
I am not a great climber (think worse than Andy Schleck on TT bike) so thought a compact crankset (34/50) and new cassette (12-28) would help get me up some big hills. Currently running a 9 speed Shimano Tiagra groupset with 39/53 and 12-21.Did a lot of research to find a compact crank (FC-4550) compatible with my 9 speed chain and sourced it from PBK. The Shimano site and this driver train chart were quite useful.
Finally got around to fitting the new cranks. Got the cranks off OK. Discovered I had an Octalink V2 BB,

whilst my brand new cranks are Hollowtech II


Options included sell the new cranks and get a new set that fits the current BB. Better option may be to replace BB-something I have never attempted before (bit new at being a bike mechanic, this was actually the first time I've had the cranks off!). Can really recommend the X-Tools toolkit from CRC-has everything i need and great price!
Worked out I had a 68mm BB with British threads so could work out which way to loosen the BB. Got the BB out okay to confirm 68-113 BB.

My Question is: Can I take the current BB out and fit a Hollowtech BB of the correct dimensions? Are the holes the BB goes into a standard size/will Hollowtech II fit?
Looks like the 6700 BB should work?
But, having made one mistake i want to avoid another.
Whilst I had the BB out, I noticed it was a bit rusty and even worse when I looked inside:


It all cleaned up OK and I used lots of grease putting it back together. Shouldn't there be a drain hole?
All advice much appreciated