Measuring for New Sealed Unit

[gazsellout]

Hi there.

I have a cracked (timber framed) double glazed window which I would like to have a go at replacing myself. I have located a firm who are able to supply a new sealed unit at a decent price and I am confident (with the help of a few previous posts on this board and some other sites) of fitting the new unit OK; however I am unsure as to how to exactly go about measuring for the new unit? Can anyone provide any pointers?
I would like to leave the unit in place, but I guess to measure accurately it would need to come out? If it needs to come out then I suppose I've answered my own question!

Any help would be greatly appreciated?

 

[denso13]

Measure to the outside of the beads, against the frame, and take off 10mm so the glass will have a 5mm gap all round to fit the packers.
The overall thickness of the unit is crucial. With one of the beads removed slide a ruler in to check the size. There are standard sizes eg 24 and 28mm.

 

[gazsellout]

Many thanks denso, a big help

I don't suppose anyone has any tips on the removal and subsequent replacement of the wooden beading?

 

[masona]

Using a panel pin punch to punch the nails through the beading or using a chisel in the middle carefully prising it apart

 

[gazsellout]

Cheers masona