Hello,
I removed my 105 cassette for the first time to clean out all the junk in it. I installed it back and when I rode my bike this morning I had trouble shifting in the middle gears and my chain even got stuck while I was going downhill. I installed it with all the spacers and the numbers facing outwards. Did I do something wrong? Is it just a matter of adding more lube? See cassette picture below.

Picture of cassette (yes it got dirty again from today's ride):

Not sure if it's an optical illusion on the pic, but cassette doesn't look "straight".

Hmm... Here is another picture:

The nobby background may cause the apparent angle 1-2 cassette rings larger than the chain. Cassettes take a surprising amount of torque to fully tighten the
lock ring, which is illusory because of the ratchet effect which makes it seem tight when it is not. If you don't have a torque wrench to get the torque in the
right range, take the wheel off, unscrew the QR nut and put the cassette tool on and lock it in place with the QR nut on the QR shaft and give the lock ring
at least another 1/4 or even 1/2 turn. If lopek is right and there is an angularity in the cassette ring-to-ring then take it apart first and clean everything up and
reassemble.

OK Thanks - i'll try this. I rode my bike to work so might have my wife pick me up (IDK if it's safe to ride like this).

Also, I did stop tightening after I heard the clicking noise. I thought that meant it was good to go!

You probably need to tighten the lockring more, well past the clicking noise.

Be sure that all of the cogs are equal distance from each other. If you look at the cogs carefully you will see that each cog has one wider spline that fits a particular wider spline gap on the freehub. Make sure that the smallest cog is properly lined up so that it will slide over the spline while you tighten the lock ring. If it's not aligned correctly it can tighten on the ends of the splines, causing the other cogs to remain loose.

Thanks - I'll do that when I get home. But is this reason it skips gears/locks when riding? Is it safe to ride home?

cog number 6 look out of place (broken or bent ) ? maybe it the angle of the photo ? not sure .

Probably OK, just use light pressure on the pedals when shifting.

I got home and tightened the heck out of it. It's better but now cog 1 & 10 are problematic. When I shift to those gears it gives a wear clicking noise and tries to skip to the next gear.

If it was my bike... At this point, I'd probably:

• Remove the cassette.
• Inspect cassette/spacers/freehub.
• Replace the cassette, ensuring that it's tightened completely. If I remember correctly, Shimano specifies lockring torque of 35 n-m, or roughly 26 foot pounds. And if you have a spacer behind your cassette, like you would if you're using a 10-speed cassette on an 11-speed hub, make sure the cassette rivets (if there are any) fit into the notches on the spacer.
• Reinspect post-assembly.
• Check derailleur settings and adjust if necessary. Park Tool's tutorial HERE will walk you through the steps.

It might be more steps than you're hoping for, but at the end of the process, it's gonna work.

Now adjust the derailer, including the limit stop screws. Hopefully it will fix everything.

This. But then I like taking things apart until I know exactly how they go back together smoothly. I could be at this for a couple hours. heh.

I figured it out. See attachment. I put spacer #14 at the end instead of the front (toward the rim). D'oh!!!

Ya know - that's how we all learn. Thanks for the post.

Let's see ... 9:19 started out with a big wrench, 2 hrs later reported assembly error corrected. Yep, that's about right. Winning.

Maybe I should've emphasized the behind part? ;-)

Glad you got it figured out!

Hahaha thanks for your help guys! I’m just glad I didn’t have to go to my LBS. I feel like I have more confidence for the next go around.