Help?!

[winjim]

I mean, even apart from explaining how the top has a recess for the bolt head and the bottom is shaped to fit with the spacers, I've literally posted a photo of the underside of a headset top cap. It's curved on top to accommodate the bolt head, although some designs are flatter but a little deeper, it's stepped to fit snugly into the spacer, and it's flat on the bottom to provide even pressure.

I now feel the need to take all my bikes' headsets apart just to prove a point but I don't have time, so here's a picture of the lid of the sugar bowl from my kid's tea set which is the same shape. Disregard the knob.

 

[Vantage]

Disregard the knob.

 

[Ajax Bay]

it's stepped to fit snugly into the spacer, and it's flat on the bottom to provide even pressure.

I feel your pain; and fervently hope that you enjoyed finishing: "Disregard the knob." as much as I laughed, reading it.
But I think the 'flat' on the bottom of the top cap is just a structural construct (to spread the load of the bolt) and that flat section contacts nothing (so no 'pressure', even or otherwise). It's the edge of the top cap which provides the necessary compression (mummy bear) for the headset bearings, controlled by the tension on the bolt. A spacer above the stem clamp provides an assured 'mating ring' for the top cap, as opposed to the top of the stem clamp, which are not all well engineered.

 

[winjim]

I feel your pain; and fervently hope that you enjoyed finishing: "Disregard the knob." as much as I laughed, reading it.
But I think the 'flat' on the bottom of the top cap is just a structural construct (to spread the load of the bolt) and that flat section contacts nothing (so no 'pressure', even or otherwise). It's the edge of the top cap which provides the necessary compression (mummy bear) for the headset bearings, controlled by the tension on the bolt. A spacer above the stem clamp provides an assured 'mating ring' for the top cap, as opposed to the top of the stem clamp, which are not all well engineered.

I should have been more clear, I meant the bottom edge of the cap is flat to even the load, the inner bit can be any shape. Your explanation is much better than mine.

Does that step-edge shape on the bottom of the cap have a name? I want to call it a flange but that seems wrong.

 

[Ajax Bay]

I think your 'stepped' is good and 'flange' might obfuscate.

 

[winjim]

Right lads, strap in and hold on tight because I'm about to take you with me on a wild ride through my collection of spare headset top caps. Let's see how they compare and what design features we can identify that might help us to install them the correct way up.

Ritchey Carbon



Curved top, quite thick in order to accommodate the cap head bolt, underside looks curved but in fact those sides are straight.

FSA



Curved top, slightly less bulbous and thinner overall than the Ritchey. Cap head bolt, underside is curved, but they are concave curves as it were. That type of curve probably has a proper name but you know what I mean.

FSA Flat



Now this one's interesting. Flat on top and very thin which they've managed by using a countersunk bolt head. Underside is ever so slightly curved, concave again, to accommodate the bolt head.

Here's a picture of the three together for comparison. Ritchey in the middle.
So. Three headset top caps, all slightly different designs, two curved on top and one flat, all various thicknesses, but interestingly none of them flat on the bottom like the one in the photo I posted earlier. However, they all have some features in common:

They all have, on the top, some sort of recess to accommodate the bolt head, be it cap head or countersunk, so that the top of the bolt head is flush with or slightly beneath the top surface of the cap. None of them have such a recess on the underside, it would be obviously wrong if you tried to pass the bolt through the wrong way and it might not even reach to the star nut. I don't know for certain but it looks like this arrangement is to minimise lateral movement of the cap, it's acting as its own washer to distribute the pressure, if you see what I mean.

They all have that step shape to enable them to fit snugly inside the spacer and give even compression. That again should be obvious if it was the wrong way up, it wouldn't seat properly and maybe even slide off to the side.

And as pointed out upthread, they all have the logo on the top.

So yes, some top caps have curved undersides but it is a different type of curve to the top. Some, but not all of them, are flat on the top. But all of the ones in my spares collection have a very obvious top and bottom and should be very difficult if not impossible to install upside down. Other headset issues notwithstanding, the top cap in the op looks to be the right way up.

If anyone's interested and wants any photos of the caps taken from different angles or with different lighting, please do let me know, won't you?

 

[CycleSafeUK]

That looks flat?
View attachment 526907

it is curved
may not be entirely visible in the photo but it did slope down into a curve as such: