the spacer it only took a slight amount more force to screw it in the rest of the way (and extrude a few years of grime out of the hole.)

So, any legitimate reason for the spacer? Can I just leave it off, or is this Chesterton’s valve nut?

trailangel

This side... other side.... can't understand which side you are refferring too. Drive side or Non-drive side would be good to know.
Maybe it's acting as a spacer so the bolt (screw) doesn't go through and hit the chain.

Litespud

There's no earthly reason for that knurled nut to be there. All I can think of is trailangel's suggestion that it's acting as a spacer to prevent the screw from protruding too far toward the drivetrain. Remove the nut, tighten up the screw. If it doesn't protrude too much, ditch the nut (and save a gram in the process )

Phil_gretz

You got it. Either select a bolt that has the proper depth reach, or offset the bolt head (outward) to prevent the bolt from overhanding within the frame. It's cleaner that way.

RobbiRobbi

Sorry, the one in the picture is drive side. And yeah, that would make sense if the bolt were longer, but it actually doesn't protrude at all without the spacer. It's just about flush without it, so I'm going to leave it like that.

[Edit: And thanks to everyone for responding!]

dedhed

That's exactly what it is. A presta valve nut to keep a longer bolt from interfering with the drive train. Probably faster and easier than finding a shorter bolt.

Crankycrank

Or.....the presta nut was put there to keep the bolt from hitting the chain while the rack was not being used. I've done this so I don't have to keep two different length bolts on hand for rack or no rack setup. The bolt keeps the threads in the eyelet from getting gunked up when not in use and you don't have to hunt for the bolt when you want to install the rack. OP, just remove the presta nut with the rack in place and the bolt should be the right length.

JohnDThompson 

Looks like a knurled nut from a presta valve stem. I agree with @trailangel; it's probably because the bolt is too long and without the spacer, it would extend through the eyelet and interfere with the chain.

Troul 

use a stud & nut. That will enable the hardware to be left in place with or without the rack.