I've just bought a bike from my father in law, he hasn't had it long and the frame is a little too big for him so he sold it to me. not long ago he replaced the straight handlebars for a riser bar and he replaced the stem too. I think the handlebars are too thin for the stem because there is some movement when you lean on them, I took the Allen bolts out to check and there is a metal ring on the handlebars (I'm guessing to bulk them out so they will fit in the stem) but the bars wobble, mainly because the metal collar round the bars isn't the same length as the stem so the bars are only held in place round the small collar, not along the whole stem. I don't really want to replace the bars as they are just right for me, is there any way I can make them fit the stem (and if so, how?) or am I looking at buying a new stem?

Just make a new shim similar to what you found out of some pop can metal but this time make it a bit wider so it reaches the ends of the saddle on the stem.

If the metal that is in there is thicker than a pop can shim then it means that he likely put a road bar stem on the bike. Road bars have a slightly bigger seating diameter than mountain bike bars. So if you need a shim that is thicker than a single layer of pop can metal then you really should replace the stem with the same length and angle but made to fit mountain bike bar sizes.

They do make shims to adapt too small bars to too big stems but stems are cheap and teeth aren't so I'd recommend getting the right stem.

he definatly didn't put a road bike stem on the bike.
just to clear up, the bars aren't loose, sorry for the confusion. but because the metal shim on the bars isn't as long as the saddle on the stem, they do wobble. other than that the bars are held securely.

These two comments are mutually exclusive.

they wobble because the collar holding them in the stem is smaller than the inside of the stem. the allen bolts on the face plate are as tight as they will go and the bars are held tight. but the bars do look quite a lot bigger than the stem.

Wobble indicates movement, tight indicates no movement. You can't have it both ways.

You should be able to pick up a stem for less than $20 that will work without shims.

$20? try about £15.

not to worry, I think a new stem is in order, as I said earlier, I don't want to replace the bars as I like them, so a new stem it is.
thanks to all for your help and advice.

So you didn't try making a new shim?

Never paid over $15 w/ S+H on eBay for decent stems.

I did but I think a new stem that is the right size to fit the bars will be better than trying to make this one fit.