I had planned on changing to a standard double next year, but I found a great deal on an Ultegra 6600 compact with bottom bracket. I have a couple of questions to see what else I would need.

1. The bottom bracket - my Felt has the older-style "internal" bb cups. Will the newer-style external cups fit my frame?

2. Rear derailleur - I currently have an Ultegra long-cage. Can this stay?

3. Is it possible to find a 39-tooth 110mm chainring if I decide I like my current gearing better?

JMO:

1. The external style will fit, but you may need to have the BB faced to get the sides perfectly parallel and square to the threading. Hopefully there's enough meat left on the BB to do this, since the width is kinda important, too.

2. Keep the long-cage Ultegra- the only thing a short cage will do for you is a slightly tighter lower run of the chain. Changing to a short cage would be primarily cosmetic, although it won't hurt anything.

3. Yes. Although I don't know where right off the top of my head...

4. Front derailleur- you may want to change to a double type- it will probably shift better. But the triple type will still work.

Thanks for the quick update. I forgot to mention I have a 6600 double front derailleur sitting on the shelf, and had planned on switching that as well.

I recently put the external cup style bb on a much older bike, a 1987 Schwinn. No problem. Good job finding a deal on that compact crank. They are on the pricey end.

+1 Keep the long cage RD. It allows you flexibility in any change you might make in the future.

Be prepared to do a lot more front shifting, depending on the terrain you ride. I used to spend most of my time on the center ring of a triple. Now I am frequently shifting from the big ring to the small ring on my compact.

3. Yes, they are available. Here is one source: https://sheldonbrown.com/harris/chainrings/110.html

The TA rings are expensive but the best choice. I believe the Suguino rings are "flat", that is they have no shifting enhancements like shaped teeth or pins and ramps. The Rocket Rings are for fixed gear/single speed use.

My experience differs.

When I swapped my Campagnolo Racing-T triple crank for an FSA carbon pro compact crank I could not adjust the front triple derailleur to both shift reliably to the big ring and not throw the chain. Swapping for a Campagnolo Chorus CT front derailleur designed for a compact crank fixed the problem.

The triple derailleur had a worn inner plate. A lower mileage part which had lost less profiling may have worked better.

The old crank and derailleur were of 8 speed vintage and chain Campagnolo 9. A derailleur designed for a 9-speed chain width/ring spacing might have worked reliably.

YMMV.

Unless you're loosing races by a fraction of a second (a whole five pounds is worth only 6 seconds on a 20 minute 7% climb and we're talking .5 pounds difference between an aluminum triple and carbon compact double) there isn't a good reason to trade your triple for a double. For the same high and low gears the triple will give you tighter spacing in your cruising range, you'll have more overlap between rings so you'll shift less up front, and there can be a smaller difference between ring diameter so it shifts better.

Doubles do look nicer.