My bike has a Dura-Ace FC-7900 crankset of 53-39T configuration. The spec sheet from Shimano says that the "Bolt circle diameter" is 130mm. The Dura-Ace FC-7950 compact crankset, which only comes in one configuration, (50-34T), and with a "Bolt circle diameter" of 110mm. The spec sheet for the FC-7950 says that the 34T chainring (part number Y-1KZ 34000) is Interchangable with the FC-7900 crankset. In fact, the footnotes at the bottom of the FC-7950 spec sheet says, "Same parts".

Is this true? I'm at the age (60 this August), and am pretty big (6' 200lb.) that I could use a little bit of help getting up the steeper, longer hills. Can I swap the FC-7950 34T chainring for my existing FC-7900 39T chainring, (knowing that the FC-7950 specsheet says "Same part", but the "Bolt circle diameter" are different)?

Thank you.

p.s. I really like the 53T and don't want to swap it out for a 50-34T compact crankset.

Nope. 130 is too large for anything smaller than 38t. FWIW, the tech docs for the 7950 on Shimano's site specify "B" interchangeability with FC6650/6650G (i.e., other compacts).

smallest that will fit on a 130mm bolt circle is a 38T if you can find one. maybe it's time to go to a wider range cassete with a midcage rear derrailleur?

Thanks guys. The bike came with an 12-27 cluster and I changed it to an 11-27, (by adding an 11T and removing the 16T). The largest Dura-Ace rear sprocket available is a 28T. Not worth the cost for one extra tooth. Maybe it's time to go with a 50-34T compact crank.

Is the right-hand crank easy and quick enough enough to swap back and forth, (depending on the ride I'll be doing), between the 53-39T and a 50-34T?

Never mind. At $500, I'll live with the 53-39T. I'll just have to get into better shape to pull the hills.

It would be cheaper just to knock off 20 pounds and keep your current setup.

(6 feet and 140 pounds here)

Working on it. 25 lbs. gone - 15 more to go to get to where I want to be.