Replacement stem cap and bolt Our oldest son has been alternating between mine and my wife’s bike (Carreras from Halfords) whenever he has to go anywhere, but over the last week or so we noticed he had stopped using my bike, saying the other one was more comfortable.  I was moving the bikes into the shed last night and noticed that the stem cap and bolt (think that's they're called) are missing!!  Don't know what he did and I've not had a chance to talk to him about it yet.  Here is a picture of the bit I'm talking about:



Is this an easy part to source and is it difficult to fit correctly?  Am I better just taking the bike into Halfords and asking them to fix it (assuming they can)?

Update, I've got what I thought I needed and went to fit it today.  But it seems there's something else missing!  I've taken pictures, can anyone see what the issue might be:


Thanks Swifty, didn't notice I'd posted the wrong picture.  Here is a better one, it looks like there should be something inside the star.  The bold is just spinning.



Thanks

Alex
The cap and bolts are readily available, if you want no need to stick to a boring round black thing - they come in all sorts of weird and wonderful shapes, colours and functions. You can get them with bottle openers and clocks. 

The cap and bolt is only needed to set the preload on the bearings when installing the stem. Once the stem bolts are done up, the cap and bolt does nothing (although for safety, I would not ride a bike with no cap). Therefore if you do not undo the stem bolts, all you need to do is put on the new cap and do the  bolt up. The bolt only needs to be tight enough it does not come undone - 5nm should be enough (that is not much). You do not want to be a gorilla on the end of the spanner. 

If you have undone the stem bolts, you need to loosen the stem, do up the cap bolt till the play is removed from the head set, line up the handle bars and tighten the stem bolts (typically 5-7nm, much less than most people think).