I have a campy record 8 speed hub/cassette,11 through 23. The rear derailleur is a 7402 Dura Ace. I was hoping to upgrade the cassette to a 13 through 26/28 campy 8 speed. Will the derailleur handle the larger gears? The crank is also campy record 42/52.
Per Shimano's original spec, the max cog is 26t. But people have been successful using a 28t cog so you may be able to get it to work "as is". If there isn't enough clearance and the jockey wheel runs into the large cog a longer B-screw will increase clearance.
The frame plays into the max cog specification as the distance from the center of the axle to the center of the derailleur hanger bolt varies with different bikes. So the 26t will be a worst case scenario where the center-to-center distance is a minimum and would be the situation where the longer B-screw will help.
Also note the maximum chain wrap for that derailleur is 28t which will be fine for what you're proposing:
- 28-13 = 15
- 52-42 = 10
- 10+15 = 25t wrap
Going to a compact crankset will easily exceed the chain wrap which you can work around by simply not using the small chainring with the smallest cog or two.