I am about to purchase a new seatpost for my 09 specialized stupmjumper comp, the seatpost I am looking at is the wrong size, the post is the
pure racing I850 Seatpost 27.2 only.

This is the size of my seatpost according to the spesh website, 30.9×350/400mm.

So my question is can I use a shim to fit the pureracing post onto my bike?

this is the shim I am looking at..

http://www.leisurelakesbikes.com/product/useseatpostshim.aspx?&id=10598

what size shim??

thanks for any help

Yes that shim will be fine… The one at the top of the list according to the dimensions you have provided, 30.9/27.2

I ordered the crank brothers joplin instead 🙂

Make sure you get a fairly long shim, it'll be better for your frame.