I have a Giant XTC SX with a Truvativ Isis crank

Just giving it a clean and I can feel play in the bottom bracket 

What size do I need and what are my options for a replacement?

on the BB its says - single bearing 68mm shell o ring, 73mm shell no o ring

completely clueless otherwise

cheers 

o rings are there

so do I measure from o ring to o ring? 

where would the axle size be written?

 can't see anything

Once out its on the body or just measure it from end to end I guess it would be 113 or 118 mm.

Why not change to HT2 ? 

cheers

is there much advantage to hollowtech 2? 

 so I need to take it to my LBS to take out the BB, then order a new one and take it back for them to fit it