Dura Ace 7400 8-speed derailleur capacity

[Spinner]

My son needs larger cassette cogs for his DA 7400 short cage set-up. A recent post discussed compatibility for a 30 tooth cog for a DA 9-speed.

Does anyone have knowledge whether an 8-speed derailleur can accommodate a 30-tooth cog?

Thanks in advance for your comments.

[fiamme red]

It says here that you can use a maximum 26t cog: http://www.velobase.com/ViewComponent.aspx?ID=B9BE2FB8-66F2-42BA-985D-A2FE24E8C20D&Enum=108&AbsPos=433.

In practice, it's likely that a 28t and even 30t cog will work. Just be careful not to exceed the derailleur capacity by using a crank with a big difference between smaller and bigger chainring (e.g., 50/34).

[KJMUNC]

I run a 28t on my Merckx that has this derailleur with no issues. I'd thought about moving up to a 30t but just haven't gotten around to it.

Interested to know if anyone else has done this successfully

[rccardr]

28 is pretty much the limit (until someone chimes in here about how they run a 34 LoL) on MOST bikes. If yours has a somewhat longer dropout then you may get lucky.

To maximize big cog/small chainring clearance, 1) screw the B screw in all the way, or even reverse it, 2) Remove the dropout screws and pull the rear axle all the way back in the dropouts, lastly 3) shorten the chain by a link or two and give up using big/big.

Might also want to try a Wolftooth dropout extender. Better yet, a Deore 9 speed RD. Right tool for the job, and all that.

[Spoker]

30 t. But cross chaining can problemon 50/34 if chain length is incorrect.

[RWL2222]

I am about to find out with an 8-speed Ultegra, on the way to moving to an Ultegra 9spd mix setup (tip o the hat to another forum memeber).

The Sheldon Brown has some good info. I found this article quite useful also https://www.bikeradar.com/advice/buyers-guides/a-complete-guide-to-rear-derailleurs/?image=1&type=gallery&gallery=1&embedded_slideshow=1

Somebody has done all of this before but I am interested in exploring a couple of experiments with the bike mounted on the stand. First up, I have a 12-34 cassette that should arrive next week. I will try it with a 50/34 crank (another tip o the hat). This suggests a wrap of 38. To bridge the gap, I will try a longer hanger with the 6400 RD. In a phone conversation yesterday with the leading hanger maker, this approach is NOT recommended in this instance. Next I will try a XT long cage RD. This should definitely work, as the poster above notes. I have a new chain and a second hand backup at the ready. This most direct line and the cheapest is probably found at RivBike, who are of course very helpful. An XTR RD would prob be closest to the dura ace setup.

This is an experiment on a the way to a more standard setup. I have two other old bikes (Mtb) to bring refresh, and I want to learn a bit. There are other experts who are a better positioned to advise on your son’s race setup. I pass this along because I had the same question as you. It is a pain how hard it is to update a perfectly working, but sadly outdated groupset, and a slippery slope to a new bike. I would love to go buy a modern bike and keep my current bone stock. But now is not the time for that.

[415km]

I think you'll find a bit of a range in answers as the hanger itself matters in what you can get away with but I have run a 32t cassette with that derailleur and it worked very good.
Rccardr's advice is good. I did not have to resort to all of that but did have to turn the B screw in a lot but again it depends on your exact setup. I didn't have enough capacity to run small/small but that was fine by me as I personally don't think that's good practice.