Stem Top Caps!
 

Let's bore down into the details here. Most of the stem top caps I've seen are not flat on the bottom side. They have a small 'cone' that extends into the steerer tube. Chris King caps come to mind. With the expander plug in the steerer tube, those with the cone require a small spacer above the stem. You can't get a really flat appearance. Are there any out there that are flat on the underside?

FSA comes to mind off the top of my cap, I mean head....
https://images-na.ssl-images-amazon....AC_SL1000_.jpg
https://www.amazon.com/Fsa-1-1-Compr...a-567649654766

did someone say top caps?

http://www.apexspeed.com/doug/random...celine_004.jpg

The spacer under the top cap is recommended by most fork and plug manufacturers afaik to ensure proper preload on the bearings and the stem is hitting the right part of the fork.

Totally flat is aesthetically pleasing but not sure it's exactly the right fit.

What Flash said. At least a 5mm spacer on top is usually a good idea - especially with carbon steerer forks. Though there are some exceptions. Cannondale SI forks/HS systems have to be installed with the top cap flat on top of the stem, but they have their own expansion plug to make this work.

Deda has a good option
 

I was always under the assumption that the 5mm above the stem was to ensure that you weren't creating a clamping pressure spot on the fork steer, and the stem's contact was 100%.

This is also a good reason - but dependent on fork manufacturer. Some are OK clamping at the end some not. Proper bearing preload another requirement. To get a proper preload without a spacer you'll have to cut the fork JUST right or have a bit of empty between the top of the fork and the top of the stem so the cap doesn't bottom out on the stem before appropriate preload is achieved.

I received a Zipp one with one of the Service Course stems, which should do what you're after. it's not flat on the underside but actually hollowed out.

looks like this:
https://www.picclickimg.com/d/l400/p...-Bolt-Road.jpg

but as others have said, I would recommend a 5mm spacer above your stem.

There are plenty of flat stem caps out there, you just have to google them.

I recently bought a titanium one that is perfectly flat both top and bottom on ebay for the same reason, minimalism that is .)

Nice. Where can those be purchased? thanks.

Thom, I *think* I may have something like what you want; a flat top cap made of carbon. If you can wait a few days until I return home from Mexico to verify that, I’d be happy to mail it off to you.

Contact me Thursday to remind me.

^ this...no shame in using a 5mm spacer...

I've been using Wolf Tooth top caps. They build the 5mm spacer in, so no need for the extra spacer. Just ends up being a bit of a thicc top cap.

this must be similar to what cannndale is using on my 2018 hi-mod synapse. its basically a 5mm top cap...the steerer is proud of the stem by a few mm.

I just looked, yup, same idea