Correct Stem Size re Steering Tube

[MacB]

Want to buy the Ritchey Adjustable Road stem, here:-

http://www.wiggle.co.uk/p/Cycle/7/Ritchey_Adjustable_Road_Stem/5360033323/

it would appear that they all come to fit a 1 and 1/8 inch diameter steering tube. Without dismantling and physically measuring is there any way to know what size my existing steering tube is? Even going to the Giant web site I can't get anymore than 'Giant A3 Aluminium Ahead Riser' as a description. I want to order the above stem but don't want it to arrive and not fit, do I need to order a shim as per below?

http://www.bikeplus.co.uk/cgi-bin/q...hits_seen=&page=search.html&and=&affiliate_id

thank....Al

 

[spandex]

If you have a Ahead set (non threaded) is the over name BTW there is a 99% chance you a 1 1/8in steerer not a 1in.

 

[MacB]

thanks Spandex, have ordered the MTB version of the Ritchey stem, as matches the 25.4mm clamp on the bars I have. Went for the 120mm version as my current stem, at 100mm, has all of the spacers below it. Figured that will give me maximum flexibility for getting right position. The butterfly bars do bring the main shifting/braking position closer to the body.