Got a new stem coming from Velo Orange. It's 1 1/8". I'll be using the 1" to 1 1/8" stem shim Velo sells as my headset/fork size is 1" (Kilo TT).

My question is, since I'm switching from a 1" stem to 1 1/8" stem what size top cap should I be using? 1" or 1 1/8"?

It depends. If you are placing the top cap directly on top of the stem, then you want to use a 1-1/8" top cap. If you are placing a 1" spacer on top of the stem first, then you want to use a 1" top cap.

Thanks.

The top cap will go directly on top of the stem without a spacer (at least that's how I'm running things now and I doubt I'll want to change that with the new stem). So, I guess I should pick up a 1-1/8" top cap.

Will I run into any issues with temporarily running a 1" top cap with the 1-1/8" stem? The stem will probably be here before I have a chance to pick up a new top cap.

It depends on the outer diameter of your top cap. If it is less than 1-1/8", then it won't sit on top of your stem and will just fall into it, so you won't be able to adjust your headset.

#kiloTT

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Makes sense. Maybe I'll rock a 5mm spacer on top until I can pick up a new stem top cap. Kind of want to get the one VO sells as it will match the stem and look all perrtty but they are currently out of stock in silver.

Best to match whatver's on top. If you mismatch inbetween getting the right stuff, it's most likely not going to kill you unless you're doing some serious sprinting or climbing. The stem clamp on the steer tube is doing the real work.