had headset fitted by mechanic, and had to change bars so removed stem etc, now i was told to do somet thing with pre load? I put all stem back on and the likes, then torqued up the stem bolts and then the top cap? What on earth is the pre load and have i done something the wrong way round with the stem or top cap? I dont fancy dying for it to work loose etc? Help to put my mind at ease please!

tightening up the top cap before the stem bolts puts preload on the headset bearings

so thats a bad thing? So if i have tightened the stem bolts first and tightened the top cap last i have done it the right way?

wrong way round. pull everything together with the top cap and then tighten the stem up.

No, you've done it the wrong way round.
Tighten the top cap first to pull everything together (preload it).
Then tighten the stem clamp bolts to hold it all in place.

Pre-load is everything being squashed tight toward the crow, by tightening the the stem bolts first you can't do it. You need to do the top cap first but be careful with the hopes their downfall is you can't tighten the topcap bolt a huge amount before it goes ping.

Baby Robin!

be careful with the hopes their downfall is you can't tighten the topcap bolt a huge amount before it goes ping.

you shouldn't need to tighten everything a huge amount…

ah right cheers- so everything goes back on, tighten top cap, the torque the stem bolts and job done? Does it just mean the bearings last longer if done correctly? Also is about 2nm torque about right for top cap?

enough to take any play out of the headset.

and yes, if you take play out of the headset then everything lasts longer

thanks guys, just out of interest though, what happens if its ridden when tightened the other way? It still feels secure and sturdy? Just ko'ing the bearings? Also if someone can tell me the torque for top cap?

There is no exact torque for the top cap.

Tighten it up and lift the front of the bike off the ground and move the stem side to side to feel the headset bearings moving. Tighten it more and feel the headset "tighten" and become stiff. Loosen it and feel the headset become too sloppy. Then tighten it 'til you get a happy medium. Then torque up the stem bolts.

It's done by feel not numbers.

you shouldn't need to tighten everything a huge amount…

I know but the op clearly doesn't.